ALBERT

Description: Amorphous silicates are common in extraterrestrial materials, especially in carbonaceous chondrites of petrologic types 1 and 2. In addition, high percentage of amorphous components and poorly crystalline phyllosilicates were found in the mudstones at Gale Crater by the CheMin instruments on-board of Mar Curiosity rover, which illustrates the importance of characterizing amorphous silicates in future planetary surface explorations. The structure of an amorphous silicate can vary in two aspects: the degree of polymerization and the degree of crystallinity. Here, we present the first phase study on characterizing synthetic alkali and alkali earth silicate glasses with different degrees of polymerization using vibration spectroscopy. Compared with crystalline silicates, their Raman and mid-IR spectra show broad spectral peaks, but have the similar peak positions. We find that a change in the degree of polymerization of these silicate glasses affects their Raman band positions, and especially the ratio of Raman band intensities, as well as the positions of the Christiansen feature and reststrahlen bands in their mid-IR absorbance spectra. Based on these observations, we establish a calibration curve that could enable semi-quantification of the polymerization degree of silicate glasses in planetary surface/subsurface materials during future robotics planetary surface exploration missions.

Description: The latitudinal motion of the Tibetan Himalaya - the northernmost continental unit of the Indian plate - is a key component in testing paleogeographic reconstructions of the Indian plate before the India-Asia collision. Paleomagnetic studies of sedimentary rocks (mostly carbonate rocks) from the Tibetan Himalaya are complicated by potentially pervasive yet cryptic remagnetization. Although traditional paleomagnetic field tests reveal some of this remagnetization, secondary remanence acquired prior to folding or tilting easily escapes detection. Here, we describe comprehensive rock magnetic and petrographic investigations of Jurassic to Paleocene carbonate and volcaniclastic rocks from Tibetan Himalayan strata (Tingri and Gamba areas). These units have been the focus of several key paleomagnetic studies for Greater Indian paleogeography. Our results reveal that while the dominant magnetic carrier in both carbonate and volcaniclastic rocks is magnetite, their magnetic and petrographic characteristics are distinctly different. Carbonate rocks have ‘wasp-waisted’ hysteresis loops, suppressed Verwey transitions, extremely fine grain sizes (superparamagnetic), and strong frequency-dependent magnetic susceptibility. Volcaniclastic rocks exhibit ‘pot-bellied’ hysteresis loops and distinct Verwey transitions. Electron microscopy reveals that magnetite grains in carbonate rocks are pseudomorphs of early diagenetic pyrite, whereas detrital magnetite is abundant and pyrite is rarely oxidized in the volcaniclastic rocks. We suggest that the volcaniclastic rocks retain a primary remanence, but oxidation of early diagenetic iron sulfide to fine-grained magnetite has likely caused widespread chemical remagnetization of the carbonate units. We recommend that thorough rock magnetic and petrographic investigations are prerequisites for paleomagnetic studies throughout southern Tibet and everywhere in general.

Description: Orbital magnetic field data show that portions of the Moon's crust are strongly magnetized, and paleomagnetic data of lunar samples suggest that Earth-strength magnetic fields could have existed during the first several hundred million years of lunar history. The origin of the fields that magnetized the crust are not understood and could be the result of either a long-lived core generated dynamo or transient fields associated with large impact events. Core-dynamo models usually predict that the field would be predominantly dipolar, with the dipole axis aligned with the rotation axis. We test this hypothesis by modeling the direction of crustal magnetization using a global magnetic field model of the Moon derived from Lunar Prospector and Kaguya magnetometer data. We make use of a model that assumes that the crust is unidirectionally magnetized. The intensity of magnetization can vary with the crust, and the best-fitting direction of magnetization is obtained from a non-negative least squares inversion. From the best fitting magnetization direction we obtain the corresponding North magnetic pole predicted by an internal dipolar field. Some of the obtained paleopoles are associated with the current geographic poles, while other well-constrained anomalies have paleopoles at equatorial latitudes, preferentially at 90° East and West longitudes. One plausible hypothesis for this distribution of paleopoles is that the Moon possessed a long-lived dipolar field, but that the dipole was not aligned with the rotation axis as a result of large scale heat flow heterogeneities at the core-mantle boundary.

Description: Both the geometry and the depth of the seismogenic zone of the North Anatolian Fault under the Marmara Sea (the Main Marmara Fault; MMF) are poorly understood, in part because of the fault's undersea location. We recorded 10 months of microseismic data with a dense array of ocean bottom seismographs and then applied double-difference relocation and 3-D tomographic modeling to obtain precise hypocenters on the MMF beneath the central and western Marmara Sea. The hypocenters show distinct lateral changes along the MMF: (1) Both the upper and lower crust beneath the Western High are seismically active and the maximum focal depth reaches 26 km, (2) seismic events are confined to the upper crust beneath the region extending from the eastern part of the Central Basin to the Kumburgaz Basin, and (3) the magnitude and direction of dip of the main fault changes under the Central Basin, where there is also an abrupt change in the depth of the lower limit of the seismogenic zone. We attribute this change to a segment boundary of the MMF. Our data show that the upper limit of the seismogenic zone corresponds to sedimentary basement. We also identified several seismically inactive regions within the upper crust along the MMF; their spatial extent beneath the Kumburgaz Basin is greater than beneath the Western High. From the comparison with seafloor extensometer data, we consider that these regions might indicate zones of strong coupling that are accumulating stress for release during future large earthquakes.

Description: Lawsonite CaAl 2 Si 2 O 7 (OH) 2 ·H 2 O is an important water carrier in subducting oceanic crusts, and the primary hydrous phase in basalt at depths greater than ~80 km. We have conducted high-pressure synchrotron single-crystal x-ray diffraction experiments on natural lawsonite at room temperature up to ~10.0 GPa to study its high-pressure polymorphism. We find that lawsonite remains orthorhombic with Cmcm symmetry up to ~9.3 GPa, and shows nearly isotropic compression. Above ~9.3 GPa, lawsonite becomes monoclinic with P 2 1 / m symmetry. Across the phase transition, the Ca polyhedron becomes markedly distorted, and the average positions of the H 2 O molecules and hydroxyls change. The changes observed in the H-atom positions under compression are different than the low temperature changes in this material. We resolve for the first time the H-bonding configuration of the high-pressure monoclinic phase of lawsonite. A bond valence approach is deployed to determine that the phase transition from orthorhombic to monoclinic is primarily driven by the Si 2 O 7 groups, and in particular their bridging oxygen atoms (O1). The changes in the structure strongly indicate that entropy increases across the symmetry-lowering transition, and hence that the slope of the phase transition is negative. Monoclinic lawsonite is thus stable under the pressure and temperature conditions that exist in the Earth, and is likely to be a major water carrier in colder, deep subducted slabs. Monoclinic lawsonite also likely has enhanced electrical conductivity along its c -axis due to its dynamically disordered hydrogen atoms.

Description: Receiver functions (RF) have been used for several decades to study structures beneath seismic stations. Although most available stations are deployed on-shore, the number of ocean bottom station (OBS) experiments has increased in recent years. Almost all OBSs have to deal with higher noise levels and a limited deployment time (∼1 year), resulting in a small number of usable records of teleseismic earthquakes. Here, we use OBSs deployed as mid-aperture array in the deep ocean (4.5-5.5 km water depth) of the eastern mid-Atlantic. We use evaluation criteria for OBS data and beam forming to enhance the quality of the RFs. Although some stations show reverberations caused by sedimentary cover, we are able to identify the Moho signal, indicating a normal thickness (5-8 km) of oceanic crust. Observations at single stations with thin sediments (300-400 m) indicate that a probable sharp lithosphere-asthenosphere boundary (LAB) might exist at a depth of ∼70-80 km which is in line with LAB depth estimates for similar lithospheric ages in the Pacific. The mantle discontinuities at ∼410 km and ∼660 km are clearly identifiable. Their delay times are in agreement with PREM. Overall the usage of beam formed earthquake recordings for OBS RF analysis is an excellent way to increase the signal quality and the number of usable events.

Description: Since the discovery of slow slip events, many methods have been successfully applied to model obvious transient events in geodetic time series, such as the widely used network strain filter. Independent seismological observations of tremors or low frequency earthquakes and repeating earthquakes provide evidence of low amplitude slow deformation but do not always coincide with clear occurrences of transient signals in geodetic time series. Here, we aim to extract the signal corresponding to slow slips hidden in the noise of GPS time series, without using information from independent datasets. We first build a library of synthetic slow slip event templates by assembling a source function with Green's functions for a discretized fault. We then correlate the templates with post-processed GPS time series. Once the events have been detected in time, we estimate their duration T and magnitude M w by modelling a weighted stack of GPS time series. An analysis of synthetic time series shows that this method is able to resolve the correct timing, location, T and M w of events larger than M w 6 in the context of the Mexico subduction zone. Applied on a real data set of 29 GPS time series in the Guerrero area from 2005 to 2014, this technique allows us to detect 28 transient events from M w 6.3 to 7.2 with durations that range from 3 to 39 days. These events have a dominant recurrence time of 40 days and are mainly located at the down dip edges of the M w 〉 7.5 SSEs.

Description: Acoustic borehole televiewer (BHTV) logs provide measurements of fracture attributes (orientations, thickness, and spacing) at depth. Orientation, censoring, and truncation sampling biases similar to those described for one-dimensional outcrop scanlines, and other logging or drilling artifacts specific to BHTV logs, can affect the interpretation of fracture attributes from BHTV logs. K -means, fuzzy K -means, and agglomerative clustering methods provide transparent means of separating fracture groups on the basis of their orientation. Fracture spacing is calculated for each of these fracture sets. Maximum likelihood estimation using truncated distributions permits the fitting of several probability distributions to the fracture attribute data sets within truncation limits, which can then be extrapolated over the entire range where they naturally occur. Akaike Information Criterion (AIC) and Schwartz Bayesian Criterion (SBC) statistical information criteria rank the distributions by how well they fit the data. We demonstrate these attribute analysis methods with a data set derived from three BHTV logs acquired from the high-temperature Rotokawa geothermal field, New Zealand. Varying BHTV log quality reduces the number of input data points, but careful selection of the quality levels where fractures are deemed fully sampled increases the reliability of the analysis. Spacing data analysis comprising up to 300 data points and spanning three orders of magnitude can be approximated similarly well (similar AIC rankings) with several distributions. Several clustering configurations and probability distributions can often characterize the data at similar levels of statistical criteria. Thus, several scenarios should be considered when using BHTV log data to constrain numerical fracture models.

