ALBERT

Description: At the end of Earth's accretion and after the core-mantle segregation, the existence of a basal magma ocean at the top of the CMB depends on the physical properties of mantle materials at relevant pressure and temperature. Present-day deep mantle structures such as ultralow-velocity zones (ULVZs) and low-shear velocity provinces (LLSVPs) might be directly linked to the still ongoing crystallization of a primordial magma ocean. We provide the first steps towards a self-consistent thermodynamic model of magma ocean crystallization at high-pressure. We build a solid-liquid thermodynamic database for silicates in the MgO-FeO-SiO 2 system from 20 GPa to 140 GPa. We use already published chemical potentials for solids, liquid MgO and SiO 2 . We derive standard state chemical potential for liquid FeO and mixing relations from various indirect observations. Using this database, we compute the ternary phase diagram in the MgO-FeO-SiO 2 system as a function of temperature and pressure. We confirm that the melt is lighter than the solid of same composition for all mantle conditions but at thermodynamic equilibrium, the iron-rich liquid is denser than the solid in the deep mantle. We compute a whole fractional crystallization sequence of the mantle and show that an iron rich and fusible layer should be left above the CMB at the end of the crystallization.

Description: Seismic noise in the period band 3-10 s is known as secondary microseism and it is generated at the ocean surface by the interaction of ocean gravity waves coming from nearly opposite directions. In this paper, we investigate the seismic content of the wavefield generated by a source at the ocean surface and three of the major wavefield shaping factors using the 2D spectral-element method: the ocean-continent boundary, the source site effect and the thickness of seafloor sediments. The seismic wavefield recorded on the vertical component seismograms below the seafloor is mainly composed of the fundamental mode and the first overtone of Rayleigh waves. A mode conversion from the first overtone to the fundamental mode of Rayleigh waves occurs at the ocean-continent boundary. The presence of a continental shelf at the ocean-continent boundary produces a negligible effect on land-recorded seismograms, whereas the source site effect, i.e. the source location with respect to the local ocean depth and sediment thickness, plays the major role. A source in shallow water mostly enhances the fundamental mode of Rayleigh waves, whereas a source in deep water mainly enhances the first overtone of Rayleigh waves. Land-recorded long period signals ( T  〉 6 s) are mostly due to deep water sources, whereas land-recorded short period signals ( T  〉 6 s) are due to sources in relatively shallow water, located close to the shelf break. Seafloor sediments around the source region trap seismic waves reducing the amplitude of land-recorded signals, especially at long periods ( T  〉 6 s).

Description: Using the unique datasets provided by satellite observations, correlated temporal variations in gravity and magnetic fields over a large area extending from the Atlantic to the Indian Ocean have been recently reported [ Mandea et al ., 2012]. On a time scale of few years to a decade, both field variations may be linked to changes at the top of the core. Here, we propose that, in addition to the topography generated by the convection in the mantle, the core mantle boundary (CMB) may be in a dynamic equilibrium state, mainly controlled by a dissolution-crystallization process of the mantle silicate rocks in the liquid alloy of the core. Due to the resulting continuous changes in CMB topography, anomalies of hundreds of nGals and tens of nTyr −2 might be produced by the corresponding mass redistribution and the secondary flow generated by the associated pressure field. Numerical modeling and both gravimetric and magnetic anomaly magnitudes suggest a rate of centimeters per year and a large range of length scales for the changes in the topography at the CMB.

Description: We implement a technique to characterize the electromagnetic properties at frequencies 100 to 165 GHz (3 cm −1 to 4.95 cm −1 ) of oriented smectite samples using an open cavity resonator connected to a sub-millimeter wave VNA (Vector Network Analyzer). We measured dielectric constants perpendicular to the bedding plane on oriented Na + and Ca ++ -ion stabilized smectite samples deposited on a glass slide at ambient laboratory conditions (room temperature and room light). The clay layer is much thinner (∼ 30 μ m) than the glass substrate (∼ 2.18 mm). The real part of dielectric constant, ε r e , is essentially constant over this frequency range but is larger in Na + - than in Ca ++ -ion infused clay. The total electrical conductivity (associated with the imaginary part of dielectric constant, ε i m ) of both samples increases monotonically at lower frequencies (〈 110 GHz), but shows rapid increase for Na + ions in the regime 〉 110 GHz. The dispersion of the samples display a dependence on the ionic strength in the clay interlayers, i.e., ζ -potential in the Stern layers.

Description: We employ 130 low-frequency-earthquake (LFE) templates representing tremor sources on the plate boundary below southern Vancouver Island to examine LFE magnitudes. Each template is assembled from 100's to 1000's of individual LFEs, representing over 269,000 independent detections from major episodic-tremor-and- slip (ETS) events between 2003 and 2013. Template displacement waveforms for direct P - and S -waves at near epicentral distances are remarkably simple at many stations, approaching the zero-phase, single pulse expected for a point dislocation source in a homogeneous medium. High spatio-temporal precision of template match-filtered detections facilitates precise alignment of individual LFE detections and analysis of waveforms. Upon correction for 1-D geometrical spreading, attenuation, free-surface magnification and radiation pattern, we solve a large, sparse linear system for 3-D path corrections and LFE magnitudes for all detections corresponding to a single ETS template. The spatio-temporal distribution of magnitudes indicates that typically half the total moment release occurs within the first 12–24 hours of LFE activity during an ETS episode when tidal sensitity is low. The remainder is released in bursts over several days, particularly as spatially extensive RTRs, during which tidal sensitivity is high. RTRs are characterized by large magnitude LFEs, and are most strongly expressed in the updip portions of the ETS transition zone and less organized at downdip levels. LFE magnitude-frequency relations are better described by power-law than exponential distributions although they exhibit very high b -values ≥ ∼ 7. We examine LFE moment-duration scaling by generating templates using detections for limiting magnitude ranges ( M W  〈 1.5, M W  ≥ 2.0). LFE duration displays a weaker dependence upon moment than expected for self-similarity, suggesting that LFE asperities are limited in fault dimension and that moment variation is dominated by slip. This behaviour implies that LFEs exhibit a scaling distinct from both large-scale slow earthquakes and regular seismicity.

Description: Back–arc extension and asthenosphere upwelling associated with oceanic lithospheric subduction affect the structure and thermal regime of the arc lithosphere, which often triggers widespread extension–related mafic magmatism. Although it is commonly accepted that the Neo–Tethyan oceanic lithosphere subducted beneath the southern Lhasa block, resulting in the well-known Late Mesozoic Gangdese magmatic arc, the possible role of contemporary back–arc extension and asthenosphere upwelling has been disputed due to a lack of evidence for extension–related mafic magmatism. Here, we report detailed petrological, geochronological, geochemical and Sr–Nd–Hf–O isotopic data for the Dagze diabases located in the north of the Gangdese district, southern Lhasa block. The zircon U–Pb analyses indicate that they were generated in the Late Cretaceous (ca. 92 Ma) instead of the Eocene (42–38 Ma) as previously believed. These mafic rocks are characterized by variable MgO (4.0–12.2 wt.%) and Mg # (42 to 71) values combined with flat to slightly enriched ([La/Yb] N = 1.87–5.23) light rare earth elements (REEs) and relative flat heavy REEs ([Gd/Yb] N = 1.36–1.87) with negative Ta, Nb and Ti anomalies (e.g., [Nb/La] PM = 0.16–0.51). They also have slightly variable ε Nd (t) (–1.25 to +4.71) and low initial 87 Sr/ 86 Sr (0.7045–0.7058) values with strong positive igneous zircon ε Hf (t) (+8.0 to +12.1) and low δ 18 O (5.31–6.12 ‰) values. The estimated primary melt compositions are similar to peridotite–derived experimental melts. Given their high melting temperature (1332 to 1372 °C) and hybrid geochemical characteristics, we propose that the Dagze mafic magmas likely represent mixtures of asthenospheric and enriched lithospheric mantle–derived melts that underwent minor crustal assimilation and fractional crystallization of clinopyroxene. Taking into account the spatial and temporal distribution of Mesozoic mafic–felsic magmatic rocks and regional paleomagnetic and basin data, we suggest that the Dagze mafic rocks resulted from asthenospheric upwelling associated with intra–continental back–arc extension during the roll–back of subducted Tethyan oceanic lithosphere in the Late Cretaceous.

Description: Dissolution of fractured rocks is often accompanied by the formation of highly localized flow paths. While the fluid flow follows existing fractures in the rock, these fissures do not in general open uniformly. Simulations and laboratory experiments have shown that distinct channels or “wormholes” develop within the fracture, from which a single highly localized flow path eventually emerges. The aim of the present work is to investigate how these emerging flow paths are influenced by the initial aperture field. We have simulated the dissolution of a single fracture starting from a spatially correlated aperture distribution. Our results indicate a surprising insensitivity of the evolving dissolution patterns and flow rates to the amplitude and correlation length characterizing the imposed aperture field. We connect the similarity in outcomes to the self-organization of the flow into a small number of wormholes, with the spacing determined by the length of the longest wormholes. We have also investigated the effect of a localized region of increased aperture on the developing dissolution patterns. A competition was observed between the tendency of the high-permeability region to develop the dominant wormhole and the tendency of wormholes to spontaneously nucleate throughout the rest of the fracture. We consider the consequences of these results for the modeling of dissolution in fractured and porous rocks.

Description: Many efforts have been made to quickly estimate the maximum run-up height of tsunamis associated with large earthquakes. This is a difficult task, because of the time it takes to construct a tsunami model using real time data from the source. It is possible to construct a database of potential seismic sources and their corresponding tsunami a priori.However, such models are generally based on uniform slip distributions and thus oversimplify the knowledge of the earthquake source. Here, we show how to predict tsunami run-up from any seismic source model using an analytic solution, that was specifically designed for subduction zones with a well defined geometry, i.e., Chile, Japan, Nicaragua, Alaska. The main idea of this work is to provide a tool for emergency response, trading off accuracy for speed. The solutions we present for large earthquakes appear promising. Here, run-up models are computed for: The 1992 Mw 7.7 Nicaragua Earthquake, the 2001 Mw 8.4 Perú Earthquake, the 2003Mw 8.3 Hokkaido Earthquake, the 2007 Mw 8.1 Perú Earthquake, the 2010 Mw 8.8 Maule Earthquake, the 2011 Mw 9.0 Tohoku Earthquake and the recent 2014 Mw 8.2 Iquique Earthquake. The maximum run-up estimations are consistent with measurements made inland after each event, with a peak of 9 m for Nicaragua, 8 m for Perú (2001), 32 m for Maule, 41 m for Tohoku, and 4.1 m for Iquique. Considering recent advances made in the analysis of real time GPS data and the ability to rapidly resolve the finiteness of a large earthquake close to existing GPS networks, it will be possible in the near future to perform these calculations within the first minutes after the occurrence of similar events. Thus, such calculations will provide faster run-up information than is available from existing uniform-slip seismic source databases or past events of pre-modeled seismic sources.

Description: Most volcanic explosions leave a crater in the surface around the center of the explosions. Such craters differ from products of single events like meteorite impacts or those produced by military testing because they typically result from multiple, rather than single, explosions. Here we analyze the evolution of experimental craters that were created by several detonations of chemical explosives in layered aggregates. An empirical relationship for the scaled crater radius as a function of scaled explosion depth for single blasts in flat test beds is derived from experimental data, which differs from existing relations and has better applicability for deep blasts. A method to calculate an effective explosion depth for non-flat topography (e.g. for explosions below existing craters) is derived, showing how multi-blast crater sizes differ from the single blast case: Sizes of natural caters (radii, volumes) are not characteristic of the number of explosions, nor therefore of the total acting energy, that formed a crater. Also the crater size is not simply related to the largest explosion in a sequence, but depends upon that explosion and the energy of that single blast and on the cumulative energy of all blasts that formed a crater. The two energies can be combined to form an effective number of explosions that is characteristic for the crater evolution. The multi-blast crater size evolution has implications on the estimates of volcanic eruption energies, indicating that it is not correct to estimate explosion energy from crater size using previously published relationships that were derived for single blast cases.

Description: The entrainment of air by turbulent mixing plays a central role in the dynamics of volcanic eruption clouds, as the amount of entrained air controls the height of the plume. In one-dimensional models of volcanic plumes, the efficiency of entrainment under a wind field is parameterized using two empirical constants. The first is the coefficient associated with the entrainment caused by the shear between the volcanic plume and the ambient air (i.e., the radial entrainment coefficient), and the other is associated with the entrainment caused by wind (i.e., the wind entrainment coefficient). In this study, we used three-dimensional numerical simulations of volcanic plumes to determine the effective values of these empirical constants from the relationship between eruption conditions and plume heights. These simulations suggest that the value of the radial entrainment coefficient is 0.05–0.06, which is slightly smaller than that of pure jets or plumes seen in laboratory experiments (0.07–0.15). The value of the wind entrainment coefficient was estimated to be 0.1–0.3, which is significantly smaller than those estimated from laboratory experiments (0.3–1.0). The entrainment coefficients derived from these simulations successfully explain the observations made during the 2011 Shinmoe-dake eruptions in which the volcanic plumes were significantly distorted by the wind.

Description: Geophysical observations suggest that mature faults weaken significantly at seismic slip rates. Thermal pressurization and thermal decomposition are two mechanisms commonly used to explain this dynamic weakening. Both rely on pore fluid pressurization with thermal pressurization achieving this through thermal expansion of native solids and pore fluid and thermal decomposition by releasing additional pore fluid during a reaction. Several recent papers have looked at the role thermal pressurization plays during a dynamically propagating earthquake, but no previous models have studied the role of thermal decomposition. In this paper we present the first solutions accounting for thermal decomposition during dynamic rupture, solving for steady-state self-healing slip pulses propagating at a constant rupture velocity. First, we show that thermal decomposition leads to longer slip durations, larger total slips, and a distinctive along-fault slip rate profile. Next, we show that accounting for more than one weakening mechanism allows multiple steady slip pulses to exist at a given background stress, with some solutions corresponding to different balances between thermal pressurization and thermal decomposition, and others corresponding to activating a single reaction multiple times. Finally, we study how the rupture properties depend on the fault properties, and show that the impact of thermal decomposition is largely controlled by the ratio of the hydraulic and thermal diffusivities χ = α h y / α t h and the ratio of pore pressure generated to temperature rise buffered by the reaction P r / E r .

Description: Faults involving phyllosilicates appear weak when compared to the laboratory-derived strength of most crustal rocks. Amongst phyllosilicates, talc, with very low friction, is one of the weakest minerals involved in various tectonic settings. As the presence of talc has been recently documented in carbonate faults, we performed laboratory friction experiments to better constrain how various amounts of talc could alter these fault's frictional properties. We used a biaxial apparatus to systematically shear different mixtures of talc and calcite as powdered gouge at room temperature, normal stresses up to 50 MPa and under different pore fluid saturated conditions, i.e. CaCO 3 -equilibrated water and silicone oil. We performed slide-hold-slide tests, 1-3000 seconds, to measure the amount of frictional healing and velocity-stepping tests, 0.1-1000 μ m/s, to evaluate frictional stability. We then analyzed microstructures developed during our experiments. Our results show that with the addition of 20% talc the calcite gouge undergoes a 70% reduction in steady-state frictional strength, a complete reduction of frictional healing and a transition from velocity-weakening to velocity-strengthening behavior. Microstructural analysis shows that with increasing talc content, deformation mechanisms evolve from distributed cataclastic flow of the granular calcite to localized sliding along talc-rich shear planes, resulting in a fully interconnected network of talc lamellae from 20% talc onwards. Our observations indicate that, in faults where talc and calcite are present, a low concentration of talc is enough to strongly modify the gouge's frictional properties and specifically to weaken the fault, reduce its ability to sustain future stress drops and stabilize slip.

