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.