Description: We investigate the dynamics of the inner core wobble (ICW), the Euler-Liouville wobbling motion of the Earth's solid inner core, under the mantle-inner core gravitational (MICG) torques within the Earth. Chao [2016] derived the full 3-D equation of motion for the MICG dynamics in terms of the spherical-harmonic multipoles of mass density, and focused on the axial component for inner-core's torsional libration. Here, aiming for the ICW, we deduce the 2-D equatorial component of the MICG torque owing to the oblateness of the mantle and the inner core. The period of the free Eulerian wobble of a hypothetical isolated rigid inner core would be a prograde +414 days. The action of the added MICG equatorial torque is found to be (negatively) strong enough to render the wobbling motion to become retrograde (with a negative frequency), which is further but slightly modified by the elastic yielding feedback of the inner core. Imposing yet further the passive effect of the hydrostatic pressure due to the fluid outer core greatly lengthens the natural period to become decadal. The final estimate of the ICW natural period in accordance with the (seismological) PREM Earth model is a retrograde P ICW ≈ −15.6 years, in contrast to a prograde +6.6 years supposed in the literature. The corresponding spring constant (per radian of wobble) is 5.5 × 10 22 N m. Our results instigate likely identification of the ICW with decadal wobbles observed in the Earth's polar motion.

Description: To understand the processes involved in rheological weakening due to phase mixing in olivine + orthopyroxene aggregates, we conducted high-strain torsion experiments on two-phase samples at a temperature of 1200°C and a confining pressure of 300 MPa. Samples composed of iron-rich olivine plus 26% orthopyroxene were deformed to outer radius shear strains of up to γ ≈ 26. Values for the stress exponent, n , and grain size exponent, p , were determined based on least-squares fits of the strain rate, stress, and grain size data to a power-law creep equation both at smaller strains (γ ≤ 3) and at larger strains (γ ≥ 24). Microstructural observations demonstrate that, with increasing shear strain, grain size decreased and mixtures of small, equant grains of olivine and pyroxene developed. The values of n and p combined with associated changes in microstructure demonstrated that our samples deformed (i) by dislocation-accommodated grain-boundary sliding with subgrains present at lower strains and (ii) by dislocation-accommodated grain-boundary sliding with subgrains absent at higher strains. The evolution of both the mechanical and the microstructural properties observed in this study provide insights into the dynamic processes associated with rheological weakening and strain localization in the plastically deforming portion of the lithosphere.

Description: We analyze three-dimensional GPS coordinate time series from continuously operating stations in Nepal and South Tibet and calculate the initial one-year postseismic displacements. We first investigate models of poroelastic rebound, afterslip and viscoelastic relaxation individually and then attempt to resolve the tradeoffs between their contributions by evaluating the misfit between observed and simulated displacements. We compare kinematic inversions for distributed afterslip with stress-driven afterslip models. The modeling results show that no single mechanism satisfactorily explains near- and far-field postseismic deformation following the Gorkha earthquake. When considering contributions from all three mechanisms, we favor a combination of viscoelastic relaxation and afterslip alone, as poroelastic rebound always worsens the misfit. The combined model does not improve the data misfit significantly, but the inverted afterslip distribution is more physically plausible. The inverted afterslip favors slip within the brittle-ductile transition zone down dip of the coseismic rupture and fills the small gap between the mainshock and largest aftershock slip zone, releasing only 7% of the coseismic moment. Our preferred model also illuminates the laterally heterogeneous rheological structure between India and the south Tibet. The transient and steady-state viscosities of the upper mantle beneath Tibet are constrained to be greater than 10 18 Pa s and 10 19 Pa s, whereas the Indian upper mantle has a high viscosity ≥ 10 20 Pa s. The viscosity in the lower crust of southern Tibet shows a clear tradeoff with its southward extent and thickness, suggesting an upper bound value of ~8×10 19 Pa s for its steady-state viscosity.

Description: The transition from sub-Rayleigh to supershear propagation of mode II cracks is a fundamental problem of fracture mechanics. It has extensively been studied in homogeneous uniform setups. When the applied shear load exceeds a critical value, transition occurs through the Burridge-Andrews mechanism at a well-defined crack length. However, velocity structures in geophysical conditions can be complex and affect the transition. Damage induced by previous earthquakes causes low-velocity zones surrounding mature faults and inclusions with contrasting material properties can be present at seismogenic depth. We relax the assumption of homogeneous media and investigate dynamic shear fracture in heterogeneous media using two-dimensional finite-element simulations and a linear slip-weakening law. We analyze the role of heterogeneities in the elastic media, while keeping the frictional interface properties uniform. We show that supershear transition is possible due to the sole presence of favorable off-fault heterogeneities. Sub-critical shear loads, for which propagation would remain permanently sub-Rayleigh in an equivalent homogeneous setup, will transition to supershear as a result of reflected waves. P-wave reflected as S-waves, followed by further reflections, affect the amplitude of the shear stress peak in front of the propagating crack, leading to supershear transition. A wave reflection model allows to uniquely describe the effect of off-fault inclusions on the shear stress peak. A competing mechanism of modified released potential energy affects transition and becomes predominant with decreasing distance between fault and inclusions. For inclusions at far distances, the wave reflection is the predominant mechanism.

Description: The physical properties of silicate melts within Earth's mantle affect the chemical and thermal evolution of its interior. Chemistry and coordination environments affect such properties. We have measured the hyperfine parameters of iron-bearing rhyolitic and basaltic glasses up to ~120 GPa and ~100 GPa, respectively, in a neon pressure medium using time-domain synchrotron Mössbauer spectroscopy. The spectra for rhyolitic and basaltic glasses are well explained by three high-spin Fe 2+ -like sites with distinct quadrupole splittings. Absence of detectable ferric iron was confirmed with optical absorption spectroscopy. The sites with relatively high and intermediate quadrupole splittings are likely a result of fivefold and sixfold coordination environments of ferrous iron that transition to higher coordination with increasing pressure. The ferrous site with a relatively low quadrupole splitting and isomer shift at low pressures may be related to a fourfold or a second fivefold ferrous iron site, which transitions to higher coordination in basaltic glass, but likely remains in low coordination in rhyolitic glass. These results indicate that iron experiences changes in its coordination environment with increasing pressure without undergoing a high-spin to low-spin transition. We compare our results to the hyperfine parameters of silicate glasses of different compositions. With the assumption that coordination environments in silicate glasses may serve as a good indicator for those in a melt, this study suggests that ferrous iron in chemically–complex silicate melts likely exists in a high-spin state throughout most of Earth's mantle.

Description: No abstract is available for this article.

Description: Analysis of fracture orientation, spacing, and thickness from acoustic borehole televiewer (BHTV) logs and cores in the andesite-hosted Rotokawa geothermal reservoir (New Zealand) highlights potential controls on the geometry of the fracture system. Cluster analysis of fracture orientations indicates four fracture sets. Probability distributions of fracture spacing and thickness measured on BHTV logs are estimated for each fracture set, using maximum likelihood estimations applied to truncated size distributions to account for sampling bias. Fracture spacing is dominantly lognormal, though two subordinate fracture sets have a power law spacing. This difference in spacing distributions may reflect the influence of the andesitic sequence stratification (lognormal) and tectonic faults (power law). Fracture thicknesses of 9–30 mm observed in BHTV logs, and 1–3 mm in cores, are interpreted to follow a power law. Fractures in thin sections (∼5 μm thick) do not fit this power law distribution, which, together with their orientation, reflect a change of controls on fracture thickness from uniform (such as thermal) controls at thin section scale to anisotropic (tectonic) at core and BHTV scales of observation. However, the ∼5% volumetric percentage of fractures within the rock at all three scales suggests a self-similar behavior in 3-D. Power law thickness distributions potentially associated with power law fluid flow rates, and increased connectivity where fracture sets intersect, may cause the large permeability variations that occur at hundred meter scales in the reservoir. The described fracture geometries can be incorporated into fracture and flow models to explore the roles of fracture connectivity, stress, and mineral precipitation/dissolution on permeability in such andesite-hosted geothermal systems.

Description: Planets and satellites can undergo physical librations, which consist of forced periodic variations in their rotation rate induced by gravitational interactions with nearby bodies. This mechanical forcing may drive turbulence in interior fluid layers such as subsurface oceans and metallic liquid cores through a libration-driven elliptical instability (LDEI) that refers to the resonance of two inertial modes with the libration-induced base flow. LDEI has been studied in the case of a full ellipsoid. Here, we address for the first time the question of the persistence of LDEI in the more geophysically-relevant ellipsoidal shell geometries. In the experimental setup, an ellipsoidal container with spherical inner cores of different sizes is filled with water. Direct side-view flow visualizations are made in the librating frame using Kalliroscope particles. A Fourier analysis of the light intensity fluctuations extracted from recorded movies shows that the presence of an inner core leads to spatial heterogeneities but does not prevent LDEI. Particle image velocimetry (PIV) and direct numerical simulations (DNS) are performed on selected cases to confirm our results. Additionally, our survey at a fixed forcing frequency and variable rotation period ( i.e. variable Ekman number, E ) shows that the libration amplitude at the instability threshold varies as ∼ E 0.65 . This scaling is explained by a competition between surface and bulk dissipation. When extrapolating to planetary interiors conditions, this leads to the E 1/2 scaling commonly considered. We argue that Enceladus' subsurface ocean and the core of the exoplanet 55 CnC e should both be unstable to LDEI.

Description: Growing evidence shows that lithospheric mantle beneath cratons may contain a certain amount of water that originated from dehydration of subducted slabs or mantle metasomatism. As water can significantly reduce the viscosity of nominally anhydrous minerals such as olivine, hydration-induced rheological weakening is a possible mechanism for the lithospheric thinning of cratons. Using 2D thermomechanical numerical models we investigated the influence of water on dislocation and diffusion creep of olivine during the evolution of cratonic lithosphere. Modeling results indicate that dislocation creep of wet olivine alone is insufficient to trigger dramatic lithospheric thinning within a timescale of tens of millions of years, even with an extremely high water content. However, if diffusion creep is incorporated, significant convective instability will occur at the base of the lithosphere and drive lithospheric mantle dripping, which results in intense lithospheric thinning. We performed semi-analytical models to better understand the influence of various parameters on the onset of convective instability. The convective instability promoted by hydration weakening drives lithospheric mantle dripping beneath cratons and thus provides a possible mechanism for cratonic thinning.