Description: This paper considers the importance of model parametrization, including dispersion, source kinematics and source discretization, in tsunami source inversion. We implement single and multiple time window methods for dispersive and non-dispersive wave propagation to estimate source models for the tsunami generated by the 2011 Tohoku-Oki earthquake. Our source model is described by sea surface displacement instead of fault slip, since sea surface displacement accounts for various tsunami generation mechanisms in addition to fault slip. The results show that tsunami source models can strongly depend on such model choices, particularly when high-quality, open-ocean tsunami waveform data are available. We carry out several synthetic inversion tests to validate the method and assess the impact of parametrization including dispersion and variable rupture velocity in data predictions on the inversion results. Although each of these effects have been considered separately in previous studies, we show that it is important to consider them together in order to obtain more meaningful inversion results. Our results suggest that the discretization of the source, the use of dispersive waves, and accounting for source kinematics are all important factors in tsunami source inversion of large events such as the Tohoku-Oki earthquake, particularly when an extensive set of high quality tsunami waveform recordings are available. For the Tohoku event, a dispersive model with variable rupture velocity results in a profound improvement in waveform fits that justify the higher source complexity and provide a more realistic source model.

Description: During 2013 Mt. Etna volcano experienced intense eruptive activity at the summit craters, foremost at the New Southeast Crater and to a minor degree at the Voragine and Bocca Nuova (BN), which took place in two cycles, February-April and September-December. In this work, we mainly focus on the period between these cycles, applying a multiparametric approach. The period from the end of April to 5 September showed a gradual increase in the amplitude of long period (LP) events and volcanic tremor, a slight inflation testified by both tilt and GPS data, and a CO 2 flux increase. Such variations were interpreted as due to a gradual pressurization of the plumbing system, from the shallowest part, where LP and volcanic tremor are located, down to about 3-9 km b.s.l., pressure source depths obtained by both geodetic and CO 2 data. On 5 September, at the same time as a large explosion at BN, we observed an instantaneous variation of the aforementioned signals (decrease in amplitude of LP events and volcanic tremor, slight deflation, and CO 2 flux decrease), and the activation of a new infrasonic source located at BN. In the light of it, the BN explosion probably caused the instantaneous end of the pressurization, and the opening of a new vent at BN, that has become a new steady source of infrasonic events. This apparently slight change in the plumbing system also led to the gradual resumption of activity at the New Southeast crater, culminating with the second lava fountain cycle of 2013.

Description: The Tonga subduction zone is among the most seismically active regions and has the highest plate convergence rate in the world. However, recorded thrust events confidently located on the plate boundary have not exceeded M w 8.0, and the historic record suggests low seismic coupling along the arc. We analyze two major thrust fault earthquakes that occurred in central Tonga in 2006 and 2009. The 3 May 2006 M w 8.0 event has a focal mechanism consistent with interplate thrusting, was located west of the trench, and caused a moderate regional tsunami. However, long-period seismic wave inversions and finite-fault modeling by joint inversion of teleseismic body waves and local GPS static offsets indicate a slip distribution centered ~65 km deep, about 30 km deeper than the plate boundary revealed by locations of aftershocks, demonstrating that this was an intraslab event. The aftershock locations were obtained using data from 7 temporary seismic stations deployed shortly after the mainshock, and most lie on the plate boundary, not on either nodal plane of the deeper mainshock. The fault plane is ambiguous and investigation of compound rupture involving co-seismic slip along the megathrust does not provide a better fit, although activation of megathrust faulting is responsible for the aftershocks. The 19 March 2009 M w 7.6 compressional faulting event occurred below the trench; finite-fault and W-phase inversions indicate an intraslab, ~50-km deep centroid, with ambiguous fault plane. This event also triggered megathrust faulting. There continues to be a paucity of large megathrust earthquakes in Tonga.

Description: Long-period (LP, 0.5-5 Hz) seismicity, observed at volcanoes worldwide, is a recognized signature of unrest and eruption. Cyclic LP “drumbeating” was the characteristic seismicity accompanying the sustained dome-building phase of the 2004–2008 eruption of Mount St. Helens (MSH), WA. However, together with the LP drumbeating was a near-continuous, randomly occurring series of tiny LP seismic events (LP “subevents”), which may hold important additional information on the mechanism of seismogenesis at restless volcanoes. We employ template matching, phase-weighted stacking, and full-waveform inversion to image the source mechanism of one multiplet of these LP subevents at MSH in July 2005. The signal-to-noise ratios of the individual events are too low to produce reliable waveform-inversion results, but the events are repetitive and can be stacked. We apply network-based template matching to 8 days of continuous velocity waveform data from 29 June to 7 July 2005 using a master event to detect 822 network triggers. We stack waveforms for 359 high-quality triggers at each station and component, using a combination of linear and phase-weighted stacking to produce clean stacks for use in waveform inversion. The derived source mechanism pointsto the volumetric oscillation (~10 m 3 ) of a subhorizontal crack located at shallow depth (~30 m) in an area to the south of Crater Glacier in the southern portion of the breached MSH crater. A possible excitation mechanism is the sudden condensation of metastable steam from a shallow pressurized hydrothermal system as it encounters cool meteoric water in the outer parts of the edifice, perhaps supplied from snow melt.

Description: The standard model of injection-induced seismicity considers changes in Coulomb strength due solely to changes in pore-pressure. We consider two additional effects: full poro-elastic coupling of stress and pore-pressure, and time dependent earthquake nucleation. We model stress and pore-pressure due to specified injection rate in a homogeneous, poroelastic medium [ Rudnicki , 1986]. Stress and pore-pressure are used to compute seismicity rate through the Dieterich [1994] model. For constant injection rate, the time to reach a critical seismicity rate scales with t  ~  r 2 /( cf c ), where r is distance from the injector, c is hydraulic diffusivity, and f c is a factor that depends on mechanical properties, and weakly on r . The seismicity rate decays following a peak, consistent with some observations. During injection poro-elastic coupling may increase or decrease the seismicity rate, depending on the orientation of the faults relative to the injector. If injection induced stresses inhibit slip, abrupt shut-in can lead to locally sharp increases in seismicity rate; tapering the flux mitigates this effect. The maximum magnitude event has been observed to occur post-injection. We suggest the seismicity rate at a given magnitude depends on the nucleation rate, the size distribution of fault segments, and if the background shear stress is low, the time varying volume of perturbed crust. This leads to a roll-over in frequency magnitude distribution for larger events, with a “corner” that increases with time. Larger events are absent at short times, but approach the background frequency with time; larger events occurring post shut-in are thus not unexpected.

Description: Compaction bands in sandstone are laterally-extensive planar deformation features that are characterized by lower porosity and permeability than the surrounding host rock. As a result, this form of localization has important implications for both strain partitioning and fluid flow in the Earth's upper crust. To better understand the time-dependency of compaction band growth, we performed triaxial deformation experiments on water-saturated Bleurswiller sandstone (initial porosity = 0.24) under constant stress (creep) conditions in the compactant regime. Our experiments show that inelastic strain accumulates at a constant stress in the compactant regime, manifest as compaction bands. While creep in the dilatant regime is characterized by an increase in porosity and, ultimately, an acceleration in axial strain rate to shear failure, compaction creep is characterized by a reduction in porosity and a gradual deceleration in axial strain rate. The global decrease in the rates of axial strain, acoustic emission energy, and porosity change during creep compaction is punctuated at intervals by higher rate excursions, interpreted as the formation of compaction bands. The growth rate of compaction bands formed during creep is lower as the applied differential stress, and hence background creep strain rate, is decreased. However, the inelastic strain associated with the growth of a compaction band remains constant over strain rates spanning several orders of magnitude (from 10 −8 to 10 −5  s −1 ). We find that, despite the large differences in strain rate and growth rate (from both creep and constant strain rate experiments), the characteristics (geometry, thickness) of the compaction bands remain essentially the same. Several lines of evidence, notably the similarity between the differential stress dependence of creep strain rate in the dilatant and compactant regimes, suggest that, as for dilatant creep, subcritical stress corrosion cracking is the mechanism responsible for compactant creep in our experiments. Our study highlights that stress corrosion is an important mechanism in the time-dependent porosity loss, subsidence, and permeability reduction of sandstone reservoirs.

Description: The Earth's long-wavelength geoid anomalies have long been used to constrain the dynamics and viscosity structure of the mantle in an isochemical, whole-mantle convection model. However, there is strong evidence that the seismically observed large low shear velocity provinces (LLSVPs) in the lower mantle underneath the Pacific and Africa are chemically distinct and likely denser than the ambient mantle. In this study, we have formulated dynamically self-consistent 3D spherical mantle convection models to investigate how chemically distinct and dense piles above the core-mantle boundary (CMB) may influence the geoid. Our dynamic models with realistic mantle viscosity structure produce dominantly spherical harmonic degree-2 convection, similar to that of the present-day Earth. The models produce two broad geoid and topography highs over two major thermochemical piles in the lower mantle, consistent with the positive geoid anomalies over the Pacific and African LLSVPs. Our geoid analysis showed that the bottom layer with dense chemical piles contributes negatively to the total geoid, while the layer immediately above the chemical piles contributes positively to the geoid, cancelling out the effect of the piles. Thus, the bottom part of the mantle, as a compensation layer, has zero net contribution to the total geoid, and the thickness of the compensation layer is ~1000 km or two to three times as thick as the chemical piles. Our results help constrain and interpret the large-scale thermochemical structure of the mantle using surface observations of the geoid and topography, as well as seismic models of the mantle.

Description: Here, we report on the first assessment of volatile fluxes from the hyper-acid crater lake hosted within the summit crater of Copahue, a very active volcano on the Argentina-Chile border. Our observations were performed using a variety of in situ and remote sensing techniques during field campaigns in March 2013; when the crater hosted an active fumarole field, and in March 2014, when an acidic volcanic lake covered the fumarole field. In the latter campaign, we found that 566 to 1373 t·d −1 of SO 2 were being emitted from the lake in a plume that appeared largely invisible. This, combined with our derived bulk plume composition, was converted into flux of other volcanic species (H 2 O~ 10989 t·d −1 , CO 2 ~ 638 t·d −1 , HCl ~66 t·d −1 , H 2 ~3.3 t·d −1 , HBr ~0.05 t·d −1 ). These levels of degassing, comparable to those seen at many open-vent degassing arc volcanoes, were surprisingly high for a volcano hosting a crater lake. Copahue's unusual degassing regime was also confirmed by the chemical composition of the plume that, although issuing from a hot (65 °C) lake, preserves a close-to-magmatic signature. EQ3/6 models of gas-water-rock interaction in the lake were able to match observed compositions, and demonstrated that magmatic gases emitted to the atmosphere were, virtually unaffected by scrubbing of soluble (S, Cl) species. Finally, the derived large H 2 O flux (10,988 t·d −1 ) suggested a mechanism in which magmatic gas stripping drove enhanced lake-water evaporation, a process likely common to many degassing volcanic lakes worldwide.

Description: No abstract is available for this article.

Description: We find that microseisms generated by Hurricane Sandy exhibit coherent energy within 1-hour time windows in the frequency band of 0.1-0.25 Hz, but with signals correlated among seismic stations aligned along close azimuths from the hurricane center. With the identification of this signal property, we show that travel-time difference can be measured between the correlated stations. These correlated seismic signals can be attributed to two types of seismic sources, with one group of the seismic signals from the hurricane center and the other from coastal region. The seismic sources in coastal region are diffusive and move northward along the coastline as Sandy moves northward. We further develop a hurricane seismic source model, to quantitatively describe the coupling among sea level pressure fluctuations, ocean waves and solid earth in the region of hurricane center, and determine the evolution of source's strength and pressure fluctuation in the region of hurricane center using seismic data. Strong seismic sources are also identified near the coastal region in New England after Sandy's dissipation, possibly related to subsequent storm surge in the area. The seismic method may be implemented as another practical means for hurricane monitoring, and seismological estimates of the hurricane seismic source model could be used as in-situ proxy measurements of pressure fluctuation in the region of hurricane center for hurricane physics studies.

Description: We explore the effects of variable material properties, phase transformations and metamorphic devolatilization reactions on the thermal structure of a subducting slab using thermodynamic phase equilibrium calculations combined with a thermal evolution model. The subducting slab is divided into three layers consisting of oceanic sediments, altered oceanic crust, and partially serpentinized peridotite. Solid–fluid equilibria and material properties are computed for each layer individually to illustrate distinct thermal consequences when chemical and mechanical homogenization within the slab is limited. Two extreme scenarios are considered for a newly forming fluid phase: complete retention in the rock pore space or instantaneous fluid escape due to porosity collapse. Internal heat generation or consumption due to variable heat capacity, compressional work, and energetics of progressive metamorphic and devolatilization reactions contribute to the thermal evolution of the slab in addition to the dominating heat flux from the surrounding mantle. They can be considered as a perturbation on the temperature profile obtained in dynamic or kinematic subduction models. Our calculations indicate that subducting sediments and oceanic crust warm by 40 and 70 °C, respectively, before the effect of wedge convection and heating is encountered at 1.7 GPa. Retention of fluid in the slab pore space plays a negligible role in oceanic crust and serpentinized peridotites. By contrast, the large volatile budget of oceanic sediments causes early fluid saturation and fluid-retaining sediments cool by up to 150 °C compared to their fluid-free counterparts.

Description: We calibrated the magnitude and symmetry of seismic anisotropy for 132 mica- or amphibole-bearing metamorphic rocks to constrain their departures from transverse isotropy (TI) which is usually assumed in the interpretation of seismic data. The average bulk V p anisotropy at 600 MPa for the chlorite schists, mica schists, phyllites, sillimanite-mica schists, and amphibole schists examined is 12.0%, 12.8%, 12.8%, 17.0%, and 12.9%, respectively. Most of the schists show V p anisotropy in the foliation plane which averages 2.4% for phyllites, 3.3% for mica schists, 4.1% for chlorite schists, 6.8% for sillimanite-mica schists, and 5.2% for amphibole schists. This departure from TI is due to the presence of amphibole, sillimanite and quartz. Amphibole and sillimanite develop strong crystallographic preferred orientations (CPO) with the fast c-axes parallel to the lineation, forming orthorhombic anisotropy with V p (X)〉V p (Y)〉V p (Z). Effects of quartz are complicated, depending on its volume fraction and prevailing slip system. Most of the mica- or amphibole-bearing schists and mylonites are approximately transversely isotropic in terms of S-wave velocities and splitting although their P-wave properties may display orthorhombic symmetry. The results provide insight for the interpretation of seismic data from the southeast Tibetan plateau. The N-S to NW-SE polarized crustal anisotropy in the Sibumasu and Indochina blocks is caused by subvertically foliated mica- and amphibole-bearing rocks deformed by predominantly compressional folding and subordinate strike-slip shear. These blocks have been rotated clockwise 70-90° around the east Himalayan Syntaxis, without finite eastward or southeastward extrusion, in responding to progressive indentation of India into Asia.