Description: We study the 3-D variation of the crustal structure of the Sikkim Himalaya using broadband seismological data acquired from a focussed network of seven stations spanning the Lesser, Higher and Tethyan Himalaya. Common conversion point stacking of receiver functions recorded along an across-strike profile of the Himalaya reveals first order northward dip on the Main Himalayan Thrust (MHT), a mid-crustal discontinuity and the Moho, along with higher order lateral variations. 3-D images generated from joint inversion of receiver functions and surface wave dispersions show that the MHT has a ramp-flat-ramp geometry. The ramps are located beneath the Lesser Himalaya and the Tethyan Himalaya with dips of ∼7 ∘ and ∼15 ∘ respectively, connected by flat segments. The ramp beneath the Lesser Himalaya forms a dome structure, up-warping the thrust sheets associated with the Peling and Main Central Thrust. The erosional surface of this dome forms the arcuate geometry of thrusts observed in the Lesser Himalaya. The thickness of the underthrust Indian crust is 35–42 km, and has an average V S of 3.63 km/s, similar to that of the Indian Shield crust. The Moho also has dome-like structures separated by elongated, deeper sections trending NW-SE. These are intersected by steeply-dipping transverse low-velocity zones, oblique to the strike of the Himalaya. We conjecture that these low-velocity zones are the dextral-strike slip faults known to be active beneath the Sikkim Himalaya. The observed alternate shallow and deep segments of the Moho must be a consequence of several cycles of strike-slip displacement on these transverse faults.

Description: We formulate the problem of fully coupled transient fluid flow and quasi-static poroelasticity in arbitrarily fractured, deformable porous media saturated with a single-phase compressible fluid. The fractures we consider are hydraulically highly conductive, allowing discontinuous fluid flux across them; mechanically they act as finite-thickness shear deformation zones prior to failure (i.e., non-slipping and non-propagating), leading to ‘apparent discontinuity’ in strain and stress across them. Local nonlinearity arising from pressure-dependent permeability of fractures is also included. Taking advantage of typically high aspect ratio of a fracture, we do not resolve transversal variations and instead assume uniform flow velocity and simple shear strain within each fracture, rendering the coupled problem numerically more tractable. Fractures are discretized as lower-dimensional zero-thickness elements tangentially conforming to unstructured matrix elements. A hybrid-dimensional, equal-low-order, two-field mixed finite element method is developed, which is free from stability issues for a drained coupled system. The fully implicit backward Euler scheme is employed for advancing the fully coupled solution in time, and the Newton-Raphson scheme is implemented for linearization. We show that the fully discretized system retains a canonical form of a fracture-free poromechanical problem; the effect of fractures is translated to the modification of some existing terms as well as the addition of several terms to the capacity, conductivity and stiffness matrices, therefore, allowing the development of independent subroutines for treating fractures within a standard computational framework. Our computational model provides more realistic inputs for some fracture-dominated poromechanical problems like fluid-induced seismicity.

Description: On 7 May 2015, a M W 4.0 earthquake occurred near Venus, northeast Johnson County, Texas, in an area of the Bend Arch-Fort Worth Basin that reports long-term, high-volume wastewater disposal and that has hosted felt earthquakes since 2009. In the weeks following the M W 4.0 earthquake, we deployed a local seismic network and purchased nearby active-source seismic reflection data to capture additional events, characterize the causative fault, and explore potential links between ongoing industry activity and seismicity. Hypocenter relocations of the resulting local earthquake catalog span ~4-6 km depth and indicate a fault striking ~230°, dipping to the west, consistent with a nodal plane of the M W 4.0 regional moment tensor. Fault plane solutions indicate normal faulting, with B-axes striking parallel to maximum horizontal compressive stress. Seismic reflection data image the reactivated basement fault penetrating the Ordovician disposal layer and Mississippian production layer, but not displacing post-Lower Pennsylvanian units. Template matching at regional seismic stations indicates that low magnitude earthquakes with similar waveforms began in April 2008, with increasing magnitude over time. Pressure data from five saltwater disposal wells within 5 km of the active fault indicate a disposal formation that is 0.9-4.8 MPa above hydrostatic. We suggest that the injection of 28,000,000 m 3 of wastewater between 2006 and 2015 at these wells led to an increase in subsurface pore fluid pressure that contributed to inducing this long-lived earthquake sequence. The 2015 M W 4.0 event represents the largest event in the continuing evolution of slip on the causative fault.

Description: We employ work optimization to predict the geometry of frontal thrusts at two stages of an evolving physical accretion experiment. Faults that produce the largest gains in efficiency, or change in external work per new fault area, Δ W ext /Δ A , are considered most likely to develop. The predicted thrust geometry matches within 1 mm of the observed position and within a few degrees of the observed fault dip, for both the first forethrust and backthrust when the observed forethrust is active. The positions of the second backthrust and forethrust that produce 〉90% of the maximum Δ W ext / Δ A also overlap the observed thrusts. The work optimal fault dips are within a few degrees of the faults dips that maximize the average Coulomb stress. Slip gradients along the detachment produce local elevated shear stresses and high strain energy density regions that promote thrust initiation near the detachment. The mechanical efficiency ( W ext ) of the system decreases at each of the two simulated stages of faulting and resembles the evolution of experimental force. The higher Δ W ex t /Δ A due to the development of the first pair relative to the second pair indicates that the development of new thrusts may lead to diminishing efficiency gains as the wedge evolves. The numerical estimates of work consumed by fault propagation overlap the range calculated from experimental force data, and crustal faults. The integration of numerical and physical experiments provides a powerful approach that demonstrates the utility of work optimization to predict the development of faults.

Description: To evaluate the role of subducted oceanic crust in the genesis of potassium-rich magmas, we report high-precision Mg isotopic data for a set of Cenozoic volcanic rocks from Northeast China. These rocks overall are lighter in Mg isotopic composition than the normal mantle, and display considerable Mg isotopic variations, with δ 26 Mg ranging from -0.61 to -0.23. The covariation of δ 26 Mg with TiO 2 in these rocks suggests their light Mg isotopic compositions were derived from recycled oceanic crust in the form of carbonated eclogite in the source region. The strong correlations between δ 26 Mg and (Gd/Yb) N ratio as well as Sr-Pb isotopes further indicate a multicomponent and multistage origin of these rocks. Magnesium isotopes may thus be used as a novel tracer of recycled oceanic crust in the source region of mantle-derived magmas.

Description: Solar wind interactions with the surfaces of asteroids and small moons eject atoms and molecules from the uppermost several n m of regolith grains through a process called sputtering. A small fraction of the sputtered species, called secondary ions, leave the surface in an ionized state, and these are diagnostic of the surface composition. Detection of secondary ions using ion mass spectrometry (IMS) provides a useful method of analysis due to low backgrounds and high instrument sensitivities. However, the sputtered secondary ion yield and the atomic composition of the surface are not 1-to-1 correlated. Thus, relative yield fractions based on experimental measurements are needed to convert measured spectra to surface composition. Here, available experimental results are combined with computationally derived solar wind sputtering yields to estimate secondary ion fluxes from asteroid-sized bodies in the Solar System. The Monte Carlo simulation code SDTrimSP is used to estimate the total sputtering yield due to solar wind ion bombardment for a diverse suite of meteorite and lunar soil compositions. Experimentally measured relative secondary ion yields are then used to determine the abundance of refractory species ( M g + , A l + , C a + , F e + ) relative to S i + , and it is shown that relative abundances can be used to distinguish whether a body is primitive or has undergone significant geologic reprocessing. Estimates of the sputtered secondary ion fluxes are used to determine the IMS sensitivity required to adequately resolve major element ratios for nominal orbital geometries.

Description: Without the protection of the atmosphere, the soils on lunar surfaces undergo a series of optical, physical, and chemical changes during micrometeorite bombardment. To simulate the micrometeorite-bombardment process and analyze the impact characteristics, four types of rocks, including terrestrial basalt and anorthosite supposed to represent lunar rock, an H-type chondrite (the Huaxi ordinary chondrite) and an iron meteorite (the Gebel Kamil iron meteorite) supposed to represent micrometeorite impactors, are irradiated by a nanosecond-pulse laser in a high vacuum chamber. Based on laser irradiation experiments, the laser pits are found to be of different shapes and sizes which vary with the rock type. Many melt and vapor deposits are found on the mineral surfaces of all the samples, and nanophase iron (npFe) or Fe-Ni alloy particles are typically distributed on the surfaces of ilmenite, kamacite or other minerals near kamacite. By analyzing the focused ion beam (FIB) ultrathin slices of laser pits with a transmission electron microscope (TEM), the results show that the subsurface structures can be divided into three classes and that npFe can be easily found in Fe-bearing minerals. These differences in impact characteristics will help determine the source material of npFe and infer the type of micrometeorite impactors. During micrometeorite bombardment, in the mare regions, the npFe are probably produced simultaneously from lunar basalt and micrometeorites with iron-rich minerals, while the npFe in the highlands regions mainly come from micrometeorites.

Description: To understand the processes involved in phase mixing during deformation and the resulting changes in rheological behavior, we conducted torsion experiments on samples of iron-rich olivine plus orthopyroxene. The experiments were conducted at a temperature, T , of 1200°C and a confining pressure, P , of 300 MPa using a gas-medium, deformation apparatus. Samples composed of olivine plus 26% orthopyroxene were deformed to outer radius shear strains up to γ ≈ 26. In samples deformed to lower strains of γ ≲ 4, elongated olivine and pyroxene grains form a compositional layering. Already by this strain, mixtures of small equant grains of olivine and pyroxene begin to develop and continue to evolve with increasing strain. The ratios of olivine to pyroxene grain size in deformed samples follow the Zener relationship, indicating that pyroxene grains effectively pin the grain boundaries of olivine and inhibit grain growth. Due to the reduction in grain size, the dominant deformation mechanism changes as a function of strain. The microstructural development forming more thoroughly mixed, fine-grained olivine-pyroxene aggregates can be explained by the difference in diffusivity among Me (Fe or Mg), O, and Si, with transport of MeO significantly faster than that of SiO 2 . These mechanical and associated microstructural properties provide important constraints for understanding rheological weakening and strain localization in upper mantle rocks.