Description: The aim of this work is to improve our understanding of the long-period (LP) source mechanism at Mt Etna (Italy) through a statistical analysis of detailed LP catalogues. The behaviour of LP activity is compared with the empirical laws governing earthquakes recurrence, in order to investigate whether any relationships exist between these two apparently different earthquake classes. We analysed a family of 8894 events detected during a temporary experiment in August 2005. For that time interval, the LP activity is sustained in time and the volcano did not exhibit any evident sign of unrest. The completeness threshold of the catalogue is established through a detection test based on synthetic waveforms. The retrieved amplitude distribution differs significantly from the Gutenberg-Richter law, and the inter-event times distribution does not follow the typical γ-law, expected for tectonic activity. In order to compare these results with a catalogue for which the source mechanism is well-established, we applied the same procedure to a dataset from Stromboli Volcano, where recurrent LP activity is closely related to Very-Long-Period pulses, in turn associated with the summit explosions. Our results indicate that the two catalogues exhibit similar behaviour in terms of amplitude and inter-event time distributions. This suggests that the Etna's LP signals are most likely driven by stress changes caused by an intermittent degassing process occurring at depth, similar to that which drives the summit explosions at Stromboli Volcano.

Description: GPS has been extensively used to estimate tidal ground displacements, but the accuracy of this has not been systematically verified. Using more than 20 sites distributed across western Europe, we show that post-processed kinematic precise point positioning GPS with appropriately-tuned process noise constraints is capable of recovering synthetic tidal displacements inserted into real data, with a typical accuracy of 0.2 mm depending on the time series noise. The kinematic method does not result in erroneous propagation of signals from one coordinate component to another, or to the simultaneously-estimated tropospheric delay parameters. It is robust to the likely effects of day-to-day equipment and reference frame changes, and to outages in the data. A minimum data span of 4 years with at least 70% availability is recommended. Finally, we show that the method of reducing apparent coordinate time series noise by constraining the tropospheric delay to values previously estimated in static batch GPS analysis, in fact results in the suppression of true tidal signals. Using our kinematic GPS analysis approach, periodic displacements can be reliably observed at the 0.2 mm level, which is suitable for the testing and refinement of ocean tide and solid Earth response models.

Description: GPS-observed vertical ocean tide loading displacements show in Cornwall, southwest England and in Brittany, northwest France, discrepancies of 2–3 mm with predicted values based on isotropic PREM for the main tidal harmonic M 2 , yet in central Europe the agreement is better than 0.5 mm. By comparison of ocean tide models and validation with tide gauge observations, we demonstrate that the uncertainties in the former are too small to cause this disagreement. Furthermore, we find that different local models of the crust and different global elastic reference models derived from seismological observations can only reduce the observed discrepancies to 1–2 mm, which still exceeds the GPS observational uncertainty of 0.2-0.4 mm. It is customary to use the elastic properties of the Earth as given by seismic models. Previously, there has been insufficient evidence to determine how to modify these properties during the transformation from seismic to tidal frequencies to account for possible anelastic dispersion in the asthenosphere, and so this effect has been ignored. If we include this effect, then our discrepancies reduce further to 0.2-0.4 mm. This value is of the same order as the sum of the remaining errors due to uncertainties in the ocean tide models and in the GPS observations themselves. This research provides evidence in western Europe of a reduction of around 8-10% of the seismic shear modulus in the asthenosphere at tidal frequencies. In addition, we find that the asthenosphere absorption band frequencies can be represented by a constant quality factor Q .

Description: This study investigated the utility of multistation waveform cross correlation to help discern induced seismicity. Template matching was applied to all Ohio earthquakes cataloged since the arrival of nearby EarthScope TA stations in late 2010. Earthquakes that were within 5 km of fluid injection activities in regions that lacked previously documented seismicity were found to be swarmy. Moreover, the larger number of events produced by template matching for these swarmy sequences made it easier to establish more detailed temporal and spatial relationships between the seismicity and fluid injection activities, which is typically required for an earthquake to be considered induced. Study results detected three previously documented induced sequences (Youngstown, Poland Township, and Harrison County) and provided evidence that suggests two additional cases of induced seismicity (Belmont/Guernsey County and Washington County). Evidence for these cases suggested that unusual swarm-like behaviors in regions that lack previously documented seismicity can be used to help distinguish induced seismicity, complementing the traditional identification of an anthropogenic source spatially and temporally correlated with the seismicity. In support of this finding, we identified 17 additional cataloged earthquakes in regions of previously documented seismicity and away from disposal wells or hydraulic fracturing that returned very few template matches. The lack of swarminess helps to indicate these events are most likely naturally occurring.

Description: The variations in the response of different state evolution laws to large velocity increases can dramatically alter the style of earthquake nucleation in numerical simulations. But most velocity step friction experiments do not drive the sliding surface far enough above steady state to probe this relevant portion of the parameter space. We try to address this by fitting 1–3 orders of magnitude velocity step data on simulated gouge using the most widely used state evolution laws. We consider the Dieterich (Aging) and Ruina (Slip) formulations along with a stress dependent state evolution law recently proposed by Nagata et al. (2012). Our inversions confirm the results from smaller velocity step tests that the Aging law cannot explain the observed response and that the Slip law produces much better fits to the data. The stress dependent Nagata law can produce fits identical to, and sometimes slightly better than, those produced by the Slip law using a sufficiently large value of an additional free parameter c that controls the stress dependence of state evolution. A Monte Carlo search of the parameter space confirms analytical results that velocity step data that are well represented by the Slip law can only impose a lower bound on acceptable values of c , and that this lower bound increases with the size of the velocity step being fit. We find that our 1–3 orders of magnitude velocity steps on synthetic gouge impose this lower bound on c to be 10–100, significantly larger than the value of 2 obtained by Nagata et al. (2012) based on experiments on initially bare rock surfaces with generally smaller departures from steady state.

Description: Magnetotelluric derived two-dimensional lithospheric resistivity structure of the Western Dharwar Craton (WDC) and adjoining Coorg Block indicates isolated low resistivity zones in the crust and three striking upper mantle conductive features within the high resistive Archean lithosphere. The crustal conductors in the WDC show good spatial correlation with the exposed supracrustal rocks conformable with the relic schist belt channels having conductive mineral grains. Conductive zones within the Coorg crust might be related to the relatively young (933 Ma) metamorphic processes in the area and/or possible fluids derived from the Cretaceous passage of Reunion plume in the proximity of Coorg area. A near vertical conductive structure extending from the lower crust into the upper mantle coincides with the transition zone between Coorg and WDC. This is interpreted as the suture zone between the two tectonic blocks and provides evidence for the individuality of the two Archean terrains. An anomalous upper mantle conductive zone found beneath the craton nucleus may indicate a modified cratonic lithosphere. This could have been derived due to the collision between Coorg and WDC, and possibly survived by the subsequent multiple episodes of melt and fluid infiltration processes experienced in the region. Thick (~190 km) and preserved lithosphere is mapped at the eastern segment of WDC. Resistive lithosphere of ~125 km thickness is imaged for the Coorg Block.

Description: The most accepted conceptual model to explain surface degassing of cold magmatic CO 2 in volcanic-geothermal systems involves the presence of a gas reservoir. In this study, numerical simulations using the TOUGH2-ECO2N V2.0 package are performed to get quantitative insights into how cold CO 2 soil flux measurements are related to reservoir and fluid properties. Although the modeling is based on flux data measured at a specific geothermal site, the Acoculco caldera (Mexico), some general insights have been gained. Both the CO 2 fluxes at the surface and the depth at which CO 2 exsolves are highly sensitive to the dissolved CO 2 content of the deep fluid. If CO 2 mainly exsolves above the reservoir within a fracture zone, the surface CO 2 fluxes are not sensitive to the reservoir size but depend on the CO 2 dissolved content and the rock permeability. For gas exsolution below the top of the reservoir, surface CO 2 fluxes also depend on the gas saturation of the deep fluid as well as the reservoir size. The absence of thermal anomalies at the surface is mainly a consequence of the low enthalpy of CO 2 . The heat carried by CO 2 is efficiently cooled down by heat conduction and to a certain extent by isoenthalpic volume expansion depending on the temperature gradient. Thermal anomalies occur at higher CO 2 fluxes (〉37,000 g m −2 day −1 ) when the heat flux of the rising CO 2 is not balanced anymore. Finally, specific results are obtained for the Acoculco area (reservoir depth, CO 2 dissolved content and gas saturation state).

Description: The sound velocities of four iron-nickel-silicon liquids (Fe-5wt%Ni-6wt%Si, Fe-5wt%Ni-10wt%Si, Fe-5wt%Ni-14wt%Si and Fe-5wt%Ni-20wt%Si) are measured between 1460 and 1925 K at ambient pressures using ultrasonic interferometry. The results constrain both the dependence on Si content of the bulk modulus of these liquids, and the temperature-dependence of their elasticity. These elastic data are utilized to assess both relatively low-pressure (to 12 GPa) compressional data on Fe-Si liquids, and to extrapolate to higher pressure and temperature conditions. If a single equation of state for Fe-Ni-Si liquids of a given composition applies from low pressure to near core conditions, then our results imply that the isothermal pressure derivative of the bulk modulus of these liquids is high: likely 8 and above at high temperatures. This high value of the pressure derivative of the bulk modulus at low-pressures in Fe-Si liquids causes marked stiffening at higher pressures, leading to notable incompressibility and apparent low values of the pressure derivative of the bulk modulus at core conditions. These results reinforce the conclusion that silicon is not a major alloying component of Earth's core.

Description: This paper describes the principles of three novel seismic imaging techniques and their application to two deep seismic reflection data sets from the vicinity of the German Continental Deep Drilling Site (KTB). These imaging techniques are based on Kirchhoff prestack depth migration and use an inherent restriction of the migration operator to focus the wavefield to its actual reflection point. For Fresnel volume migration, the emergent angle at the receivers is estimated and then used to propagate the wavefield back into the subsurface along which the Fresnel volume is determined. The migration operator is restricted to this volume, thereby focusing the image to the part of the isochrone which physically contributes to the reflection. For Coherency migration, the coherency of the wavefield at neighboring traces is calculated and used as a weighting factor within the migration integral, leading to a comparable focusing to the reflection point. For Coherency based Fresnel volume migration, both approaches are combined, resulting in an even more focused seismic image with significantly increased image quality. We applied these methods to two seismic data sets from the area around the KTB: a survey with standard split spread geometry (KTB8502), and a sparse data set with a small number of source points in combination with short receiver lines (INSTRUCT93). The focusing approaches yield major improvements in the final images for both data sets. Incoherent noise and migration artifacts are reduced and the visibility of crustal structures is strongly enhanced, allowing for an improved geologic and tectonic characterization.

Description: Double-difference methods applied to cross-correlation measured Rayleigh wave time shifts are an effective tool to improve epicentroid locations and relative origin-time shifts in remote regions. We apply these methods to seismicity offshore of southwestern Canada and the U.S. Pacific Northwest, occurring along the boundaries of the Pacific and Juan de Fuca (including the Explorer plate and Gorda block) plates. The Blanco, Mendocino, Revere-Dellwood, Nootka, and Sovanco fracture zones host the majority of this seismicity, largely consisting of strike-slip earthquakes. The Explorer, Juan de Fuca, and Gorda spreading ridges join these fracture zones and host normal faulting earthquakes. Our results show that at least the moderate-magnitude activity clusters along fault strike, supporting suggestions of large variations in seismic coupling along oceanic transform faults. Our improved relative locations corroborate earlier interpretations of the internal deformation in the Explorer and Gorda plates. North of the Explorer Plate, improved locations support models that propose northern extension of the Revere-Dellwood fault. Relocations also support interpretations that favor multiple parallel active faults along the Blanco Transform Fault Zone. Seismicity of the western half of the Blanco appears more scattered and less collinear than the eastern half, possibly related to fault maturity. We use azimuthal variations in the Rayleigh-wave cross-correlation amplitude to detect and model rupture directivity for a moderate-size earthquake along the eastern Blanco fault. The observations constrain the seismogenic zone geometry and suggest a relatively narrow seismogenic zone width of 2 to 4 km.

Description: The Slichter mode is the triplet oscillational mode of the 3-D translation of the Earth's solid inner core; despite much effort it has so far eluded observation. Here we revisit the search by applying new data-processing methods upon the latest set of available superconducting gravimeter (SG) data. The new methods are the AR-z spectrum [ Ding and Chao, 2015a] that is sensitive in detecting weak harmonic signals, and the OSE data stacking scheme [ Ding and Shen, 2013] proven to be effective in identifying seismic normal mode singlets. The SG data consist of 19 records from 14 worldwide stations spanning up to 15 years. We arrive at three candidate sets of frequencies for the Slichter triplet that satisfy the theoretical splitting rule, namely (3.952, 4.432, 4.908) cpd, (5.136/5.032/5.020, 5.592, 6.040) cpd, and (5.704, 6.208, 6.748) cpd, of which the first set is regarded to be the more likely. A series of synthetic experiments shows that the Slichter triplet signals with RMS received amplitude of ~0.3 ngal (10 −11 m/s 2 ) can be detected to the extent of the present combination of data and methodology.

Description: Impoundment of the Zipingpu Reservoir (ZR), China, began in September 2005, and was followed 2.7 years later by the 2008 Mw 7.9 Wenchuan Earthquake (WE) rupturing the Longmen Shan Fault (LSF), with its epicenter ~12 kilometers away from the ZR. Based on the poroelastic theory, we employ three-dimensional finite element models to simulate the evolution of stress and pore pressure due to reservoir impoundment, and its effect on the Coulomb failure stress on the LSF. The results indicate that the reservoir impoundment formed a pore pressure front that slowly propagated through the crust with fluid diffusion. The reservoir loading induced either moderate or no increase of the Coulomb failure stress at the hypocenter prior to the WE. The Coulomb failure stress, however, grew ~9.3-69.1 kPa in the depth range of 1-8 km on the LSF, which may have advanced tectonic loading of the fault system by ~60-450 years. Due to uncertainties of fault geometry and hypocenter location of the WE, it is inconclusive whether impoundment of the ZR directly triggered the WE. However, a small event at the hypocenter could have triggered large rupture elsewhere on fault, where the asperities were weakened by the ZR. The micro-seismicity around the ZR also showed an expanding pattern from the ZR since its impoundment, likely associated with diffusion of a positive pore-pressure pulse. These results suggest a poroelastic triggering effect (even if indirectly) of the WE due to the impoundment of the ZR.

Description: Characterizing the evolution of seismicity rate of early aftershocks can yield important information about earthquake nucleation and triggering. However, this task is challenging because early aftershock seismic signals are obscured by those of the mainshock. Previous studies of early aftershocks employed high-pass filtering and template matching, but had limited performance and completeness at very short times. Here we take advantage of repeating events previously identified on the San Andreas fault at Parkfield and apply empirical Green's function deconvolution techniques. Both Landweber and sparse deconvolution methods reveal the occurrence of aftershocks as early as few tenths of a second after the mainshock. These events occur close to their mainshock, within one to two rupture lengths away. The aftershock rate derived from this enhanced catalog is consistent with Omori's law, with no flattening of the aftershock rate down to the shortest resolvable time scale ∼ 0.3 s. The early aftershock rate decay determined here matches seamlessly the decay at later times derived from the original earthquake catalog, yielding a continuous aftershock decay over time scales spanning nearly 8 orders of magnitude. Aftershocks of repeating micro-earthquakes may hence be governed by the same mechanisms from the earliest time resolved here, up to the end of the aftershock sequence. Our results suggest that these early aftershocks are triggered by relatively large stress perturbations, possibly induced by aseismic afterslip with very short characteristic time. Consistent with previous observations on bimaterial faults, the relative location of early aftershocks shows asymmetry along-strike, persistent over long periods.

Description: Pyroclastic flows are characterized by their high mobility, which is often attributed to gas-fluidization of the usually fine and/or low density particles. However, the physical mechanism that might drive sustained fluidization of pyroclastic flows over extraordinarily long runout distances is elusive. In this letter it is proposed that a powerful mechanism to weaken the frictional resistance of pyroclastic flows would arise from the prolonged and intense mechanical vibrations that commonly accompany these dense gravitational fluid-particle flows. The behavior of fine powders in a slowly rotating drum subjected to vibrations suggests that fluid-particle relative oscillations in granular beds can effectively promote the pore gas pressure at reduced shear rates. Dynamical weakening, as caused by the enhancement of pore fluid pressure, may be a powerful mechanism in any geophysical process that involves vibrations of granular beds in a viscous fluid. This is particularly relevant for granular flows involvinglarge amounts of fine and/or light particles such as pyroclastic density currents.

Description: One of the challenges faced today in a variety of geophysical applications is the need to understand the changes of elastic properties due to time-variant chemo-mechanical processes. The objective of this work is to model carbonate rock elastic properties as functions of pore geometry changes that occur when the solid matrix is dissolved by carbon dioxide. We compared two carbonate microstructures: porous micrite (“mudstone”) and grain-supported carbonate (“packstone”). We formulated a mathematical model that distinguishes the effects of micro- and macro- porosity on stiffness changes. We used measures of mechanical and chemical porosity changes recorded during injection tests to compute elastic moduli and compare them to moduli obtained from wave velocity measurements. In mudstones, both experimental and numerical results indicate that bulk moduli change by less than 5%. The evolution of elastic moduli is controlled by macropore enlargement. In packstones, model predictions under-estimate changes of elastic moduli with total porosity by 10% to 80%. The total porosity variation is 60% to 75% smaller than the chemical porosity variation, which indicates that pore expansion due to dissolution is counter-balanced by pore shrinkage due to compaction. Packstone elastic properties are controlled by grain sliding. The methodology presented in this paper can be generalized to other chemo-mechanical processes studied in rocks, such as dislocations, glide, diffusive mass transfer, recrystallization and precipitation.

Description: The Chinese Loess Plateau red clay sequences display a continuous alternation of sedimentary cycles that represent recurrent climatic fluctuations from 2.58 Ma to the Miocene. Deciphering such a record can provide us with vital information on global and Asian climatic variations. Lack of fossils and failure of absolute dating methods made magnetostratigraphy a leading method to build age models for the red clay sequences. Here we test the magnetostratigraphic age model against the cyclostratigraphy. For this purpose we investigate the climate cyclicity recorded in magnetic susceptibility and sedimentary grain size in a red clay section previously dated 11 Myr old with magnetostratigraphy alone. Magnetostratigraphy dating based on only visual correlation could potentially lead to erroneous age model. In this study the correlation is executed through the iteration procedure until it is supported by cyclostratigraphy; i.e. Milankovitch cycles are resolved in the best possible manner. Our new age model provides an age of 5.2 Ma for the Shilou profile. Based on the new age model, wavelet analysis reveals the well preserved 400 kyr and possible 100 kyr eccentricity cycles on the eastern Chinese Loess Plateau. Further, paleomonsoon evolution during 2.58–5.2 Ma is reconstructed and divided into three intervals (2.58–3.6 Ma, 3.6–4.5 Ma, and 4.5–5.2 Ma). The upper part, the youngest stage is characterized by a relatively intensified summer monsoon, the middle stage reflects an intensification of the winter monsoon and aridification in Asia, and the earliest stage indicates that summer and winter monsoon cycles may have rapidly altered. The use of cyclostratigraphy along with magnetostratigraphy gives us an effective method of dating red clay sequences and our results imply that many presently published age models for the red clay deposits should be perhaps re-evaluated.

Description: Long period global scale electromagnetic induction studies of deep Earth conductivity are based almost exclusively on magneto-variational methods, and require accurate models of external source spatial structure. We describe approaches to inverting for both the external sources and three-dimensional (3D) conductivity variations, and apply these methods to long period ( T ≥1.2 days) geomagnetic observatory data. Our scheme involves three steps: (1) Observatory data from 60 years (only partly overlapping and with many large gaps) are reduced and merged into dominant spatial modes using a scheme based on frequency domain principal components. (2) Resulting modes are inverted for corresponding external source spatial structure, using a simplified conductivity model with radial variations overlain by a two-dimensional thin sheet. The source inversion is regularized using a physically-based source covariance, generated through superposition of correlated tilted zonal (quasi-dipole) current loops, representing ionospheric source complexity smoothed by Earth rotation. Free parameters in the source covariance model are tuned by a leave-one-out cross-validation scheme. (3) The estimated data modes are inverted for 3D Earth conductivity, assuming the source excitation estimated in step two. Together, these developments constitute key components in a practical scheme for simultaneous inversion of the catalogue of historical and modern observatory data for external source spatial structure and 3D Earth conductivity.

Description: No abstract is available for this article.

Description: We present a new GPS velocity field covering the peri-Adriatic tectonically-active belts and the entire Balkan Peninsula. From the velocities, we calculate consistent strain rate and interpolated velocity fields. Significant features of the crustal deformation include: (1) the eastward motion of the northern part of the Eastern Alps together with part the Alpine foreland and Bohemian Massif towards the Pannonian Basin, (2) shortening across the Dinarides, (3) a clock-wise rotation of the Albanides-Hellenides and (4) a southward motion South of 44 ∘ N of the inner Balkan lithosphere between the rigid Apulia and Black Sea, towards the Aegean domain. Using this new velocity-field, we derive the strain rate tensor to analyze the regional style of the deformation. Then, we devise a simple test based on the momentum balance equation, to investigate the role of horizontal gradients of gravitational potential energy in driving the deformation in the Peri-Adriatic tectonically-active mountain belts : the Eastern Alps, the Dinarides, the Albanides and the Apennines. We show that the strain rate fields observed in the Apennines and Albanides are consistent with a fluid, with viscosity η ∼ 3×10 21 Pas, deforming in response to horizontal gradients of gravitational potential energy. Conversely, both the Dinarides and Eastern Alps are probably deforming in response to the North and North-East oriented motion of the Adria-Apulia indenter respectively, and as a consequence of horizontal lithospheric heterogeneity.

Description: Large rockslide-debris avalanches, resulting from flank collapses that shape volcanoes and mountains on Earth and other telluric planets, are rapid and dangerous gravity-driven granular flows that travel abnormal distances. During the last 50 year, numerous physical models have been put forward to explain their extreme mobility. The principal models are based on fluidisation, lubrication or dynamic disintegration. However, these processes remain poorly constrained. To identify precisely the transport mechanisms during debris avalanches, we examined morphometric (fractal dimension, circularity), grain-size and exoscopic characteristics of the various types of particles (clasts and matrix) from volcanic debris avalanche deposits of La Réunion Island (Indian Ocean). From these data we demonstrate for the first time that syn-transport dynamic disintegration continuously operates with the increasing runout distance from the source down to a grinding limit of 0.5 mm. Below this limit, the particle size reduction exclusively results from their attrition by frictional interactions. Consequently, the exceptional mobility of debris avalanches may be explained by the combined effect of elastic energy release during the dynamic disintegration of the larger clasts, and frictional reduction within the matrix due to interactions between the finer particles.

Description: Slow slip events (SSEs) have been observed in GPS time series for many subduction zones worldwide, but not in decade-long GPS time series from the Sumatran GPS Array (SuGAr). An outstanding question has been whether SSEs have simply not occurred on the Sunda megathrust, or whether they have been obscured by the prodigious number of earthquakes and their ensuing postseismic deformation within the time of geodetic observation. We remove all known tectonic signals from the time series to search for evidence of SSEs. The residuals are essentially flat at the centimeter scale. To search for signals at the millimeter scale we test various filtering and visualization techniques. Despite these efforts, we conclude that it is difficult to confirm that SSEs exist at this scale using the current data, although we do see a few suspicious signals. The lack of evidence for events may reflect SSEs occurring at a magnitude, location, or time scale that renders them undetectable with the current resolution of the SuGAr, that the properties of this megathrust are not conducive to SSEs, or because the megathrust is in an active period of the earthquake cycle.

Description: Proximal infrasound arrays can robustly track rapidly moving gravity-driven mass wasting, which occurs commonly at erupting volcanoes. This study reports on detection, localization, and quantification of frequent small rockfalls and infrequent pyroclastic density currents descending the southeast flanks of Santiaguito's active Caliente Dome in January of 2014. Such activities are identified as moving sources, which descend several hundred meters at bulk flow speeds of up to ~10 m/s, which is considerably slower than the descent velocity of individual blocks. Infrasound rockfall signal character is readily distinguishable from explosion infrasound, which is manifested by a relatively fixed location source with lower frequency content. In contrast, the rockfalls of Santiaguito possess higher frequencies dominated by 7.5 to 20 Hz energy. During our observation periods typical rockfall signals occurred ~10 times per hour and lasted tens of seconds or more. Array beamforming permitted detection of rockfall transients with amplitudes of only a few tens of mPa that would be impossible to distinguish from noise using a single sensor. Conjoint time-synchronized video is used to corroborate location and to characterize various gravity-driven events.

Description: The proposal that the seismically observed hemispherical asymmetry of Earth's inner core is controlled by the heat flux structure imposed on the outer core by the lower mantle is tested with numerical dynamo models driven by mixed thermochemical convection. We find that models driven by a single core-mantle boundary (CMB) spherical harmonic of degree and mode 2, the dominant mode in lower mantle seismic shear velocity tomography, produce a similar structure at the inner-core boundary (ICB) shifted 30degrees westward. The sensitivity of the ICB to the CMB is further tested by increasing the CMB heterogeneity amplitude. In addition, two seismic tomographic models are tested: first with CMB resolution up to degree and order 4, and second with resolution up to degree and order 8. We find time averaged ICB heat flux in these cases to be similar at large scale, with small scale differences due to higher CMB harmonics (above degree 4). The tomographic models produce “Earth-like" magnetic fields, while similar models with twice the CMB heat flow amplitudes produce less Earth-like fields, implying that increasing CMB heterogeneity forces the model out of an Earth-like regime. The dynamic ICB heat fluxes are compared to the proposed translation mode of the inner core to test whether the CMB controls inner core growth and structure. This test indicates that, although CMB tomography is unlikely to be driving inner core translation, the ICB heat flux response is weak enough to not interfere with the most unstable translation mode, if it is occurring.

Description: We review three data stacking methods developed for attaining the splitting parameters of normal modes of the Earth's free oscillation, namely SHS (spherical harmonic stacking), OSE (optimal sequence estimation) and MSE (multi-station experiment), that take advantage of the spatial orthogonality of the (scalar and vector) spherical harmonic functions. We further extend these methods to broader applicability as appropriate, in particular SHS extended to matrix -SHS (MSHS) to better accounting for the non-global distribution of recording stations, and OSE and MSE extended to transverse components and arbitrary harmonic degrees. We conduct synthetic numerical experiments and conclude that OSE and MSHS stackings yield superior results. Based on the lessons learned we apply the methods on real global seismic records after the 2004 Sumatra earthquake, and report the complete resolution of all singlets for 13 selected multiplets: 8 are mantle-sensitive ( 1 S 2 , 0 S 4 , 1 S 4 , 0 S 5 , 0 S 6 , 1 T 2 , 0 T 4 , 0 T 6 ) and 5 inner-core sensitive and of anomalous splitting ( 13 S 2 , 10 S 2 , 2 S 3 , 3 S 2 , 11 S 1 ), among them the singlet resolution of 1 T 2 , 0 T 4 , 10 S 2 , 2 S 3 , 3 S 2 , and 11 S 1 are reported for the first time.

Description: The impact of self-attraction and loading (SAL) on Earth rotation has not been previously considered except at annual timescales. We estimate Earth rotation excitations using models of atmospheric, oceanic, and land hydrology surface mass variations and investigate the importance of including SAL over monthly to interannual timescales. We assess SAL effects in comparison with simple mass balance effects where net mass exchanged with the atmosphere and land is distributed uniformly over the global ocean. For oceanic polar motion excitations, SAL impacts are important even though mass balance impact is minor except at the annual period. This is true of global (atmosphere+land+ocean) polar motion excitations as well, although the SAL impacts are smaller. When estimating length-of-day excitations, mass balance effects have a dominant impact, particularly for oceanic excitation. Although SAL can have a significant impact on estimated Earth rotation excitations, its consideration generally did not improve comparisons with geodetic observations. This result may change in the future as surface mass models and Earth rotation observations improve.

Description: Taiwan experiences high deformation rates, particularly along its eastern margin where a shortening rate of about 30 mm/yr is experienced in the Longitudinal Valley and the Coastal Range. Four Sacks-Evertson borehole strainmeters have been installed in this area since 2003. Liu et al . [2009] proposed that a number of strain transient events, primarily coincident with low barometric pressure during passages of typhoons, were due to deep triggered slow slip. Here we extend that investigation with a quantitative analysis of the strain responses to precipitation as well as barometric pressure and the earth tides in order to isolate tectonic source effects. Estimates of the strain responses to barometric pressure and ground water level changes for the different stations vary over the ranges -1~-3 nanostrain/millibar(hPa) and -0.3~-1.0 nanostrain/hPa, respectively, consistent with theoretical values derived using Hooke's law. Liu et al . [2009] noted that during some typhoons, including at least one with very heavy rainfall, the observed strain changes were consistent with only barometric forcing. By considering a more extensive data set we now find the strain response to rainfall is about -5.1 nanostrain/hPa. A larger strain response to rainfall compared to that to air pressure and water level may be associated with an additional strain from fluid pressure changes that take place due to infiltration of precipitation. Using a state-space model, we remove the strain response to rainfall, in addition to those due to air pressure changes and the earth tides and investigate whether corrected strain changes are related to environmental disturbances or tectonic-original motions. The majority of strain changes attributed to slow earthquakes seem rather to be associated with environmental factors. However some events show remaining strain changes after all corrections. These events include strain polarity changes during passages of typhoons (a characteristic that is not anticipated from our estimates of the precipitation transfer function) that are more readily explained in terms of tectonic-origin motions but clearly the triggering argument is now weaker than that presented in Liu et al . [2009]. Additional on-site water level sensors and rain gauges will provide data critical for a more complete understanding including the currently unresolved issue of why, for some typhoons, there appears to be a much smaller transfer function for precipitation induced strain changes.