Description: We have used spherical harmonic coefficients that describe Earth's gravity anomaly and topography fields to quantify the role of isostasy in contributing to crustal and upper mantle structure. Power spectra reveal that the gravity effect of topography and its flexural compensation contributes significantly to the observed free-air gravity anomaly spectra for spherical harmonic degree 33 〈 n 〈 400, which corresponds to wavelength 100 〈 λ 〈 1200 km. The best fit is for an elastic plate (flexure) model with an elastic thickness, T e , of 34.0 ± 4.0 km. Smaller values underpredict the observed gravity spectra while higher values overpredict. The best fit T e is a global average and so there will be regions where T e is lower and higher. This is confirmed in studies of selected regions such as the Hawaiian-Emperor seamount chain and the Himalaya fold and thrust belt where we show that flexural isostatic anomalies are near zero in regions where T e ~34.0 km and of large amplitude in regions of lower and higher T e . Plate flexure may also contribute at higher ( n 〉 400) and lower ( n 〈 33) degrees, but topography appears either uncompensated or fully compensated at these degrees, irrespective of the actual T e . All isostatic models underpredict the spectra at 2 〈 n 〈 12 and so we interpret the low order Earth's gravity field as caused, at least in part, by non-isostatic processes due to dynamic motions such as those associated with convective upwellings and downwellings in Earth's mantle.

Description: This paper proposes a stochastic approach to model the earthquake uncertainties in terms of the rupture location and the slip distribution for a future event, with an expected earthquake magnitude. Once the statistical properties of earthquake uncertainties are described, they are then propagated into the tsunami response and the inundation at assessed coastal areas. The slip distribution is modeled as a random field within a non-rectangular rupture area. The Karhunen-Lòeve (K-L) expansion method is used to generate samples of the random slip, and a translation model is employed to obtain target probability properties. A strategy is developed to specify the accuracy of the random samples in terms of numbers of sub-faults of the rupture area and the truncation of the K-L expansion. The propagation of uncertainty into the tsunami response is performed by means of a Stochastic Reduced Order Model. To illustrate the methodology, we investigated a study case in north Chile. We first demonstrate that the stochastic approach generates consistent earthquake samples with respect to the target probability properties. We also show that the results obtained from SROM are more accurate than those obtained with classic Monte Carlo simulations. To validate the methodology, we compared the simulated tsunamis and the tsunami records for the 2014 Chilean earthquake. Results show that leading wave measurements fall within the tsunami sample space. At later times, however, there are mismatches between measured data and the simulated results, suggesting that other sources of uncertainties are as relevant as the uncertainty of earthquakes.

Description: Intersections in a fracture network control the connectivity of the flow paths through rock. The long near-linear geometric nature of fractures makes them difficult to identify and characterized. We present a new type of elastic wave, an intersection wave, which travels along an intersection and is sensitive to the coupling between two orthogonal fractures that define the intersection. Group theory for C 2 v and C 4 v point groups predict sets of propagating elastic waves confined to the fracture intersection. Along with the use of the wave equation and displacement discontinuity boundary conditions, the dispersion relationships for intersection waves were predicted. Experimental ultrasonic measurements on a non-welded linear intersection between two orthogonal, synthetic fractures in aluminum confirm the existence of multiple modes that travel between the speed of wedge waves (sub-Rayleigh waves) when the intersection is completely open or decoupled, and bulk shear waves, when the intersection is closed, as predicted by theory. In between these two limits, the intersection behaves as a non-welded contact and yields these new intersection waves that are dispersive and sensitive to the coupling along the intersection. Intersection waves provide the foundation for new geophysical approaches for characterizing the hydraulic connectivity of fracture networks.

Description: The Indian subcontinent comprises of geological terranes of varied age and structural character. In this study, we provide new constraints to existing crustal models by inverting the P-to-s receiver functions (RFs) at 317 broadband seismic stations. Inversion results fill crucial gaps in existing velocity models (CRUST1.0 and SEAPS) by capturing regions which are less represented. The final model produced is much more heterogeneous and is able to capture the structural variations between closely spaced seismic stations. In comparison to the global models, major differences are seen for seismic stations located over various rift zones (e.g. Godavari, Narmada and Cambay) and those close to the coastal regions where transition from oceanic to continental crust is expected to create drastic changes in the crustal configuration. Seismic images are produced along various profiles using 49682 individual RFs recorded at 442 seismic stations. Lateral variations captured using migrated images across the Himalayan collisional front revealed the hitherto elusive southern extent of the Moho and intracrustal features south of the Main Central Thrust (MCT). Poisson's ratio and crustal thickness estimates obtained using H- k stacking technique and inversion of RFs are grossly similar lending credence to the robustness of inversions. An updated crustal thickness map produced using 1525 individual data points from controlled source seismics and RFs reveals a a) thickened crust (〉55 km) at the boundary of Dharwar Craton and Southern Granulite Terrain b) clear difference in crustal thickness estimates between Eastern Dharwar Craton and Western Dharwar Craton c) thinner crust beneath Cambay Basin between southwest Deccan Volcanic Province and Delhi Aravalli Fold Belt d) thinner crust (〈35 km) beneath Bengal Basin e) thicker crust (〉40 km) beneath paleo-rift zones like Narmada Son Lineament and Godavari Graben f) very thick crust beneath central Tibet (〉 65 km) with maximum lateral variations along the Himalayan collision front.

Description: The Lichi Mélange in the Coastal Range, eastern Taiwan, is considered as sheared forearc sequences. However, the age of these sequences is still uncertain, and their significance for the arc-continent collision has been overlooked. Based on field surveys and micropaleontological analysis, four independent biostratigraphic units ranging from 〈8.5 to 3.0 Ma are discerned in the Lichi Mélange. These units are obviously older than the remnant forearc sequences in the east (3.4–1.2 Ma), supporting the interpretation that the Lichi Mélange arose from the shearing of the lower forearc basin sequences. The stratigraphy also suggests that the older forearc strata (〈8.5–3.4 Ma) were deformed and uplifted as a bathymetric high similar to the Huatung Ridge in the western North Luzon Trough by arcward backthrusting before 3.4 Ma, while sedimentation continued in the remnant forearc basin in the east during 3.4–1.2 Ma. The older forearc strata possess vitrinite reflectance values even lower than those of the remnant forearc sequences, which also supports that they were uplifted by backthrusting and therefore did not experience significant burial. Neodymium isotope analysis shows that the 〈8.5–6.4 Ma forearc sediments were sourced from both the volcanic arc and the accretionary prism, indicating the emergence of the accretionary prism within 〈8.5–6.4 Ma. It might have resulted from the underplating of thinned continental crust and provides a good time constraint for the onset of arc-continent collision. The post-6.4-Ma forearc sediments were mainly derived from the accretionary prism, reflecting its constant uplift and large-scale exposure.

Description: The main purpose of this study is to construct a new 3D-model of the Central Asian Orogenic Belt (CAOB) crust, which can be used as a starting point for future lithospheric studies. The CAOB is a Paleozoic accretionary orogen surrounded by the Siberian Craton to the north and the North China and Tarim cratons to the south. This area is of great interest due to its enigmatic and still not completely understood geodynamic evolution. Firstly, we estimate an initial crustal thickness by inversion of the vertical gravity component of the GOCE and DTU10 models. Secondly, 3D forward modelling of the GOCE gravity gradients is performed, which determines the topography of the Moho, the geometry and the density distribution of the deeper parts of the CAOB and its surroundings, taking into account the lateral and vertical density variations of the crust. The model is constrained by seismic refraction, reflection and receiver function studies as well as geological studies. In addition, we discuss the isostatic implications of the differences between the seismic Moho and the resulting 3D gravity Moho, complemented by the analysis of the lithostatic load distribution at the upper mantle level. Finally, the correlation between the contrasting tectonic domains and the thickness of the crust reveals the inheritance of Paleozoic and Mesozoic geodynamics, particularly the magmatic provinces and the orocline which preserve their crustal features.

Description: The presence of oxidants such as hydrogen peroxide (H 2 O 2 ) and perchlorate (ClO 4 - ), which have been detected on Mars, has significant implications for chemistry and astrobiology. These oxidants can increase the reactivity of the martian soil, accelerate the decomposition of organic molecules, and depress the freezing point of water. The study by Crandall et al “Can Perchlorates be Transformed to Hydrogen Peroxide Products by Cosmic Rays on the Martian Surface” reveals a new formation mechanism by which hydrogen peroxide and other potential oxidants can be generated via irradiation of perchlorate by cosmic rays. This study represents an important next step in developing a full understanding of martian surface and subsurface chemistry, particularly with respect to degradation of organic molecules and potential biosignatures.

Description: We introduced a P-velocity model into the traditional joint inversion of P receiver function (RF) and surface wave dispersions to reduce model ambiguity. The method was implemented using a global search-based algorithm and a flexible parameterization of a sedimentary layer and spline-based parameterization that can represent sharp discontinuities. We applied the method to a dense array in SE Tibet (longitude ~97.5 °E to 107 °E, latitude ~25.3 °N). Extensive tests using synthetic and real data suggest that the method is suitable and robust for a variety of velocity structures and Moho discontinuities, and can simultaneously provide the crustal Vp/Vs profile and better-constrained Moho depth. The flexibility of the parameterization and the inclusion of the Vp constraint are crucial in the improved model recovery. Artifacts may be created without including the sedimentary layer. Even when it is less perfect, a reasonable Vp model is valuable in such a joint inversion. We showed that crustal multiples in RFs may bias the traditional H-k results when the crust structure is complex and should be avoided in a joint inversion before appropriate corrections can be made. The results from the joint inversion show two low-velocity zones (LVZs) reported previously and were identified as channels of crustal flow. A prominent isolated LVZ is observed in the mid-lower crust under the Xiaojiang fault area, which correlates with anomalously high Vp/Vs ratios, indicating possible partial melting. However, the other LVZ is imaged to be in the brittle shallow upper crust without very high Vp/Vs ratios, which is likely associated with crustal fault zones rather than partial melting. We observe clear low-velocity structures in the mantle beneath the two crustal LVZs, which also correlate with zones of low resistivity. The crust-mantle correlation may suggest influence of mantle processes on crustal deformation.

Description: Observations of a water vapor exosphere around Ceres suggest the dwarf planet may be episodically outgassing at a rate of ~6 kg s -1 from unknown sources. With data from the Dawn mission as constraints, we use a coupled thermal and vapor diffusion model to explore three different configurations of water ice (global buried pore-filling ice, global buried excess ice, and local exposed surface ice) that could be present on Ceres. We conclude that a buried ice table cannot alone explain the vapor production rates previously measured, but newly exposed surface ice, given the right conditions, can exceed that vapor production rate. Sublimation lag deposits form that bury and darken this surface ice over a large range of timescales (from 〈1 yr to ~100s of kyrs) that depend on latitude and ice regolith content. Sublimating water vapor can loft regolith particles from the surface of exposed ice, possibly prolonging the visible lifespan of those areas. We find this process is only effective for regolith grains smaller than ~1s of μm.

Description: O 2 , H 2 and H 2 O 2 radiolysis from water ice is pervasive on icy astrophysical bodies, but the lack of a self-consistent, quantitative model of the yields of these water products versus irradiation projectile species and energy has been an obstacle to estimating the radiolytic oxidant sources to the surfaces and exospheres of these objects. A major challenge is the wide variation of O 2 radiolysis yields between laboratory experiments, ranging over four orders of magnitude from 5×10 -7 to 5×10 -3 molecules per eV for different particles and energies. We revisit decades of laboratory data to solve this long standing puzzle, finding an inverse projectile range dependence in the O 2 yields, due to preferential O 2 formation from an ~30 Å thick oxygenated surface layer. Highly penetrating projectile ions and electrons with ranges ≳30 Å are therefore less efficient at producing O 2 than slow/heavy ions and low-energy electrons (≲ 400 eV) which deposit most energy near the surface. Unlike O 2 the H 2 O 2 yields from penetrating projectiles fall within a comparatively narrow range of (0.1-6)×10 -3 molecules per eV, and do not depend on range, suggesting H 2 O 2 forms deep in the ice uniformly along the projectile track, e.g. by reactions of OH radicals. We develop an analytical model for O 2 , H 2 and H 2 O 2 yields from pure water ice for electrons and singly-charged ions of any mass and energy, and apply the model to estimate possible O 2 source rates on several icy satellites. The yields are upper limits for icy bodies on which surface impurities may be present.

Description: Rock materials often display long-time relaxation, commonly termed aging or “slow dynamics”, after the cessation of acoustic perturbations. In this paper, we focus on unconsolidated rock materials and propose to explain such nonlinear relaxation through the Shear-Transformation-Zone (STZ) theory of granular media, adapted for small stresses and strains. The theory attributes the observed relaxation to the slow, irreversible change of positions of constituent grains, and posits that the aging process can be described in three stages: fast recovery before some characteristic time associated with the subset of local plastic events or grain rearrangements with a short time scale, log-linear recovery of the elastic modulus at intermediate times, and gradual turnover to equilibrium steady-state behavior at long times. We demonstrate good agreement with experiments on aging in granular materials such as simulated fault gouge after an external disturbance. These results may provide insights into observed modulus recovery after strong shaking in the near surface region of earthquake zones.

Description: The conventional “asperity model” posits that faults are partitioned into fixed velocity-weakening (VW) patches (asperities) that are locked interseismically and velocity-strengthening (VS) regions that creep stably without accumulating stress. However, studies of GPS-derived deformation in northern Japan have shown that interseismic strain in the Tohoku region did not accumulate at a constant rate (as expected), but gradually decreased from 1996 to 2011. This change in strain rate is consistent with locked asperities shrinking by ∼75% in area during this period. Here, we consider a modification to the conventional asperity model, such that thermal pressurization (TP) is active over an area that encompasses a VW region and part of the surrounding VS region. In our quasi-dynamic simulations, TP causes shear stress during rapid slip to decrease to very low levels. During the interseismic period, stress gradually recovers to steady-state friction at the plate rate, at which point stable creep initiates. The creep front propagates inward, effectively eroding the locked asperity. For uniform properties, the locked area shrinks roughly linearly in time through the VS region. Locked asperities shrink more slowly with higher nominal friction coefficient or background effective normal stress in the VS region, lower hydraulic diffusivity, as well as larger TP zones. Lateral heterogeneity in properties can give rise to nonlinear erosion. Predictions from this model can be compared against GPS data to test whether the model can explain the observed changes in interseismic strain rate in Tohoku.

Description: This study analyzes surface displacements generated by a low magnitude crustal earthquake in the Ecuadorian Andes by combining analysis of SAR Interferometry, geological field investigations and seismological data. In March 2010, a significant surface faulting event occurred in the Pisayambo area (Eastern cordillera), along the major dextral fault zone bounding the North Andean Sliver and the South-America Plate. Interferograms were inverted to determine fault plane geometry and slip displacement distribution. The event affected a 9 km-long previously unknown fault, referred as the Laguna Pisayambo Fault (LPF), with purely dextral movement reaching 45 cm and concentrated in the top 3 km of the crust. Geological investigations confirm both the fault mechanism and the amplitude of displacements. While these large displacements would be related to an event with a magnitude of 5.44 if using a standard crustal rigidity, we show that they can be convincingly associated with an Mw5.0 earthquake, that occurred on 2010/03/26. Reconciling the apparent differences in magnitude requires the existence of a low rigidity medium at shallow depths and/or postseismic activity of the fault. However, considering only the latter hypothesis would imply an unusually active postseismic process, in which  400-500% of the coseismic moment is released in the 6 days following the earthquake. Our observations highlight that the scaling laws relating surface observations to earthquake magnitude, classically used for seismic hazard assessment, should be carefully used. This study also illustrates how systematic InSAR analysis, even in places where no clues of ground deformation are present, can reveal tectonic processes.

Description: We investigate possible biasing effects of inaccurate timing corrections on teleseismic P-wave back-projection imaging of large earthquake ruptures. These errors occur because empirically-estimated time shifts based on aligning P-wave first arrivals are exact only at the hypocenter and provide approximate corrections for other parts of the rupture. Using the Japan subduction zone as a test region, we analyze 46 M6–7 earthquakes over a ten-year period, including many aftershocks of the 2011 M9 Tohoku earthquake, performing waveform cross-correlation of their initial P-wave arrivals to obtain hypocenter timing corrections to global seismic stations. We then compare back-projection images for each earthquake using its own timing corrections with those obtained using the time corrections from other earthquakes. This provides a measure of how well sub-events can be resolved with back-projection of a large rupture as a function of distance from the hypocenter. Our results show that back-projection is generally very robust and that the median sub-event location error is about 25 km across the entire study region (∼700 km). The back-projection coherence loss and location errors do not noticeably converge to zero even when the event pairs are very close (〈20 km). This indicates that most of the timing differences are due to 3D structure close to each of the hypocenter regions, which limits the effectiveness of attempts to refine back-projection images using aftershock calibration, at least in this region.

Description: The accuracy of Green's functions retrieved from seismic noise correlations in the microseism frequency band is limited by the uneven distribution of microseism sources at the surface of the Earth. As a result, correlation functions are often biased as compared to the expected Green's functions, and they can include spurious arrivals. These spurious arrivals are seismic arrivals that are visible on the correlation and do not belong to the theoretical impulse response. In this article, we propose to use Rayleigh wave spurious arrivals detected on correlation functions computed between European and United States seismic stations to locate microseism sources in the Atlantic Ocean. We perform a slant stack on a time-distance gather of correlations obtained from an array of stations that comprises a regional deployment and a distant station. The arrival times and the apparent slowness of the spurious arrivals lead to the location of their source, which is obtained through a grid search procedure. We discuss improvements in the location through this methodology as compared to classical back-projection of microseism energy. This method is interesting because it only requires an array and a distant station on each side of an ocean, conditions that can be met relatively easily.

Description: Passive seismic monitoring of microseismic events induced in oil or gas reservoirs is known as microseismic monitoring. Microseismic monitoring is used to understand the process of hydraulic fracturing, which is a reservoir stimulation technique. We use a new geomechanical model with bedding plane slippage induced by hydraulic fractures within shale reservoirs to explain seismicity observed in a typical case study of hydraulic fracturing of a shale gas play in North America. Microseismic events propagating from the injection point are located at similar depths (within the uncertainty of their locations), and their source mechanisms are dominated by shear failure with both dip-slip and strike-slip senses of motion. The prevailing dip-slip mechanisms have one nearly vertical nodal plane perpendicular to the minimum horizontal stress axis, while the other nodal plane is nearly horizontal. Such dip-slip mechanisms can be explained by slippage along bedding planes activated by the aseismic opening of vertical hydraulic fractures. The model explains the observed prevailing orientation of the shear planes of the microseismic events, as well as the large difference between seismic and hydraulic energy.

Description: Lithology distribution across the Moon is pivotal for understanding lunar evolution. However, so far, the distribution of lunar rock suites is still uncertain; as a result, many related core issues on lunar evolution have long been in dispute. This work reports on a new lithology distribution map across the Moon and discusses some critical issues of the lunar evolutionary process. The oxide abundances derived from Chang’E-1 Interference Imaging Spectrometer imagery and the Th contents inferred by Lunar Prospector Gamma Ray and Neutron Spectrometer data are employed to generate the lithological map by using the decision tree C5.0 algorithm. The following conclusions are inferred from this new map. (1) Magnesian suite is widely distributed across the Feldspathic Highlands Terrane and in the periphery of the South Pole-Aitken Terrane. Thus, the viewpoints have been validated that the early magnesian magmatism may be a global phenomenon and that KREEP basalt is not necessary for the petrogenesis of Mg-rich rocks. Moreover, the regions of Dryden, Chaffee S, Theophilus and Moscoviense are confirmed as magnesian suite exposures. (2) The observation that the Feldspathic Highlands Terrane has a higher Mg/(Mg+Fe) value than the maria is related to the flood of the Mg-suite across the Feldspathic Highlands Terrane. (3) The specific exposed locations of the alkali suite across the Moon has long been unsolved, and this work discloses that the alkali suite prevails in the outskirts of the Procellarum KREEP Terrane, the center of the South Pole-Aitken Terrane and some isolated locales. (4) Focusing on distinguishing mare basalts from other mafic rocks, 7 geochemical indices are proposed to determine the potential exposures of mare basalts across the Moon. (5) In Mare Insularum, a series of KREEP volcanisms may have occurred and lasted longer than the mare volcanism.

Description: The Bagnold dunes in Gale Crater, Mars are the first active aeolian dune field explored in situ on another planet. The Curiosity rover visited the Bagnold dune field to understand modern winds and modern aeolian processes, rates, and structures; to constrain dune material composition, provenance, and the extent and type of compositional sorting of materials; and to collect knowledge that informs the interpretation of past aeolian processes that are preserved in the Martian sedimentary rock record. The Curiosity rover conducted a coordinated campaign of activities lasting four months, interspersed with other rover activities, and employing all of the rover's science instruments and several engineering capabilities. Described in thirteen manuscripts and summarized here, the major findings of the Bagnold Dunes Campaign, phase I include: the characterization of and explanation for a distinctive, meter-scale size of sinuous aeolian bedform formed in the high kinetic viscosity regime of Mars’ thin atmosphere; articulation and evaluation of a grain splash model that successfully explains the occurrence of saltation even at wind speeds below the fluid threshold; determination of the dune sands’ basaltic mineralogy and crystal chemistry in comparison with other soils and sedimentary rocks; and characterization of chemically distinctive volatile reservoirs in sand-sized versus dust-sized fractions of Mars soil, including the importance of amorphous phase(s) as volatile carriers.