Description: A non-linear viscoelastoplastic theory is developed for porous rate-dependent materials filled with a fluid in the presence of gravity. The theory is based on a rigorous thermodynamic formalism suitable for path-dependent and irreversible processes. Incremental evolution equations for porosity, Darcy's flux, and volumetric deformation of the matrix represent the simplest generalization of Biot's equations. Expressions for pore compressibility and effective bulk viscosity are given for idealized cylindrical and spherical pore geometries in an elastic-viscoplastic material with low pore concentration. We show that plastic yielding around pores leads to decompaction weakening and an exponential creep law. Viscous and plastic end members of our model are consistent with experimentally verified models. In the poroelastic limit, our constitutive equations reproduce the exact Gassmann's relations, Biot's theory, and Terzaghi's effective stress law. The nature of the discrepancy between Biot's model and the True Porous Media (TPM) theory is clarified. Our model provides a unified and consistent formulation for the elastic, viscous, and plastic cases that have previously been described by separate “end member” models.

Description: GRACE provides monthly solutions for the Earth's gravity field in the form of spherical harmonic coefficients. These can be used to infer changes in mass at the Earth's surface. The pole tide (the response of the Earth and oceans to polar motion) causes gravity signals dominated by harmonics of degree 2, order 1. If the pole tide is not removed from GRACE data, it affects the coefficients of those harmonics (C 21 , S 21 ), and introduces errors when using those coefficients to determine surface mass variations. The pole tide is partially removed by GRACE processing centers before solving for the gravity field. But long-period pole tide signals are not usually included in the GRACE pole tide correction, and so those signals are still present in the GRACE coefficients. We discuss this issue from the standpoint of somebody who uses the GRACE gravity fields to infer changes in surface mass. We arrive at a recommendation for an optimal GRACE pole tide correction. We describe how to modify the C 21 , S 21 coefficients provided by the processing centers, so that they conform with our recommendation. We discuss the size of the pole tide contributions to C 21 , S 21 , compared to those of the direct load-induced contributions. As an example, we show how an incompletely removed pole tide can impact GRACE results for the trend in ocean mass. We consider the impact of mantle anelasticity on long period pole tide corrections, and conclude that it is unlikely to affect those corrections by more than 20%.

Description: Northern Japan is a tectonically active area, with the presence of several volcanoes, and with frequent earthquakes among which the destructive M w = 8.9 - 9.0 Tohoku-oki occurred on 11 March 2011. Tectonic activity leaves an imprint on the crustal structures, on both the upper and the lower layers. To investigate the crust in Northern Japan, we construct a receiver function data-set using teleseismic events recorded at 58 seismic stations belonging to the Japanese National (Hi-net) network. We isolate the signals, in the receiver function wavelet, that witness the presence of anisotropic structures at depth, with the aim of mapping the variation of anisotropy across the northern part of the island. This study focuses on the relation among anisotropy detected in the crust, stresses induced by plate convergence across the subduction zone, and the intrinsic characteristics of the rocks. Our results show how a simple velocity model with two anisotropic layers reproduces the observed data at the stations. We observe a negligible or small amount of signal related to anisotropy in the eastern part of the study area (i.e. the outer arc) for both upper and lower crust. Distinct anisotropic features are observed at the stations onthe western part of the study area (i.e. the inner arc) for both upper and lower crust. The symmetry axes are mostly E-W oriented. Deviation from the E-W orientation is observed close to the volcanic areas, where the higher geothermal gradient might influence the deformation processes.

Description: Real-time high-rate geodetic data have been shown to be useful for rapid earthquake response systems during medium to large events. The 2014 M w 6.1 Napa, California earthquake is important because it provides an opportunity to study an event at the lower threshold of what can be detected with GPS. We show the results of GPS-only earthquake source products such as peak ground displacement (PGD) magnitude scaling, centroid moment tensor (CMT) solution and static slip inversion. We also highlight the retrospective real-time combination of GPS and strong motion data to produce seismogeodetic waveforms that have higher precision and longer period information than GPS-only or seismic-only measurements of ground motion. We show their utility for rapid kinematic slip inversion and conclude that it would have been possible, with current real-time infrastructure, to determine the basic features of the earthquake source. We supplement the analysis with strong motion data collected close to the source to obtain an improved post-event image of the source process. The model reveals unilateral fast propagation of slip to the north of the hypocenter with a delayed onset of shallow slip. The source model suggests that the multiple strands of observed surface rupture are controlled by the shallow soft sediments of Napa Valley and do not necessarily represent the intersection of the main faulting surface and the free surface. We conclude that the main dislocation plane is westward dipping and should intersect the surface to the east, either where the easternmost strand of surface rupture is observed or at the location where the West Napa fault has been mapped in the past.

Description: In this paper we present a penalized likelihood-based method for spatial estimation of Gutenberg-Richter's b-value. Our method incorporates a non-arbitrary partitioning scheme based on Voronoi tessellation, which allows for the optimal partitioning of space using a minimum number of free parameters. By random placement of an increasing number of Voronoi nodes we are able to explore the whole solution space in terms of model complexity. We obtain an overall likelihood for each model by estimating the b-values in all Voronoi regions and calculating its joint likelihood using Aki's formula. Accounting for the number of free parameters we then calculate the Bayesian Information Criterion for all random realizations. We investigate the ensemble of the best performing models and demonstrate the robustness and validity of our method through extensive synthetic tests. We apply our method to the seismicity of California using two different time spans of the ANSS catalog (1984–2014 and 2004–2014). The results show that for the last decade the b-value variation in the well-instrumented parts of mainland California is limited to the range of [0.94 ± 0,04-1.15 ± 0.06]. Apart from the Geysers region, the observed variation can be explained by network related discrepancies in the magnitude estimations. Our results suggest that previously reported spatial b-value variations obtained using classical fixed radius or nearest neighbor methods are likely to have been overestimated, mainly due to subjective parameter choices. We envision that the likelihood-based model selection criteria used in this study can be a useful tool for generating improved earthquake forecasting models.

Description: The stable Ordos Plateau, extensional Weihe Graben and Qinling orogenic belt locate at the northeast margin of the Tibetan Plateau. They have been thought to play different roles in the eastward expanding of the Tibetan Plateau. Peking University deployed a linear seismic array across the western end of the Weihe Graben to investigate the crustal structures of the tectonic provinces of this structure. Receiver function analyses revealed low-to-moderate Poisson's ratios and anti-correlations between Poisson's ratios and topography beneath the Qinling Orogen. These features may indicate a tectonic thickening of the felsic upper crust by folding and thrusting within the Qinling Orogen. We observed a strong horizontal negative signal at the mid-crust beneath the Ordos Plateau which may indicate a low velocity zone. This observation would suggest the stable cratonic Ordos Plateau had been modified due to the compression between the Tibetan Plateau and the Ordos Plateau. We also observed an abrupt 4-km Moho offset across the Weihe Fault, changing from ~44 km beneath the Ordos Plateau to ~40 km beneath the Qinling Orogen. We conclude that the Weihe Fault is a lithosphere-scale fault/shear zone, which extends into the upper mantle beneath the Weihe Graben. It acts as the major boundary separating the stable Ordos Plateau and the active Qinling Orogen.

Description: The inner shelf of the East China Sea is a river-dominated margin characterized by fine-grained mud deposits and a rapid sedimentation rate. Three short sediment cores (~2.7 m in length) were examined to characterize spatial variations in magnetic mineral diagenesis. The sediment cores were analyzed for sedimentation rates, magnetic properties, particle size distribution, organic carbon and total sulfur content. The two more proximal cores with higher sedimentation rates (~2.2 cm/yr and ~0.96 cm/yr) do not exhibit obvious effects of reductive dissolution of magnetite with increasing depth, which is consistent with their lower total sulfur content. The offshore core, A12-4, which has a lower sedimentation rate, contains clear evidence of magnetite dissolution and increasing total sulfur content with depth. The three cores have a similar sediment source and organic matter input, therefore, we suggest that a higher sedimentation rate will lead to less reductive diagenesis of magnetite, assuming that other factors are constant. The iron- to sulfate-reduction boundary, i.e., revealed by the onset of a rapid decline of magnetic susceptibility, is located 1.0 m below sea floor in core A12-4. This is much deeper than is reported in many other coastal marine environments, and can be explained by the higher sedimentation rate, the presence of refractory terrestrial organic matter and an abundant input of detrital iron oxides. This study demonstrates that analyses of the magnetic mineral zonation provides a straightforward approach to assess diagenetic organic carbon decomposition pathways in marine environments.

Description: The first-order characteristics of collisional mountain belts and the potential feedback with surface processes are predicted by critical taper theory. While the feedback between erosion and mountain-belt structure has been fairly extensively studied, less attention has been given to the potential role of synorogenic deposition. For thin-skinned fold-and-thrust belts, recent studies indicate a strong control of syntectonic deposition on structure, as sedimentation tends to stabilize the thin-skinned wedge. However, the factors controlling basement deformation below fold-and-thrust belts, as evident for example in the Zagros Mountains or in the Swiss Alps, remain largely unknown. Previous work has suggested that such variations in orogenic structure may be explained by the thermo-tectonic “age” of the deforming lithosphere and hence its rheology. Here we demonstrate that sediment loading of the foreland basin area provides an additional control and may explain the variable basement involvement in orogenic belts. When examining the role of sedimentation we identify two end-members: 1) sediment-starved orogenic systems with thick-skinned basement deformation in an axial orogenic core and thin-skinned deformation in the bordering forelands; and 2) sediment-loaded orogens with thick packages of synorogenic deposits, derived from the axial basement zone, deposited on the surrounding foreland fold-and-thrust belts and characterized by basement deformation below the foreland. Using high resolution thermo-mechanical models, we demonstrate a strong feedback between deposition and crustal scale thick-skinned deformation. Our results show that the loading effects of syntectonic sediments lead to long crustal-scale thrust sheets beneath the orogenic foreland and explain the contrasting characteristics of sediment-starved and sediment-loaded orogens, showing for the first time how both thin- and thick-skinned crustal deformation are linked to sediment deposition in these orogenic systems. We show that the observed model behavior is consistent with observations from a number of natural orogenic systems.

Description: Po/So waves are characterized by their high-frequency content and long-duration travel over great distances (up to 3000 km) through the oceanic lithosphere. Po/So waves are developed by the multiple forward scattering of P- and S-waves due to small-scale stochastic random heterogeneities. To study the nature of these heterogeneities, Po/So waves are analyzed in the Philippine Sea Plate, which consists of three regions with different lithospheric ages. In the Philippine Sea Plate, Po/So waves propagate in the youngest region (15 Ma) and propagate more effectively in older regions. We investigate the mechanism of this propagation efficiency using numerical Finite Difference Method simulations of 2-D seismic wave propagation. The results of this study demonstrate that the increase in propagation efficiency of Po/So waves depends on the age of the oceanic lithosphere, and this relationship can be qualitatively explained by thickening of the oceanic lithosphere including small-scale heterogeneities and a reduction in the intrinsic attenuation. These small-scale heterogeneities may form continuously in oceanic lithosphere from the time of its formation at a spreading ridge, via the solidification of melts distributed in the asthenosphere.

Description: As a result of the comminution that takes place over numerous earthquake cycles, mature faults are characterized by thick layers of pulverized gouge with finite porosity that is saturated with water at seismogenic depths. The heat generated during earthquakes raises the gouge temperature and thermal expansion of the pore fluid and surrounding solids produces elevated pore pressures that cause fault strength to decrease in the process known as thermal pressurization. Building upon this framework, we describe a model that imposes a plane-strain configuration and shows that the stress variations caused by porothermoelasticity promote the Mohr-Coulomb failure of previously undeformed regions. Except in special cases where the friction is rate-strengthening, we find that the frictional strength must vary throughout the post-failure region, which we identify in our model with the shear zone. We introduce a strain-rate function that describes the overall influence of distributed slip on energy dissipation and fault strength as the shear-zone thickness expands. Using typical fault parameters at 7 depth, the shear zone reaches several millimeters thickness after 1 sliding at an overall rate of 1 m/s. The expansion of the shear zone limits the temperature rise to several hundred degrees Celsius, and the average fault strength falls to about a tenth of the static frictional strength.

Description: Most crustal rocks present some amount of directional dependence of seismic speeds, particularly mudstones (also termed “shales”). Hence, accurate imaging of the subsurface requires anisotropic models, and integration of rock physics information in order to constrain the inherent non-uniqueness of the inversion from seismic data. Resonant Ultrasound Spectroscopy provides estimates of the full anisotropic stiffness tensor from resonant frequencies of geological core samples, along with a measure of intrinsic attenuation. We have developed new functionality to existing codes which enable horizontal transversely isotropic samples to be analyzed. We compared and discussed estimations of the elastic properties of mudstone samples with hexagonal symmetry; one sample was drilled parallel and one perpendicular to the layering. While spatial heterogeneity in the mudstone prevented a direct correlation of the elastic parameters of each sample, time-of-flight measurements reveal frequency dispersion of the elastic parameters that is consistent between the samples.

Description: The Western Canada Sedimentary Basin marks the transition from the old North American continental lithosphere to young accreted terranes. Earlier studies in this region have suggested a large number of intricate basement domains as well as major seismic velocity gradients in the mantle. To investigate the effect of the accretion and subduction on the mantle structure beneath the western margin of the North American craton, we analyze P-to-S converted waves from upper mantle discontinuities from the Canadian Rockies and Alberta Network, a regional broadband seismic array based in Alberta, Canada. The depths of the 410 and 660 km seismic discontinuities are correlated and they are, on average, 9 km and 7 km greater than the respective global estimates. The largest depression is observed beneath the Rocky Mountain foreland belt in southern Alberta, which highlights a steep south/westward structural gradient from the cratons to Cordillera at lithospheric mantle depths. The severity of the depressions, especially in the southernmost Alberta, may be triggered by diffuse partial melt or increased water content above the 410-km discontinuity. This result is jointly corroborated by locally increased impedance contrast across the 410-km discontinuity and a strongly depressed 660-km discontinuity. A relic Mesozoic slab fragment may be partially responsible for the deep olivine phase boundary at the base of the upper mantle.

Description: The Siling Co lake is the largest endorheic lake in Central Tibet. Altimetric measures, combined with lake contours, show that in 1972-1999 its water level remained stable, while it increased by about 1.0 m/yr in the period 2000-2006. The increased rate gradually stepped down to 0.2 m/yr in 2007-2011. The ground motion associated with the water load increase is studied by InSAR using 107 ERS and Envisat SAR images during the period 1992-2011. The deformation amplitude closely follows the lake level temporal evolution, except that subsidence continues in 2008-2011, while the lake level stagnated. This temporal evolution suggests a non elastic relaxation process taking place at a decade time-scale. Phase delay maps are used to constrain possible layered visco-elastic rheological models. An elastic model could partly explain the observed subsidence rate if elastic moduli are about twice lower than those extracted from Vp/Vs profiles. The surface deformation pattern is also extracted by projecting the phase delay maps against the best-fit model temporal behavior. It shows that deep relaxation in the asthenosphere is negligible at the decade time-scale and favors the existence of a ductile channel in the deep crust above a more rigid mantle. Overall, the best fit model includes a ductile lower crust, with a viscosity of 1-3×10 18 Pa.s between 25-35 km and the Moho (at 65 km), overlying a rigid mantle.

Description: The challenge of characterizing subsurface fluid flow has motivated extensive laboratory studies, yet fluid-flow through rock specimens in which fractures are created and maintained at high-stress conditions remains under-investigated at this time. The studies of this type that do exist do not include in situ fracture geometry measurements acquired at stressed conditions, which would be beneficial for interpreting the flow behavior. Therefore, this study investigates the apparent permeability induced by direct-shear fracture stimulation through Utica shale (a shale gas resource and potential caprock material) at high triaxial-stress confinement and for the first time relates these values to simultaneously acquired in situ X-ray radiography and microtomography images. Change in fracture geometry and apparent permeability was also investigated at additional reduced stress states. Finite element and combined finite discrete element modeling were used to evaluate the in situ observed fracturing process. Results from this study indicate that the increase in apparent permeability through fractures created at high-stress (22.2 MPa) was minimal relative to the intact rock (less than 1 order of magnitude increase) while fractures created at low stress (3.4 MPa) were significantly more permeable (2 to 4 orders of magnitude increase). This study demonstrates the benefit of in situ X-ray observation coupled with apparent permeability measurement to analyze fracture creation in the subsurface. Our results show that the permeability induced by fractures through shale at high stress can be minor and therefore favorable in application to CO 2 sequestration caprock integrity but unfavorable for hydrocarbon recovery from unconventional reservoirs.

Description: Moving beyond the widely used kinematic models for the deformation sources, we present a new dynamic model to describe the process of injecting magma into an existing magma reservoir. To validate this model, we derive an analytical solution and compare its results to those calculated using the Finite Element Method. A Newtonian fluid characterized by its viscosity, density, and overpressure (relative to the lithostatic value) flows through a vertical conduit, intruding into a reservoir embedded in an elastic domain, leading to an increase in reservoir pressure and time-dependent surface deformation. We apply our injection model to Interferometric Synthetic Aperture Radar (InSAR) data from the ongoing unrest episode at Laguna del Maule (Chile) volcanic field that started in 2007. Using a grid search optimization, we minimize the misfit to the InSAR displacement data and vary the three parameters governing the analytical solution: the characteristic timescale τ P for magma propagation, the maximum injection pressure, and the inflection time when the acceleration switches from positive to negative. For a spheroid with semi-major axis a = 6200 m, semi-minor axis c = 100 m, located at a depth of 4.5 km in a purely elastic half-space, the best fit to the InSAR displacement data occurs for τ P = 9.5 years and an injection pressure rising up to 11.5 MPa for two years. The volume flow rate increased to 1.2 m 3 /s for two years and then decreased to 0.7 m 3 /s in 2014. In 7.3 years, at least 187 million cubic meters of magma was injected.

Description: We study the detailed mantle transition zone structure beneath the active Changbai intraplate volcano in Northeast China using a receiver-function method. A total of 3005 teleseismic receiver functions recorded by 70 broadband stations are obtained by using a common-conversion-point stacking method. For conducting the time-to-depth conversion, we use a three-dimension velocity model of the study region so as to take into account the influence of structural heterogeneities. Our results reveal significant depth variations of the 410, 520 and 660-km discontinuities. A broad depression of the 410 km discontinuity and a low-velocity anomaly are revealed beneath the Changbai volcano, which may reflect a large-scale hot mantle upwelling around the 410 km discontinuity with a positive Clapeyron slope. The 520 km discontinuity is identified clearly and its uplift occurs above the stagnant Pacific slab. We also find a prominent depression of the 660 km discontinuity, which is elongated along the trend of deep-earthquake clusters in a range of 39°N ~ 44°N latitude, and the depression area has a lateral extent of about 400 km. Because the 520 and 660 km discontinuities correspond to positive and negative Clapeyron slopes, respectively, we think that the 520 uplift and the 660 depression are caused by the cold subducting Pacific slab. A part of the Pacific slab may have penetrated into the lower mantle and so caused the large-scale 660 depression in front of the deep-earthquake clusters. Our results also reveal a part of the upper boundary of the subducting Pacific slab in the mantle transition zone.

Description: Fault gouge deformation likely plays a significant role in controlling the strength of mature, large-displacement faults. Experiments show that intact gouge deforms in an overall ductile and stable manner, readily compacting, but dilates and experiences brittle failure under large strain rate. Inelastic gouge compaction and dilatancy are modeled here using a combined Mohr-Coulomb and end-cap yield criterion in a dynamic rupture model of a strike-slip fault with strongly velocity-weakening friction. We show that large shear stress concentration ahead of the rupture associated with the rupture front causes the gouge layer to compact (e.g., by structural collapse and comminution), leading to rapidly elevated pore pressure and significant weakening of the principal fault surface. Shortly after the rupture front passes, strong dilatancy during strength drop and rapid sliding reduces pore pressure and strengthens the fault, promoting slip pulses. Large strain localization in the gouge layer occurs as a result of rapid gouge dilatancy and strain softening. The combination of pre-rupture weakening from compaction and restrengthening from dilatancy hardening leads to a smaller strength drop, and limits the stress concentration outside the gouge layer. This leads to a reduction of inelastic shear strain in the damage zone, which is more consistent with geological observations and high-speed frictional experiments. With the presence of well-developed fault gouge, the strength of mature faults may be limited by end-cap, rather than Mohr-Coulomb failure; thus, their frictional strengths are significantly smaller than Byerlee friction.

Description: High-resolution, near-surface, shear wave reflection seismic measurements were carried out in November 2013 at the CO2CRC Otway Project site, Victoria, Australia with the aim to determine, and if so, where deeper faults reach the near subsurface. From a previous P wave 3-D reflection seismic data set that was concentrated on a reservoir at 2 km depth, we can only interpret faults up to 400 m below sea level. For the future monitoring in the overburden of the CO 2 reservoir it is important to know whether and how the faults continue in the subsurface. We prove that two regional fault zones do in fact reach the surface instead of dying out at depth. Individual first break signatures in the shot gathers along the profiles support this interpretation. However, this finding does not imply perforce communication between the reservoir and the surface in the framework of CO 2 injection. The shear wave seismic sections image with high resolution (better than 3 m vertically), and complementary to existing P wave volumes different tectonic structures. Similar structures also outcrop on the southern coast of the Otway Basin. Both the seismic and the outcrops evidence the complex youngest structural history of the area.

Description: The processes and deformation mechanisms (e.g. dislocation creep, pressure solution, grain-boundary sliding, recrystallization) of rock salt are still a matter of debate. In order to fill this gap, high strain constriction experiments at 345 °C, atmospheric pressure and a strain rate of ~10 −7 s −1 have been conducted on natural halite cuboids (60x60x45mm) from the Morsleben mine of Northern Germany. Most samples were almost single crystals and contain a small amount of smaller grains (10-26%). The grain boundaries are decorated with fluid inclusions. The experiments were stopped at different final strains (ε y=z of ~10, 20, 30 and 40%) corresponding to a maximum strain (ε x ) range of 20-170%. The halite is deformed by dislocation creep, and the size of developed subgrains corresponds to the applied stress. The combined Schmid factor and subgrain boundary analysis indicate that slip was largely accommodated by the {110} 〈 110 〉 slip systems, with possible minor contribution by slip on the {100} 〈 110 〉 slip systems. Some of the deformed samples show evidence of grain boundary migration. In addition, subgrains with small misorientations form, that result in large cumulative misorientations within a single grain (〉40°). However, no subgrain rotation recrystallization is observed (i. e. misorientation angles are 〈10°). All the experiments show strain hardening, suggesting that recrystallization by grain boundary migration was not extensive and did not reset the microstructure. The experiments show that high finite strain in coarse grained relatively dry rock salt can be accommodated by dislocation creep, without extensive dynamic recrystallization.

Description: We present a 3D interpretation of the deep magnetic sources beneath the main geological structure of Central-Eastern Europe, the “Trans European Suture Zone” (TESZ). We used a multiscale analysis of aeromagnetic data, based on a multiscale dataset generated by upward continuation of the EMMP European dataset from 5 to 100 km altitude. We also computed the multiscale Total Gradient |∇ T | of the multiscale field. Both of the multiscale datasets allow us to discriminate the main crustal contributions to the field at various scales. and we showed that at large altitudes the field is dominated by the sole effect of the TESZ. The multiridge geometric method was very useful in studying the complex features of the main crustal interfaces, either shallow or deep. The interpreted interfaces are in substantial agreement with geological models based on seismic surveys and, in some cases, complete the models out of the analyzed region. In order to estimate the deepest source depths in the TESZ region, we applied the multiridge method to the large scales (50-100 km altitude), obtaining a set of singular points at depths ranging between 35-40 km. Considering the trend of the heat flow and the geological models around the TESZ area, we found a meaningful correspondence among the location of the estimated singular points and the most abrupt variations and complex morphology features of the Moho boundary. The multiridge estimates are so consistent with known structural information and can be used for a 3D representation of the Moho depth along the TESZ.

Description: We investigate theoretical limits on detection and reliable estimates of source characteristics of small earthquakes using synthetic seismograms for shear/tensile dislocations on kinematic circular ruptures, observed seismic noise and properties of several acquisition systems (instrument response, sampling rate). Simulated source time functions for shear/tensile dislocation events with different magnitudes, static stress drops and rupture velocities provide estimates for the amplitude and frequency content of P and S phases at various observation angles. The source time functions are convolved with a Green's function for a homogenous solid assuming given P-, S- wave velocities and attenuation coefficients and a given instrument response. The synthetic waveforms are superposed with average levels of the observed ambient seismic noise up to 1-kHz. The combined seismograms are used to calculate signal-;to-noise ratios and expected frequency content of P and S phases at various locations. The synthetic simulations of signal-;to-noise ratio reproduce observed ratios extracted from several well recorded datasets. The results provide guidelines on detection of small events in various geological environments, along with information relevant to reliable analyses of earthquake source properties.

Description: Stochastic discrete fracture networks (DFNs) are classically simulated using stochastic point processes which neglect mechanical interactions between fractures and yield a low spatial correlation in a network. We propose a sequential parent-daughter Poisson point process that organizes fracture objects according to mechanical interactions while honoring statistical characterization data. The hierarchical organization of the resulting DFNs has been investigated in 3D by computing their correlation dimension. Sensitivity analysis on the input simulation parameters shows that various degrees of spatial correlation emerge from this process. A large number of realizations has been performed in order to statistically validate the method. The connectivity of these correlated fracture networks has been investigated at several scales and compared to those described in the literature. Our study quantitatively confirms that spatial correlations can affect the percolation threshold and the connectivity at a particular scale.

Description: Through glacial moulins, meltwater is routed from the glacier surface to its base. Moulins are a main feature feeding subglacial drainage systems and thus influencing basal motion and ice dynamics, but their geometry remains poorly known. Here, we show that analysis of the seismic wavefield generated by water falling into a moulin can help constrain its geometry. We present modeling results of hour-long seimic tremors emitted from a vertical moulin shaft, observed with a seismometer array installed at the surface of the Greenland Ice Sheet. The tremor was triggered when the moulin water level exceeded a certain height, which we associate with the threshold for the waterfall to hit directly the surface of the moulin water column. The amplitude of the tremor signal changed over each tremor episode, in close relation to the amount of inflowing water. The tremor spectrum features multiple prominent peaks, whose characteristic frequencies are distributed like the resonant modes of a semi-open organ pipe and were found to depend on the moulin water level, consistent with a source composed of resonant tube waves (water pressure waves coupled to elastic deformation of the moulin walls) along the water-filled moulin pipe. Analysis of surface particle motions lends further support to this interpretation. The seismic wave field was modeled as a superposition of sustained wave radiation by pressure sources on the side walls and at the bottom of the moulin. The former was found to dominate the wave field at close distance and the latter at large distance to the moulin.

Description: The present-day stress state is a key parameter in numerous geoscientific research fields including geodynamics, seismic hazard assessment and geomechanics of geo-reservoirs. The Taranaki Basin of New Zealand is located on the Australian Plate and forms the western boundary of tectonic deformation due to Pacific Plate subduction along the Hikurangi margin. This paper presents the first comprehensive wellbore-derived basin-scale in-situ stress analysis in New Zealand. We analyse borehole image and oriented caliper data from 129 petroleum wells in the Taranaki Basin to interpret the shape of boreholes and determine the orientation of maximum horizontal stress (S Hmax ). We combine these data (151 S Hmax data records) with 40 stress data records derived from individual earthquake focal mechanism solutions, six from stress inversions of focal mechanisms and one data record using the average of several focal mechanism solutions. The resulting data set has 198 data records for the Taranaki Basin and suggests a regional S Hmax orientation of N068°E (±22°), which is in agreement with NW-SE extension suggested by geological data. Furthermore, this ENE-WSW average S Hmax orientation is subparallel to the subduction trench and strike of the subducting slab (N50°E) beneath the central western North Island. Hence, we suggest that the slab geometry and the associated forces due to slab rollback are the key control of crustal stress in the Taranaki Basin. In addition, we find stress perturbations with depth in the vicinity of faults in some of the studied wells, which highlight the impact of local stress sources on the present-day stress rotation.

Description: Continuous gravity data collected near the summit eruptive vent at Kīlauea Volcano, Hawaiʻi, during 2011–2015 show a strong correlation with summit-area surface deformation and the level of the lava lake within the vent over periods of days to weeks, suggesting that changes in gravity reflect variations in volcanic activity. Joint analysis of gravity and lava level time series data indicates that, over the entire time period studied, the average density of the lava within the upper tens to hundreds of meters of the summit eruptive vent remained low—approximately 1,000–1,500 kg/m 3 . The ratio of gravity change (adjusted for Earth tides and instrumental drift) to lava level change measured over 15-day windows rose gradually over the course of 2011–2015, probably reflecting either 1) a small increase in the density of lava within the eruptive vent, or 2) an increase in the volume of lava within the vent due to gradual vent enlargement. Superimposed on the overall time series were transient spikes of mass change associated with inflation and deflation of Kīlauea's summit and coincident changes in lava level. The unexpectedly strong mass variations during these episodes suggest magma flux to and from the shallow magmatic system without commensurate deformation, perhaps indicating magma accumulation within, and withdrawal from, void space—a process that might not otherwise be apparent from lava level and deformation data alone. Continuous gravity data thus provide unique insights into magmatic processes, arguing for continued application of the method at other frequently active volcanoes.

Description: Precursory aseismic slip lasting days to months prior to the initiation of earthquakes has been inferred from seismological observations. Similar precursory slip phenomena have also been observed in laboratory studies of shear rupture nucleation on frictional interfaces. However the mechanisms that govern rupture nucleation, even in idealized laboratory settings, have been widely debated. Here we show that a numerical model incorporating rate-and-state friction laws and elastic continuum can reproduce the behaviours of rupture nucleation seen in laboratory experiments. In particular, we find that both in laboratory experiments and simulations with a wide range of normal stresses, the nucleation consists of two distinct phases: initial slow propagation phase and faster acceleration phase, both of which are likely aseismic processes, followed by dynamic rupture propagation that radiates seismic waves. The distance at which the rupture transitions from the initial slow phase to the acceleration phase can be roughly predicted by a theoretical estimate of critical nucleation length. Our results further show that the critical nucleation length depends on the background loading rate. In addition, our analysis suggests that critical nucleation length and breakdown power derived from the Griffith-crack energy balance control the scaling of nucleating ruptures. Moreover, the background loading rate and loading configuration significantly affect the rupture propagation speed. Furthermore, if the same nucleation mechanism applies to natural faults, the migration speed of foreshocks triggered by the propagation of slow rupture within the nucleation zone would depend on the effective normal stress and hence fluid pressure in the fault zone.