Description: Waveforms of SS precursors recorded by global stations are analyzed to investigate lateral heterogeneities of upper-mantle discontinuities on a global scale. A sporadic low-velocity layer immediately above the 410-km discontinuity (LVL-410) is observed worldwide, including East Asia, western North America, eastern South America, the Pacific Ocean, and possibly the Indian Ocean. Our best data coverage is for the Pacific Ocean, where the LVL-410 covers 33–50% of the resolved region. Lateral variations of our LVL-410 observations show no geographical correlation with 410-km discontinuity topography or tomographic models of seismic velocity, suggesting that the LVL-410 is not caused by regional thermal anomalies. We interpret the LVL-410 as partial melting due to dehydration of ascending mantle across the 410-km discontinuity, which is predicted by the transition zone water filter hypothesis. Given the low vertical resolution of SS precursors, it is possible that the regions without a clear LVL-410 detection also have a thin layer. Therefore, the strong lateral heterogeneity of the LVL-410 in our observations suggests partial melting with varying intensities across the Pacific, and further provides indirect evidence of a hydrous mantle transition zone with laterally varying water content.

Description: Methane hydrate-bearing sediments with different amounts of fines content and at three densities were artificially prepared under controlled temperature and pressure conditions. The void ratios of specimens after isotropic consolidation tend to decrease with a rise in fines content. The fines particles enter into the pore space between sand grains and densify the specimens. A series of triaxial compression tests were performed to systematically investigate the influences of fines content and density on the shear properties of hydrate-free sediments and methane hydrate-bearing sediments. The test results demonstrate that a rise in fines content within methane hydrate-bearing sediments significantly enhances peak shear strength and promotes dilation behavior. These influences are particularly prominent for specimens at loose packing state. A decrease in void ratio increases the shear strength and stiffness of hydrate-free sediments and methane hydrate-bearing sediments containing fines content of 0% and 8.9%. It is noted that the formation of methane hydrate in samples with varying amounts of fines content increases the stress ratios at the critical state. The addition of fines particles into coarse-grained sand grains alters the internal microstructure of sand matrix and the hydrate formation pattern in the pore space between sand grains and fines particles.

Description: We investigate the cycling of water (regassing, dehydration and degassing) in mantle convection simulations as a function of the strength of the oceanic lithosphere and its influence on the evolution of mantle water content. We also consider pseudo-plastic yielding with a friction coefficient for simulating brittle behavior of the plates and the water-weakening effect of mantle materials. This model can generate long-term plate-like behavior as a consequence of the water-weakening effect of mantle minerals. This finding indicates that water cycling plays an essential role in generating tectonic plates. In vigorous plate motion, the mantle water content rapidly increases by up to approximately 4–5 ocean masses, which we define as the ‘burst' effect. A ‘burst' is related to the mantle temperature and water solubility in the mantle transition zone. When the mantle is efficiently cooled down, the mantle transition zone can store water transported by the subducted slabs that can pass through the ‘choke-point' of water solubility. The onset of the ‘burst' effect is strongly dependent on the friction coefficient. The ‘burst' effect of the mantle water content could have significantly influenced the evolution of the surface water if the burst started early, in which case the Earth's surface cannot preserve the surface water over the age of the Earth.

Description: We compile and analyze a dataset of secondary microseismic P-wave spectra that were observed by North American seismic arrays. Two distinct frequency bands, 0.13–0.15Hz and 0.19–0.21Hz, with enhanced P-wave energy characterize the dataset. Cluster analysis allows to classify the spectra and to associate typical spectral shapes with geographical regions: Low frequency dominated spectra (0.13-0.15Hz) are mostly detected in shallower regions of the North Atlantic and the South Pacific, as well as along the Central and South American Pacific coast. High frequency dominated spectra (0.19-0.21Hz) are mostly detected in deeper regions of the North-Western Pacific and the South Pacific. For a selected subset of high quality sources, we compute synthetic spectra from an ocean wave hindcast. These synthetic spectra are able to reproduce amplitude and shape of the observed spectra, but only if P-wave resonance in the water column at the source site is included in the model. Our datasets therefore indicate that the spectral peaks at 0.13-0.15Hz and 0.19-0.21 Hz correspond to the first and second harmonics of P-wave resonance in the water column that occur in shallower ocean depths (〈3000m) and in the deep ocean (∼5000m), respectively. This article demonstrates the important effect of water column resonance on the amplitude and frequency of P-waves that are generated by secondary microseisms, and that the amplitude of high quality sources can be predicted from ocean wave hindcasts within a factor of 0.4 − 6.

Description: Sinus Aestuum is the only known location on the Moon where orbital data has detected Fe- and/or Cr-spinel. We analyzed Moon Mineralogy Mapper (M 3 ) visible to near-infrared data of the largest and strongest spinel signatures and determined these locations always correspond to impact craters. M 3 spectra show at least three types of spinels may be present, all of which exhibit a strong and broad absorption at ~2100 nm, and also one of the following: (1) a narrow 700-750 nm absorption; (2) a broad 600-900 nm absorption; or (3) both a weaker 700 nm and stronger 1000 nm absorption. All the spinel detections occur on either larger highland massifs that make up Sinus Aestuum east and west, or smaller highland kīpukas and buried highlands within the mare. Almost all of the spinel signatures occur within the mapped pyroclastic dark mantle deposit (DMD). The strong correlation between spinel and DMD distribution on the highlands at Sinus Aestuum is best explained if the spinels were emplaced during the same explosive eruption(s) that deposited the pyroclastics in the Sinus Aestuum DMD. Our observations are most consistent with models of melt-rock reactions in the anorthositic lunar crust that produce contaminated (high-Al) regions within a volcanic dike or magmatic reservoir that was capable of erupting pyroclastic glass beads containing pleonaste spinel [Mg,Fe]Al 2 O 4 . Over billions of years, this surface layer of spinels and pyroclastics became heterogeneously mixed into and partially buried within the highland regolith where younger impact craters may sometimes expose it.

Description: The Queen Charlotte plate boundary, near Haida Gwaii, B.C., includes the dextral, strike-slip, Queen Charlotte Fault (QCF) and the subduction interface between the downgoing Pacific and overriding North American plates. In this study, we present a comprehensive repeating earthquake catalogue that represents an effective slip meter for both structures. The catalogue comprises 712 individual earthquakes (0.3≤ M W ≤3.5) arranged into 224 repeating earthquake families on the basis of waveform similarity and source-separation estimates from coda wave interferometry. We employ and extend existing relationships for repeating earthquake magnitudes and slips to provide cumulative slip histories for the QCF and subduction interface in 6 adjacent zones within the study area between 52.3° N and 53.8° N. We find evidence for creep on both faults; however, creep rates are significantly less than plate motion rates, which suggests partial locking of both faults. The QCF exhibits the highest degrees of locking south of 52.8° N, which indicates that the seismic hazard for a major strike-slip earthquake is highest in the southern part of the study area. The October 28, 2012, M W 7.8 Haida Gwaii thrust earthquake occurred in our study area and altered the slip dynamics of the plate boundary. The QCF is observed to undergo accelerated, right-lateral slip for 1-2 months following the earthquake. The subduction interface exhibits afterslip thrust motion that persists for the duration of the study period ( i.e. , 3 years and 2 months after the Haida Gwaii earthquake). Afterslip is greatest (5.7 - 8.4 cm/yr) on the periphery of the main rupture zone of the Haida Gwaii event.

Description: The central section of the San Andreas fault hosts tectonic tremor and low-frequency earthquakes (LFEs) similar to subduction zone environments. LFEs are often interpreted as persistent regions that repeatedly fail during the aseismic shear of the surrounding fault allowing them to be used as creepmeters. We test this idea by using the recurrence intervals of individual LFEs within LFE families to estimate the timing, duration, recurrence interval, slip, and slip rate associated with inferred slow slip events. We formalize the definition of a creepmeter and determine whether this definition is consistent with our observations. We find that episodic families reflect surrounding creep over the interevent time while the continuous families and the short timescale bursts that occur as part of the episodic families do not. However, when these families are evaluated on timescales longer than the interevent time these events can also be used to meter slip. A straight-forward interpretation of episodic families is that they define sections of the fault where slip is distinctly episodic in well defined SSEs that slip 16 times the long term rate. In contrast, the frequent short-term bursts of the continuous and short-timescale episodic families likely do not represent individual creep events but rather are persistent asperities that are driven to failure by quasi-continuous creep on the surrounding fault. Finally we find that the moment-duration scaling of our inferred creep events are inconsistent with the proposed linear moment-duration scaling. However, caution must be exercised when attempting to determine scaling with incomplete knowledge of scale.

Description: Groundwater management typically relies on water level data and spatially limited deformation measurements. While InSAR has been used to study hydrological deformation, its limited temporal sampling can lead to biases in rapidly changing systems. Here we use 2011-2017 COSMO-SkyMed data with revisit intervals as short as one day to study the response of the Santa Clara Valley (SCV) aquifer in California to the unprecedented 2012-2015 drought. Cross correlation and independent component analyses of deformation time series enable tracking water though the aquifer system. The aquifer properties are derived prior to and during the drought to assess the success of water-resource management practices. Subsidence due to groundwater withdrawal dominates during 2011-2017, limited to the confined aquifer and west of the Silver Creek Fault, similar to pre-drought summer periods. Minimum water levels and elevations were reached in mid-2014, but thanks to intensive groundwater management efforts the basin started to rebound in summer 2015, during the deepening drought. By 2017, water levels were back to their pre-drought levels, while elevations had not yet fully rebounded due to the delayed poroelastic response of aquitards and their large elastic compressibility. As water levels did not reach a new low stand, the drought led to only elastic and recoverable changes in the SCV. The SCV lost 0.09 km 3 during the drought while seasonal variations amount to 0.02 km 3 . Analysis of surface loads due to water mass changes in the aquifer system suggests that groundwater drawdowns could influence the stress on nearby faults.

Description: The large scale stress patterns observed in intra-plate areas are often considered to result from far-field boundary forces that drive plate tectonics. However, no present-day deformation has been detected in the French Paris Basin, yet significant deviatoric stresses are measured in limestone formations observed above soft argillite layers encountered in this region. Further, the pore pressure measured in the argillite is larger than that measured in the surrounding permeable zones. These observations suggest a presently active source of stress in this sedimentary system. We propose that this stress field is not related to tectonics but only to gravity acting on the series of visco-elastic orthotropic geomaterials that fill up the basin. This visco-elastic response is linked to pressure solution effects activated by pore pressure transients related to climatic variations. These pressure transients develop in the fracture system that affects some of the geomaterials and imply a time dependant deformation field, with time constants related to those of climatic variations. This model outlines the influence of time-dependent material properties on the present day stress and pore pressure fields that prevail at various depths. It may be considered as a possible loading mechanism for the analysis of intra-plate seismicity.

Description: The India-Eurasia collision zone is the largest deforming region on the planet; direct measurements of present-day deformation from Global Positioning System (GPS) have the potential to discriminate between competing models of continental tectonics. But the increasing spatial resolution and accuracy of observations have only led to increasingly complex realizations of competing models. Here we present the most complete, accurate and up-to-date velocity field for India-Eurasia available, comprising 2576 velocities measured during 1991-2015. The core of our velocity field is from the Crustal Movement Observation Network of China (CMONOC-I/II): 27 continuous stations observed since 1999; 56 campaign stations observed annually during 1998-2007; 1000 campaign stations observed in 1999, 2001, 2004, 2007; 260 continuous stations operating since late 2010; 2000 campaign stations observed in 2009, 2011, 2013, 2015. We process these data and combine the solutions in a consistent reference frame with stations from the Global Strain Rate Model compilation, then invert for continuous velocity and strain-rate fields. We update geodetic slip rates for the major faults (some vary along strike), and find those along the major Tibetan strike-slip faults are in good agreement with recent geological estimates. The velocity field shows several large undeforming areas, strain focused around some major faults, areas of diffuse strain, and dilation of the high plateau. We suggest that a new generation of dynamic models incorporating strength variations and strain weakening mechanisms is required to explain the key observations. Seismic hazard in much of the region is elevated, not just near the major faults.

Description: Fluid-filled fractures and fissures often determine the pathways and volume of fluid movement. They are critically important in crustal seismology and in the exploration of geothermal and hydrocarbon reservoirs. We introduce a model for tube-wave scattering and generation at dipping, parallel-wall fractures intersecting a fluid-filled borehole. A new equation reveals the interaction of tube wavefield with multiple, closely-spaced fractures, showing that the fracture dip significantly affects the tube waves. Numerical modeling demonstrates the possibility of imaging these fractures using a focusing analysis. The focused traces correspond well with the known fracture density, aperture and dip angles. Testing the method on a VSP dataset obtained at a fault-damaged zone in the Median Tectonic Line, Japan presents evidences of tube waves being generated and scattered at open fractures and thin cataclasite layers. This finding leads to a new possibility for imaging, characterizing and monitoring in-situ hydraulic properties of dipping fractures using the tube wavefield.

Description: While deep moonquakes are seismic events commonly observed on the Moon, their source mechanism is still unexplained. The two main issues are poorly constrained source parameters and incompatibilities between the thermal profiles suggested by many studies and the apparent need for brittle properties at these depths. In this study, we reinvestigated the deep moonquake data to re-estimate its source parameters and uncover the characteristics of deep moonquake faults that differ from those on Earth. We first improve the estimation of source parameters through spectral analysis using “new” broadband seismic records made by combining those of the Apollo long- and short-period seismometers. We use the broader frequency band of the combined spectra to estimate corner frequencies and DC values of spectra, which are important parameters to constrain the source parameters. We further use the spectral features to estimate seismic moments and stress drops for more than 100 deep moonquake events from three different source regions. This study revealed that deep moonquake faults are extremely smooth compared to terrestrial faults. Second, we re-evaluate the brittle-ductile transition temperature that is consistent with the obtained source parameters. We show that the source parameters imply the tidal stress is the main source of the stress glut causing deep moonquakes and the large strain rate from tides makes the brittle-ductile transition temperature higher. Higher transition temperatures open a new possibility to construct a thermal model that is consistent with deep moonquake occurrence and pressure condition and thereby improve our understandings of the deep moonquake source mechanism.

Description: We present new viscosity measurements of a synthetic silicate system considered an analogue for the lava erupted on the surface of Mercury. In particular, we focus on the northern volcanic plains (NVP), which correspond to the largest lava flows on Mercury and possibly in the Solar System. High-temperature viscosity measurements were performed at both superliquidus (up to 1736 K) and subliquidus conditions (1569–1502 K) to constrain the viscosity variations as a function of crystallinity (from 0 to 28%) and shear rate (from 0.1 to 5 s -1 ). Melt viscosity shows moderate variations (4 –16 Pa s) in the temperature range 1736–1600 K. Experiments performed below the liquidus temperature show an increase in viscosity as shear rate increases from 0.1 to 5 s -1 , resulting in a shear thinning behaviour, with a decrease in viscosity of ca. 1 log unit. The low viscosity of the studied composition may explain the ability of NVP lavas to cover long distances, on the order of hundreds of kilometres in a turbulent flow regime. Using our experimental data we estimate that lava flows with thickness of 1, 5 and 10 m are likely to have velocities of 4.8, 6.5 and 7.2 m/s respectively, on a 5° ground slope. Numerical modelling incorporating both the heat loss of the lavas and its possible crystallization during emplacement allows us to infer that high effusion rates (〉 10000 m 3 /s) are necessary to cover the large distances indicated by satellite data from the MESSENGER spacecraft.

Description: Chloride-bearing deposits on Mars record high-elevation lakes during the waning stages of Mars’ wet era (mid-Noachian to late Hesperian). The water source pathways, seasonality, salinity, depth, lifetime, and paleoclimatic drivers of these widespread lakes are all unknown. Here we combine reaction-transport modeling, orbital spectroscopy, and new volume estimates from high-resolution digital terrain models, in order to constrain the hydrologic boundary conditions for forming the chlorides. Considering a T = 0 °C system, we find: (1) individual lakes were 〉100 m deep and lasted decades or longer; (2) if volcanic degassing was the source of chlorine, then the water-to-rock ratio or the total water volume were probably low, consistent with brief excursions above the melting point and/or arid climate; (3) if the chlorine source was igneous chlorapatite, then Cl-leaching events would require a (cumulative) time of 〉10 yr at the melting point; (4) Cl masses, divided by catchment area, give column densities 0.1 – 50 kg Cl/m 2 , and these column densities bracket the expected chlorapatite-Cl content for a seasonally-warm active layer. Deep groundwater was not required. Taken together, our results are consistent with Mars having a usually cold, horizontally segregated hydrosphere by the time chlorides formed.

Description: No abstract is available for this article.

Description: Similar to other subduction zones, tectonic tremors (TT) and slow slip events (SSE) take place in the deep segment of the plate interface in Guerrero, Mexico. However, their spatial correlation in this region is not as clear as the Episodic Tremor and Slip observed in Cascadia and Japan. In this study we provide insights into the causal relationship between TTs and SSEs in Guerrero by analyzing the evolution of the deformation fields induced by the long-term 2006 SSE together with new locations of TTs and low-frequency earthquakes (LFE). Unlike previous studies we find that the SSE slip rate modulates the TT and LFE activity in the whole tremor region. This means that the causal relationship between the SSE and the TT activity directly depends on the stressing rate history of the tremor asperities that is modulated by the surrounding slip rate. We estimated that the frictional strength of the asperities producing tremor downdip in the sweet spot is around 3.2 kPa, which is ~2.3 times smaller than the corresponding value updip in the transient zone, partly explaining the overwhelming tremor activity of the sweet spot despite that the slow slip there is smaller. Based on the LFE occurrence-rate history during the inter-long-term SSE period we determined that the short-term SSEs in Guerrero take place further downdip (about 35 km) than previously estimated, with maximum slip of about 8 mm in the sweet spot. This new model features a continuum of slow slip extending across the entire tremor region of Guerrero.

Description: We studied the elastic properties of a fault zone intersecting the Opalinus Clay formation at 300m depth in the Mont Terri Underground Research Laboratory (Switzerland). Four controlled water injection experiments were performed in borehole straddle intervals set at successive locations across the fault zone. A three-component displacement sensor, which allowed capturing the borehole wall movements during injection, was used to estimate the elastic properties of representative locations across the fault zone, from the host rock to the damage zone to the fault core. Young's moduli were estimated by both an analytical approach and numerical finite difference modeling. Results show a decrease in Young's modulus from the host rock to the damage zone by a factor of 5 and from the damage zone to the fault core by a factor of 2. In the host rock, our results are in reasonable agreement with laboratory data showing a strong elastic anisotropy characterized by the direction of the plane of isotropy parallel to the laminar structure of the shale formation. In the fault zone, strong rotations of the direction of anisotropy can be observed. The plane of isotropy can be oriented either parallel to bedding (when few discontinuities are present), parallel to the direction of the main fracture family intersecting the zone, and possibly oriented parallel or perpendicular to the fractures critically oriented for shear reactivation (when repeated past rupture along these plane has created a zone of weakness).

Description: We performed a systematic investigation of mechanical compaction, strain localization and permeability in Leitha limestone. This carbonate from the area of Vienna (Austria) occurs with a broad range of grain sizes and porosity, due to changes in depositional regime and degree of cementation. Our new mechanical data revealed a simple relation between porosity and mechanical strength both in the brittle and ductile regimes. Increasing cementation and decreasing porosity led to a significant increase of the rock strength in both regimes. Micromechanical modelling showed that the dominant micromechanisms of inelastic deformation in Leitha limestone are pore-emanated microcracking in the brittle regime, and grain crushing and cataclastic pore collapse in the ductile regime. Microstructural analysis and X-ray computed tomography revealed the development of compaction bands in some of the less cemented samples, while more cemented end-members failed by cataclastic flow in the compactant regime. In contrast to mechanical strength, permeability of Leitha limestone was not significantly impacted by increasing cementation and decreasing porosity. Our microstructural and tomography data showed that this was essentially due to the existence of a backbone of connected large macropores in all our samples, which also explained the relatively high permeability (in the range of 2-5 Darcy) of Leitha limestone in comparison to other carbonates with significant proportion of micropores.