Description: The Asal-Ghoubbet Rift (AG Rift) in Djibouti lies in the subaerial continuation of the Aden ridge system, thereby constituting a unique location to study rifting processes and mechanisms involved in continental break up and oceanic spreading. Continually upgraded and expanded geodetic technology has been used to record the 1978 Asal rifting event and post-diking deformation. In light of recent results obtained for the Manda Hararo–Dabbahu rifting event (2005–2010), we propose that the horizontal and vertical geodetic data can be modeled with a double source, involving a dyke-like inflation component aligned along the rift axis and a spherical pressure source located at mid-segment below the Fieale caldera. By revisiting the co-dyking data, we propose that the reservoir below Fieale could have fed, at least partially, the 1978 injection and the contemporaneous Ardoukôba eruption, and potentially induced local subsidence due to magma draining out of the central reservoir. As an alternative to previously proposed visco-elastic relaxation models, we re-interpret post-dyking observations using a purely elastic rheology. We determine the relative contribution of a mid-segment reservoir inflation and a dyke-like opening component, together with their respective time evolutions. Our results suggest that interactions between steadily accumulating tectonic strain and temporal variations in melt supply to the shallow magma plumbing system below the AG Rift may entirely explain the geodetic observations and that visco-elastic deformation processes played a minor role in the 30 years following the 1978 rifting event.

Description: It has recently been reported, on the basis of extrapolated experimental data, that the iron carbide, Fe 7 C 3 , has shear wave velocities and a Poisson's ratio consistent with the seismological values for the Earth's inner core, and thus that Fe 7 C 3 is a strong candidate for the inner core composition. In this study, using ab initio molecular dynamics simulations, we report the thermoelastic properties of Fe 7 C 3 at 350 GPa up to its melting temperature. Due to significant elastic softening prior to melting, the calculated elastic properties, including wave velocities, do indeed agree well with those from seismology. However, the density was found to be much too low (by ~8%) when compared to geophysical data, and therefore Fe 7 C 3 must be ruled out as a major component of the Earth's inner-core.

Description: Shear deformation, accompanied with fluid activity inside the subduction interface, is related to many tectonic energy-releasing events, including regular and slow earthquakes. We have numerically examined the fluid-rock interactions inside a deforming subduction interface using state-of-the-art 2D hydro-mechanical numerical models, which incorporate the rock fracturing behavior as a plastic rheology which is dependent on the pore fluid pressure. Our modeling results suggest that two typical dynamical regimes of the deforming subduction interface exist, namely a “coupled” and a “decoupled” regime. In the coupled regime the subduction interface is subdivided into multiple rigid blocks, each separated by a narrow shear zone inclined at an angle of 15-20 degrees with respect to the slab surface. In contrast, in the decoupled regime the subduction interface is divided into two distinct layers moving relative to each other along a pervasive slab surface parallel shear zone. Through a systematic parameter study, we observe that the tensile strength (cohesion) of the material within the subduction interface dictates the resulting style of deformation within the interface: high cohesion (~60 MPa) results in the coupled regime, whilst low cohesion (~10 MPa) leads to the decoupled regime. We also demonstrate that the lithostatic pressure and inflow/outflow fluid fluxes (i.e., fluid-fluxed boundary condition) influence the location and orientation of faults. Predictions from our numerical models are supported by experimental laboratory studies, geological data, and geophysical observations from modern subduction settings.

Description: We use virtual deep seismic sounding (VDSS) and data from ~1,000 broadband seismic stations to provide high-resolution estimates of crustal structure in the western Cordillera of the United States (US). The most robust result is the geographic distribution of residual topography (that is, the difference between observed elevation and that expected from crustal buoyancy alone) and, by implication, thermal or petrologic anomalies in the mantle. Overall, residual topography of the western US Cordillera varies considerably; with contrasts of up to about 3 km across distances of 200 km or less. High residual topography, indicating large mantle effects, is evident along the periphery of the Colorado Plateau and the surroundings of the Great Basin. In contrast, the central Colorado Plateau and the Wyoming Basin show low residual topography, close to what is expected of a geologically stable lithosphere. Overall, in regions to the east of the Wasatch hinge line (the eastern limit of significant extension in the North American cratonic basement) patterns of high residual topography and anomalies of low seismic wave-speeds in the upper mantle are similar, suggestive of a common, thermal origin. In contrast, such a similarity is absent in regions to the west of the hinge line, suggesting substantial effects of petrological heterogeneities in the mantle. Finally, joint analyses of VDSS and conventional receiver functions reveal a wide range of crustal P -wave speeds, locally as high as 6.7 km/s, perhaps indicating magmatic modification of the crust.

Description: We present an improved neotectonic numerical model of the complex NW Africa – SW Eurasia plate boundary segment that runs from West to East along the Gloria Fault up to the Northern Algerian margin. We model the surface velocity field and the ongoing lithospheric deformation using the most recent version of the thin-shell code SHELLS , and updated lithospheric model and fault map of the region. To check the presence versus the absence of an independently-driven Alboran domain, we develop two alternative plate models: one does not include an Alboran plate; another includes it and determines the basal shear tractions necessary to drive it with known velocities. We also compare two alternative sets of Africa-Eurasia velocity boundary conditions, corresponding to geodetic and geological-scale averages of plate motion. Finally, we perform an extensive parametric study of fault friction coefficient, trench resistance and velocities imposed in Alboran nodes. The final run comprises 5240 experiments, each scored to geodetic velocities (estimated for 250 stations and here provided), stress direction data and seismic strain rates. The model with the least discrepancy to the data includes the Alboran plate driven by a basal WSW-directed shear traction, slightly oblique to the westward direction of Alboran motion. We provide estimates of long-term strain rates and slip rates for the modeled faults, which can be useful for further hazard studies. Our results support that a mechanism additional to the Africa-Eurasia convergence is required to drive the Alboran domain, which can be related to subduction processes occurring within the mantle.

Description: Deep tectonic tremors detected in many subduction zones worldwide are often accompanied by very low frequency (VLF) signals, detectable by broadband seismometers but usually hidden in large ambient noise. By stacking broadband seismograms relative to tremor hypocentral times, we can recover the VLF signals. The stacked signals are then inverted to determine a moment tensor solution, using a procedure previously applied to VLF signals in Japan, Taiwan, and Mexico. Here we apply this method to the Cascadia subduction zone, where tremors and slow slip events (SSE) are clearly observed. As expected, we successfully recover VLF signals for almost the entire tremor region beneath southern Vancouver Island and northern Washington State. The moment tensors are mostly well determined as low-angle thrust type, but source depths are poorly constrained. The slip direction is slightly rotated counterclockwise with respect to the local plate motion direction, probably due to bending of the subducting plate. The seismic moment measured in VLF band is proportional to the seismic energy of tremors, with a scaled energy of about 3 × 10 −9 . The widespread observability of VLF signals suggests that the deformation associated with tremors and SSEs is actually a very broadband phenomenon, as suggested by stochastic models.

Description: Temporal variations of the fault frictional strength was investigated based on the diversity of focal mechanisms in the source area of the Yamagata-Fukushima border earthquake swarm, a significant earthquake swarm that occurred in central Tohoku, NE Japan, which started just after the 2011 M9.0 Tohoku-Oki earthquake. The focal mechanisms of events in this swarm activity were determined using P-wave polarity data as well as short-period (1.5 - 2.5 Hz) waveform data from the direct P-wave. The stress field in the source area of this swarm was estimated by applying stress tensor inversions to these focal mechanism data. Based on the estimated stress field, and under the assumption of uniform stress, we calculated relative frictional strengths for individual focal mechanisms. The calculated relative frictional strengths vary over a wide range, but their mean value exhibits a characteristic temporal variation, which is at first small, but steadily increases with time for 100 to 150 days, and then becomes approximately constant. We confirmed this characteristic temporal variation of the mean relative frictional strength by assuming the stress to be non-uniform. Similar temporal variations of the mean relative frictional strength are obtained for even these cases, confirming the variation. The most likely cause for the observed temporal variation of the mean relative frictional strength is the temporal variation of the pore fluid pressure in the source area of the swarm, facilitated by the Tohoku-Oki earthquake and the subsequent fluid diffusion.

Description: The new hexagonal aluminous phase, named the NAL phase, is expected to be stable at depths of 〈1200 km in subducted slabs and believed to constitute 10 ~ 30 wt % of subducted MORB together with the CaFe 2 O 4 -type aluminous phase. Here elasticity of the single-crystal NAL phase is investigated using Brillouin light scattering coupled with diamond anvil cells up to 20 GPa at room temperature. Analysis of the results shows that the substitution of iron lowers the shear modulus of the NAL phase by ~5 % (~6 GPa), but does not significantly affect the adiabatic bulk modulus. The NAL phase exhibits high velocity anisotropies with AV P  = 14.7 % and AV S  = 15.12 % for the Fe-bearing phase at ambient conditions. The high AV S of the NAL phase mainly results from the high anisotropy of the faster V S1 (13.9 ~ 15.8 %), while the slower V S2 appears almost isotropic (0.1 ~ 2.8 %) at ambient and high pressures. The AV P and AV S of the NAL phase decrease with increasing pressure, but still have large values with AV P  = 11.4 % and AV S  = 14.12 % for the Fe-bearing sample at 20.4 GPa. The extrapolated AV P and AV S of the Fe-free and Fe-bearing NAL phase at 40 GPa are larger than those of bridgmanite at the same pressure. Together with its spin transition of iron and structural transition to the CF phase, the presence of the NAL phase with high velocity anisotropies may contribute to the observed seismic anisotropy around subducted slabs in the uppermost lower mantle.

Description: Slow slip events (SSEs) are identified as the quasi-stable fault deformation in the deep transition zone from locked to continuous sliding in many subduction zones. In the well-instrumented Cascadia margin, a class of Mw6.0 slow slip events arise beneath Port Angeles every ∼14 months, as inferred from two decades of continuous geodetic monitoring. The along-strike bending of the incoming oceanic plate beneath north Washington is a unique geometric feature whose influence on slow slip processes is still unknown. Here we incorporate a realistic fault geometry of northern Cascadia in the framework of rate- and state-dependent friction law, to simulate the spatiotemporal evolution of slow slip events on a non-planar subduction fault. The modeled SSEs capture the major characteristics revealed by GPS observations. The central 150km-long fault segment beneath Port Angeles acts as a repetitive slip patch, where SSEs appear every ∼1.5 years with a maximum slip of ∼2.5 cm. Two minor slip patches with smaller areas and cumulative slips straddle this central slip patch. The along-strike segmentation of slow slip is inversely related to the local fault dip and strike angles of the slow slip zone, suggesting strong geometrical control on the slow slip process. This correlation holds even after removing the effect of W / h ∗ , ratio between velocity-weakening SSE fault width and characteristic nucleation size. Besides the GPS-detectable fast-spreading phase, we find that each SSE cycle consists of deep pre-SSE preparation and post-SSE relaxation phases, which may be the driving mechanism for the deep inter-ETS tremor activity discovered in Cascadia.

Description: Short-duration, pulse-like long-period (LP) events are a characteristic type of seismicity accompanying eruptive activity at Mount Etna in Italy in 2004 and 2008 and at Turrialba Volcano in Costa Rica and Ubinas Volcano in Peru in 2009. We use the discrete wave number method to compute the free surface response in the near field of a rectangular tensile crack embedded in a homogeneous elastic half space and to gain insights into the origin of the LP pulses. Two source models are considered, including (1) a vertical fluid-driven crack, and (2) a unilateral tensile rupture growing at a fixed sub-Rayleigh velocity with constant opening on a vertical crack. We apply cross correlation to the synthetics and data to demonstrate that a fluid-driven crack provides a natural explanation for these data with realistic source sizes and fluid properties. Our modeling points to shallow sources (〈 1 km depth), whose signatures are representative of the Rayleigh pulse sampled at epicentral distances 〉∼1 km. While a slow-rupture failure provides another potential model for these events, the synthetics and resulting fits to the data are not optimal in this model compared to a fluid-driven source. We infer that pulse-like LP signatures are parts of the continuum of responses produced by shallow fluid-driven sources in volcanoes.

Description: Observations of dunite channels in ophiolites and uranium-series disequilibria in mid-ocean ridge basalt suggest that melt transport in the upper mantle beneath mid-ocean ridges is strongly channelized. We present experimental evidence that spatial variations in mineralogy can also focus melt on the grain-scale. This lithologic melt partitioning, which results from differences in the interfacial energies associated with olivine-melt and orthopyroxene-melt boundaries, may complement other melt focusing mechanisms in the upper mantle such as mechanical shear and pyroxene dissolution. We document here lithologic melt partitioning in olivine/orthopyroxene-basaltic melt samples containing nominal olivine to orthopyroxene ratio of 3 to 2 and melt fractions of 0.02 to 0.20. Experimental samples were imaged using synchrotron-based X-ray micro-computed tomography at a resolution of 700 nm per voxel. By analyzing the local melt fraction distributions associated with olivine and orthopyroxene (opx) grains in each sample, we found that the melt partitioning coefficient, i.e. , the ratio of melt fraction around olivine to that around orthopyroxene grains, varies between 1.1 and 1.6. The permeability and electrical conductivity of our digital samples were estimated using numerical models and compared to those of samples containing only olivine and basaltic melt. Our results suggest that lithologic melt partitioning and preferential localization of melt around olivine grains might play a role in melt focusing, potentially enhancing average melt ascent velocities.

Description: The mass discharge rate is a key parameter for initializing volcanic ash dispersal models. Commonly used empirical approaches derive the discharge rate by the plume height as estimated by remote sensors. A novel approach based on the combination of weather radar observations and thermal camera imagery is presented here. It is based on radar ash concentration estimation and the retrieval of the vertical exit velocities of the explosive cloud using thermal camera measurements. The applied radar retrieval methodology is taken from a revision of previously presented work. Based on the analysis of four eruption events of the Mt. Etna volcano (Sicily, Italy) that occurred in December 2015, the proposed methodology is tested using observations collected by three radar systems (at C- and X-band) operated by the Italian Department of Civil Protection. The total erupted mass was estimated to be about 9·10 9  kg and 2.4·10 9  kg for the first and second events, respectively, while it was about 1.2·10 9  kg for both the last two episodes. The comparison with empirical approaches based on radar-retrieved plume height shows a reasonably good agreement. Additionally, the comparative analysis of the polarimetric radar measurements provides interesting information on the vertical structure of the ash plume, including the size of the eruption column and the height of the gas thrust region.

Description: Induced earthquakes often accompany fluid injection, and the seismic hazard they pose threatens various underground engineering projects. Models to monitor and control induced seismic hazard with traffic light systems should be probabilistic, forward-looking, and updated as new data arrive. In this study, we propose an Induced Seismicity Test Bench to test and rank such models; this test bench can be used for model development, model selection, and ensemble model building. We apply the test bench to data from the Basel 2006 and Soultz-sous-Forêts 2004 geothermal stimulation projects, and we assess forecasts from two models: Shapiro and Smoothed Seismicity (SaSS) and Hydraulics and Seismics (HySei). These models incorporate a different mix of physics-based elements and stochastic representation of the induced sequences. Our results show that neither model is fully superior to the other. Generally, HySei forecasts the seismicity rate better after shut-in, but is only mediocre at forecasting the spatial distribution. On the other hand, SaSS forecasts the spatial distribution better and gives better seismicity rate estimates before shut-in. The shut-in phase is a difficult moment for both models in both reservoirs: the models tend to underpredict the seismicity rate around, and shortly after, shut-in.

Description: GPS geodesy provides very precise velocities of benchmarks on decadal timescales, and geodesists often describe their uncertainties with a velocity covariance matrix. However, those who model neotectonic deformation to estimate long-term seismic hazard want constraints on the interseismic velocities of stable bedrock on multi-thousand-year timescales. When the former (available data) are used as proxies for the latter (desired constraints), it is necessary to increase uncertainties to characterize a variety of transient and/or surficial noise processes, including magma-chamber recharge, post-seismic relaxation, pore-fluid motion, extremely slow landsliding, and glacial isostatic adjustment. The effects of transient noise on distant reference benchmarks also add to uncertainty of the long-term velocity reference frame. We augment the reported velocity covariance matrix with transpose-products of velocity-perturbation vectors from simple models approximately describing anticipated transient and surficial noise sources. No artifacts are introduced by this method because the velocity-vector data are unchanged. When the inverse of the augmented covariance matrix (the “diminished normal matrix”) is used in the objective function of a neotectonic deformation model partially driven by GPS data, the perturbing effects of transient and surficial signals are greatly reduced. Improvement occurs even when the prior estimates of noise processes are rather crude. We present two examples computed with synthetic data.

Description: Backarc spreading center characteristics reflect interactions between plate-driven mantle advection and melting and slab-driven hydrous melting and buoyant upwelling in the mantle wedge. At the Fonualei Rift and Spreading Center (FRSC) in the Lau Basin spreading rates decrease from slow to ultraslow, providing an opportunity to examine crustal accretion as the plate-driven component is minimized. A new Lau Basin kinematic analysis predicts FRSC spreading rates of ~32-8 mm/yr southward, much slower than previous estimates of ~95-47 mm/yr. Here we examine FRSC morphology and geophysical characteristics as it approaches the Tofua arc volcanic front southward and spreading rates decrease, minimizing the plate-driven component of mantle advection and maximizing buoyant hydrous flux melting. Axial morphology changes abruptly from a deep, flat, faulted axis ~100 km away from the arc to a volcanic ridge that shoals and increases in relief southward. Within ~50 km of the arc at the south end, the volcanic ridge is abruptly replaced by isolated volcanic cones bisected by volcanic rift zones and surrounded by anomalously deep seafloor. These morphologic changes likely reflect along-axis focusing of mantle upwelling and melting similar to that seen at ultraslow mid-ocean ridges, causing the change in morphology from a segmented ridge to spaced axial cones. We propose that as opening rates slow and the ridge approaches the arc, more of the inherently three-dimensional pattern of hydrous flux melting and buoyant upwelling in the mantle wedge is expressed volcanically. With faster opening, two-dimensional plate-driven mantle advection dominates melt production, favoring ridges over point-source features.

Description: Recent geological observations support the subduction of some oceanic arcs into the mantle. For example, the Izu-Bonin arc is considered to be partially subducting underneath the island of Honshu, Japan, despite its lower density than that of surrounding mantle, after the vertical collision with Honshu at ~17 Ma. Seismological surveys have clarified the internal structure of the arc, which consists of the basaltic and boninitic upper crust, the felsic middle crust, the intermediate upper-lower crust, and the mafic lower crust. This structure extends and moves northward toward Honshu. Among these components, part of the middle, the intermediate upper-lower, and the lower crust is thought to be subducting. Here, in order to estimate the subduction rate of granitic materials in oceanic arcs to the deep mantle, we have conducted numerical simulations of the subduction of arcs based on the finite element method and investigated the effect of arcs’ sizes and shapes and slab temperature. The results show that the subduction rate heavily depends on the geometry of the arcs or temperature profiles of the subducting slabs. When the size of the arc or the temperature of the slab is smaller, the subduction rate grows larger owing to competition between upward buoyancy and downward viscous drag from slabs. The results also show that about 20% of the felsic crust materials in oceanic arcs that are comparable in size to the Izu-Bonin arc can be subducted into the deep mantle when the temperature of the subducting slab is of the usual value.

Description: The lithosphere of Italy is exposed to a number of different short-term strain transients, including but not limited to landslides, post-seismic relaxation, and volcanic inflation/deflation. These transients affect GPS velocities and complicate the assessment of the long-term tectonic component of the surface deformation. In a companion paper we present a method for anticipating the principal patterns of non-tectonic, short-term strains and building this information into the covariance matrix of the geodetic velocities. In this work we apply this method to Italian GPS velocities to build an augmented covariance matrix that characterizes all expected discrepancies between short- and long-term velocities. We find that formal uncertainties usually reported for GPS measurements are smaller than the variability of the same benchmarks across a geologic time-span. Furthermore, we include in our modeling the azimuths of most-compressive horizontal principal stresses (SH max ) because GPS data can not resolve the active kinematics of coastal and offshore areas. We find that the final tectonic model can be made relatively insensitive to short-term interfering processes if the augmented covariance matrix and SH max data records are used in the objective function. This results in a preferred neotectonic model that is also in closer agreement with independent geologic and seismological constraints and has the advantage of reducing short-term biases in forecasts of long-term seismicity.

Description: We present new crust and lithosphere thickness maps of the African mainland based on integrated modeling of elevation and geoid data and thermal analysis. The approach assumes local isostasy, thermal steady-state, and linear density increase with depth in the crust and temperature-dependent density in the lithospheric mantle. Results are constrained by a new comprehensive compilation of seismic Moho-depth data consisting of 551 data points, and by published tomography models relative to LAB-depth. The crustal thickness map shows a N-S bimodal distribution with higher thickness values in the cratonic domains of southern Africa (38–44 km) relative to those beneath northern Africa (33–39 km). The most striking result is the crustal thinning (28–30 km thickness) imaged along the Mesozoic West and Central African Rift Systems. Our crustal model shows noticeable differences compared to previous models. After excluding the Afar plume region, where the modeling assumptions are not fulfilled, our model better fits the available seismic data (76.3% fitting; RMSE = 4.3 km). The LAB-depth map shows large spatial variability (90 to 230 km), with deeper LAB related to cratonic domains and shallower LAB related to Mesozoic and Cenozoic rifting domains, in agreement with tomography models. Though crustal and lithosphere thickness maps show similar regional patterns, major differences are found in the Atlas Mountains, the West African Rift System, and the intracratonic basins. The effects of lateral variations in crustal density as well as the non-isostatic contribution to elevation in the Afar plume region, which we estimate to be ~1.8 km, are also discussed.

Description: We present high-quality focal mechanisms based on a refined earthquake location catalog for the Island of Hawai‘i, focusing on Mauna Loa and Kı̄lauea volcanoes. The relocation catalog is based on first-arrival times and waveform data of both compressional and shear waves for about 180,000 events on and near the Island of Hawai‘i between 1986 and 2009 recorded by the seismic stations at the Hawaiian Volcano Observatory. We relocate all the earthquakes by applying ray-tracing through an existing three-dimensional velocity model, similar event cluster analysis and a differential-time relocation method. The resulting location catalog represents an expansion of previous relocation studies, covering a longer time period and consisting of more events with well-constrained absolute locations. The focal mechanisms are obtained based on the compressional-wave first motion polarities and compressional-to-shear wave amplitude ratios by applying the HASH program to the waveform cross-correlation relocated earthquakes. Overall, the good-quality (defined by the HASH parameters) focal solutions are dominated by normal faulting in our study area, especially in the active Ka‘ōiki and Hı̄lea seismic zones. Kı̄lauea caldera is characterized by a mixture of approximately equal numbers of normal, strike-slip, and reverse faults, whereas its south flank has slightly fewer strike-slip events. Our relocation and focal mechanism results will be useful for mapping the seismic stress and strain fields and for understanding the seismic-volcanic-tectonic relationships within the magmatic systems.

Description: Progressive development of opening-mode splay or branch fractures along a permeable fault in an elastic medium, subject to elevated fluid pressure from a constant influx fluid source, is studied numerically using a plane-strain hydraulic fracturing model that couples fracture deformation and fluid flow. In situ stresses are imposed so that their resultant shear stress on the fault is lower than the frictional strength. New splay fractures are initiated based on satisfying a dual criterion for both tensile strength and fracture toughness and meeting a minimum fracture spacing requirement. Numerical results demonstrate that spatial variations in permeability along faults can cause arrest of local slip and the created slip gradient can result in splay fracture initiation at a significant distance inwards from the fault tips. One splay fracture generally grows first and the kinematic coherence in displacements at the junction between it and the fault can reduce the downstream flow rate to prevent the nucleation of other splay fractures. However, the number of splay fractures can be increased when the branch growth extent is limited to a certain size, when the fault is divided into many segments, each with a linear distributed initial aperture and if the main fault is curved. The generation of a number of splay fractures can act to increase the permeability of the rock mass. When the splay fractures are constrained by two faults, a rhomb-shaped fault zone involving multiple high-angle branches forms. The development of the second-generation splay fractures on a first-generation one is promoted by fluid penetration. Multiple fluid-driven splay fractures can be created under the condition that fluid pressure is below or slightly above the fault confining stress. This implies that generation of complex splay fracture patterns requires less energy than generating a single opening mode fracture. Multiple fluid-driven splay fracture nucleation and growth into a horsetail pattern occur when the fault-parallel in situ normal stress becomes more tensile. In addition, the effects of fluid viscosity and tensile fracture toughness are investigated to determine their role in splay fracture initiation and growth.

Description: The cratonic cores of the continents are remarkably stable and long-lived features. Their ability to resist destructive tectonic processes is associated with their thick (∼250 km), cold, chemically-depleted, buoyant lithospheric keels that isolate the cratons from the convecting mantle. The formation mechanism and tectonic stability of cratonic keels remains under debate. To address this issue, we use P- and S-wave relative arrival-time tomography to constrain upper-mantle structure beneath southeast Canada and the northeast USA, a region spanning three quarters of Earth's geological history. Our models show three distinct, broad zones: Seismic wavespeeds increase systematically from the Phanerozoic coastal domains, through the Proterozoic Grenville Province, to the Archean Superior craton in central Québec. We also recover the NW-SE-trending track of the Great Meteor hotspot that cross-cuts the major tectonic domains. The decrease in seismic wavespeed from Archean to Proterozoic domains across the Grenville Front is consistent with predictions from models of two-stage keel formation, supporting the idea that keel growth may not have been restricted to Archean times. However, while crustal structure studies suggest that Archean Superior material underlies Grenvillian-age rocks up to ∼300 km SE of the Grenville Front, our tomographic models show a near-vertical boundary in mantle wavespeed directly beneath the Grenville Front. We interpret this as evidence for subduction-driven metasomatic enrichment of the Laurentian cratonic margin, prior to keel stabilization. Variable chemical depletion levels across Archean-Proterozoic boundaries worldwide may thus be better explained by metasomatic enrichment than inherently less-depleted Proterozoic composition at formation.

Description: Lithosphere delamination is believed to have played a major role in mountain building; however, the mechanism and dynamics of delamination remain poorly understood. Using a 2-D high-resolution thermo-mechanical model, we systematically investigated the conditions for the initiation of lithosphere delamination during orogenesis of continental collision, and explored the key factors that control the various modes of delamination. Our results indicate that the negative buoyancy from lithosphere thickening during orogenesis could cause delamination, when the reference density of the lithospheric mantle is not lower than that of the asthenosphere. In these cases, compositional rejuvenation of depleted continental lithosphere by magmatic/metasomatic plume- and/or subduction-induced processes may play crucial roles for subsequent lithosphere delamination. If the reference density of the lithospheric mantle is less than that of the asthenosphere, additional promoting factors, such as lower crust eclogitization, are required for delamination. Our numerical simulations predict three basic modes of lithosphere delamination: pro-plate delamination, retro-plate delamination, and a transitional double-plates (both the pro- and retro-plate) delamination. Pro-plate delamination is favored by low convergence rates, high lithospheric density and relatively strong retro-plate, whereas retro-plate delamination requires a weak retro-plate. The Northern Apennines and Central-Northern Tibetan plateau are possible geological analogues for the pro-plate and retro-plate delamination modes, respectively. Our model also shows significant impact of delamination on the topographic evolution of orogens. Large-scale lithosphere delamination in continental collision zones would lead to wide and flat plateaus, whereas relatively narrow and steep mountain belts are predicted in orogens without major delamination.

Description: The effect of finite strain geometry on crystallographic preferred orientation (CPO) is poorly constrained in the upper mantle. Specifically, the relationship between shape preferred orientation (SPO) and CPO in the mantle rocks remains unclear. We analyzed a suite of 40 spinel peridotite xenoliths from Marie Byrd Land, west Antarctica. X-ray computed tomography allows for quantification of spinel SPO, which ranges from prolate to oblate shape. Electron backscatter diffraction analysis reveals a range of olivine CPO patterns, including A-type, axial-[010], axial-[100], and B-type patterns. Until now, these CPO types were associated with different deformation conditions, deformation mechanisms, or strain magnitudes. Microstructures and deformation mechanism maps suggest that deformation in all studied xenoliths is dominated by dislocation-accommodated grain boundary sliding. For the range of temperatures (779–1198 °C), extraction depths (39–72 km), differential stresses (2–60 MPa), and water content (up to 500 H/10 6 Si) of the xenolith suite, variations in olivine CPO do not correlate with changes in deformation conditions. Here we establish for the first time in naturally deformed mantle rocks that finite strain geometry controls the development of axial-type olivine CPOs; axial-[010] and axial-[100] CPOs form in relation to oblate and prolate fabric ellipsoids, respectively. Girdling of olivine crystal axes results from intracrystalline slip with activation of multiple slip systems, and grain boundary sliding. Our results demonstrate that mantle deformation may deviate from simple shear. Olivine texture in field studies and seismic anisotropy in geophysical investigations can provide critical constraints for the 3D strain in the upper mantle.

Description: Space geodetic measurements from the Envisat satellite between 2003 and 2010 show that subsidence rates near the southeastern shoreline of the Salton Sea in Southern California are up to 52 mm yr −1 greater than the far-field background rate. By comparing these measurements with model predictions, we find that this subsidence appears to be dominated by poroelastic contraction associated with ongoing geothermal fluid production, rather than the purely fault-related subsidence proposed previously. Using a simple point-source model, we suggest that the source of this proposed volumetric strain is at depths between 1.0 km and 2.4 km (95% confidence interval), comparable to generalized boundaries of the Salton Sea geothermal reservoir. We find that fault slip on two previously imaged tectonic structures, which are part of a larger system of faults in the Brawley Seismic Zone, is not an adequate predictor of surface velocity fields because the magnitudes of the best fitting slip rates are often greater than the full plate-boundary rate, and at least two times greater than characteristic sedimentation rates in this region. Large-scale residual velocity anomalies indicate that spatial patterns predicted by fault slip are incompatible with the observations.