Description: The India-Asia collision resulted in the Cenozoic framework of faults, ranges, and tectonic basins and the high topography of the northeastern Tibetan Plateau, but how and when these features formed remains poorly understood, leading to conflicting tectonic models. However, information on the tectonic evolution of these active orogenic belts is well preserved in synorogenic basin sediments. In this study, we carefully analyze the detrital apatite fission-track (AFT) ages of Cenozoic synorogenic sediments from the Jiuquan Basin to decipher the entire exhumation process of the adjacent Qilian Shan throughout the Cenozoic. Our data indicate that initially rapid Cenozoic exhumation occurred in the Qilian Shan during the late Paleocene-early Eocene (~60-50 Ma), almost synchronous with the India-Asia collision. The Qilian Shan subsequently experienced long-lived exhumation that continued until at least the middle Miocene (~45-10 Ma). During this period of exhumation in the Qilian Shan, tectonic deformation occurred throughout the northeastern Tibetan Plateau. The early Cenozoic deformation in the northeastern Tibetan Plateau may have been caused by the transfer of tectonic stress from the distant India-Asia collision boundary through the complex lithospheric environment of the Tibetan Plateau. The present tectonic configuration and topography of the Qilian Shan and the northeastern Tibetan Plateau likely became established since the middle Miocene and after the long-lived deformation began in the early Cenozoic.

Description: Polycrystalline ice weakens significantly after a few percent strain, during high homologous temperature deformation. Weakening is correlated broadly with the development of a crystallographic preferred orientation (CPO). We deformed synthetic polycrystalline ice at −5 ∘ C under uniaxial compression, whilst measuring ultrasonic p-wave velocities along several ray paths through the sample. Changes in measured p-wave velocities ( V p ) and in the velocities calculated from microstructural measurements of CPO (by cryo-electron backscatter diffraction), both show that velocities along trajectories parallel and perpendicular to shortening decrease with increasing strain, whilst velocities on diagonal trajectories increase. Thus, in these experiments, velocity data provide a continuous measurement of CPO evolution in creeping ice. Samples reach peak stresses after 1% shortening. Weakening corresponds to the start of CPO development, as indicated by divergence of p-wave velocity changes for different ray paths, and initiates at ≈3% shortening. Selective growth by strain induced grain boundary migration (GBM) of grains favourably oriented for basal-slip may initiate weakening through the formation of an interconnected network of these grains by 3% shortening. After weakening initiates, CPO continues to develop by GBM and nucleation processes. The resultant CPO has an open cone (small circle) configuration, with the cone axis parallel to shortening. The development of this CPO causes significant weakening under uniaxial compression, where the shear stresses resolved on the basal planes (Schmid factors) are high.

Description: The basin-filling materials of the northern lowlands, which cover ~1/3 of Mars’ surface, record the long-term evolution of Mars' geology and climate. The buried stratigraphy was inferred through analyses of impact crater mineralogy, detected using data acquired by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). Examining 1045 impact craters across the northern lowlands, we find widespread olivine and pyroxene and diverse hydrated/hydroxylated minerals, including Fe/Mg smectite, chlorite, prehnite, and hydrated silica. The distribution of mafic minerals is consistent with infilling volcanic materials across the entire lowlands (~1–4⋅10 7 km 3 ), indicating a significant volume of volatile release by volcanic outgassing. Hydrated/hydroxylated minerals are detected more frequently in large craters, consistent with the scenario that the hydrated minerals are being excavated from deep basement rocks, beneath 1-2 km thick mafic lava flows or volcaniclastic materials. The prevalences of different types of hydrated minerals are similar to statistics from the southern highlands. No evidence of concentrated salt deposits has been found, which would indicate a long-lived global ocean. We also find significant geographical variations of local mineralogy and stratigraphy in different basins (geological provinces), independent of dust cover. For example, many hydrated and mafic minerals are newly discovered within the polar Scandia region (〉 60°N), and Chryse Planitia has more mafic mineral detections than other basins, possibly due to a previously unrecognized volcanic source.

Description: A regional approach using Poisson wavelets is applied for gravity field recovery using the GOCE (Gravity Field and Steady-State Ocean Circulation Explorer) gravity gradient tensor, heterogeneous gravimetry data, and altimetry measurements. The added value to the regional model introduced by GOCE data is validated and quantified. The performances of the solutions modeled with different diagonal components of GOCE data and their combinations are investigated. Numerical experiments in a region in Europe show that the effects introduced by GOCE data demonstrate long-wavelength patterns on the centimeter scale in terms of quasi-geoid heights, which may allow reducing the remaining long-wavelength errors in ground-based data, and improve the regional model. The accuracy of the gravimetric quasi-geoid computed with a combination of three diagonal components is improved by 0.6 cm (0.5 cm) in the Netherlands (Belgium) compared to that derived from gravimetry and altimetry data alone, when GOCO05s is used as the reference model. Moreover, the added value from GOCE data reduces the mean values of the misfit between the gravimetric solution and GPS/leveling data. Performances of different components and their combinations are not identical, and the solution with vertical gradients is best when a single component is used. The incorporation of multiple components shows further improvements, and the combination of three components best fits the local GPS/leveling data. Further comparison shows our solution is the highest quality, and may be substituted for existing models for engineering purposes and geophysical investigations over the target area.

Description: Field, geochemical, and geochronological data show that the southern segment of the ancestral Cascades arc advanced into the Oregon back-arc region from 30 to 20 Ma. We attribute this event to thermal uplift of the Farallon slab by the Yellowstone mantle plume, with heat diffusion, decompression, and the release of volatiles promoting high-K calc-alkaline volcanism throughout the back-arc region. The greatest degree of heating is expressed at the surface by a broad ENE-trending zone of adakites and related rocks generated by melting of oceanic crust from the Farallon slab. A hiatus in eruptive activity began at ca. 22-20 Ma, but ended abruptly at 16.7 Ma with renewed volcanism from slab rupture occurring in two separate regions. The eastern rupture resulted in the extrusion of Steens Basalt during the ascent and melting of a dry mantle (plume) source contaminated with depleted mantle. The contemporaneous western rupture resulted in renewed subduction, melting of a wet mantle source, and the rejuvenation of high-K calc-alkaline volcanism near the Nevada-California border at 16.7 Ma. Here, the initiation of slab rollback is evident in the westward migration of arc volcanism at 7.8 km/Ma. Today, the uplifted slab is largely missing beneath the Oregon back-arc region, replaced instead by a seismic hole which is bound on the south by the adakite hotspot track. We attribute slab destruction to thermal uplift and mechanical dislocation that culminated in rapid tearing of the slab from 17-15 Ma, and possible foundering and sinking of slab segments from 16-10 Ma.

Description: Unconsolidated granular earth materials exhibit softening behavior due to external perturbations such as seismic waves; namely, the wave speed and elastic modulus decrease upon increasing the strain amplitude above dynamics strains of about 10 −6 under near-surface conditions. In this letter, we describe a theoretical model for such behavior. The model is based on the idea that shear transformation zones (STZs) – clusters of grains that are loose and susceptible to contact changes, particle displacement, and rearrangement – are responsible for plastic deformation and softening of the material. We apply the theory to experiments on simulated fault gouge composed of glass beads, and demonstrate that the theory predicts nonlinear resonance shifts, reduction of the P-wave modulus, and attenuation, in agreement with experiments. The theory thus offers insights on the nature of nonlinear elastic properties of a granular medium, and potentially into phenomena such as triggering on earthquake faults.

Description: We numerically compute seismoelectric wavefields generated at a fluid/porous medium interface by an explosive source in the fluid. Our numerical experiments show that electromagnetic (EM) signals accompanying the P, S and interface waves can be observed at receivers located in the fluid regions near the interface. Such accompanying EM signals are produced by the inhomogeneous EM waves that are generated by the seismic waves at the interface and their amplitudes decrease with the distance from interface. Under the excitation of an explosive source whose strength is within the capability of industry air-guns, electric and magnetic fields that accompany the Scholte wave are on the order of 1 μV/m and 0.01 nT, respectively. This means that the EM signals arising from the electrokinetic effect at an ocean bottom are detectable and suggests that it is possible to measure the EM signals during marine seismic explorations to study the properties of the seafloor material. EM signals that accompany the P, S and interface waves are also observed in the porous medium region near the interface. Component analysis shows that they contain contributions from multiple modes of waves, among which the slow compressional wave contributes significantly to the vertical electric field, leading to a much stronger vertical electric field than the horizontal electric field during the passage of a seismic wave along the interface.

Description: The knowledge of the permeability of porous media is crucial to understand fluid flow in various natural and artificial materials. Due to the complex nature of pore structure, the pore characteristic (porosity, pore radius) determining the permeability has long been under discussion. Here, we determined the critical pore radius, which is the radius of the largest sphere that can freely pass through a porous medium, using the water-expulsion method, an experimental technique measuring the pressure at which gas passes through a water-saturated porous medium. We demonstrate that the critical pore radius correlates well with the permeability for a variety of porous granular media and volcanic products with an extensive range of porosities (0.71%–50%) and permeabilities (10 −20 –10 −10 m 2 ). We also obtained a porosity–critical pore radius–permeability relationship that provides a better prediction of the permeability compared with predictions obtained by previous correlations.

Description: We present a new 3D lithospheric Vs model for the NE Tibetan Plateau (NETP) and the western North China Craton (NCC). First, high frequency receiver functions (RFs) were inverted using the Neighborhood algorithm to estimate the sedimentary structure beneath each station. Then a 3D Vs model with unprecedented resolution was constructed by jointly inverting RFs and Rayleigh wave dispersions. A low velocity sedimentary layer with thicknesses varying from 2 to 10 km is present in the Yinchuan–Hetao graben, Ordos block, and western Alxa block. Velocities from the mid-lower crust to the uppermost mantle are generally high in the Ordos block and low in the Alxa block, indicating that the Alxa block is not part of the NCC. The thickened crust in southwestern Ordos block and western Alxa block suggests that they have been modified. Two crustal low-velocity zones (LVZs) were detected beneath the Kunlun Fault (KF) zone and western Qilian Terrane (QLT). The origin of the LVZ beneath the KF zone may be the combined effect of shear heating, localized asthenosphere upwelling and crustal radioactivity. The LVZ in the western QLT, representing an early stage of the LVZ that has developed in the KF zone, acts as a decollement to decouple the deformation between the upper and lower crust and plays a key role in seismogenesis. We propose that the crustal deformation beneath the NETP is accommodated by a combination of shear motion, thickening of the upper-middle crust, and removal of lower crust.

Description: We investigate the stress sensitivity of velocity changes at Izu-Oshima in Japan using seismic interferometry method. We calculate cross correlation functions (CCFs) of ambient noises recorded by four seismic stations on the active volcano from 1 January 2012 to 31 December 2015 at 0.5 – 1 Hz, 1 – 2 Hz, and 2 – 4 Hz. Applying moving time windows to calculate cross spectrum between daily CCFs and reference CCF, which is the average for the all observation period, we compute daily velocity changes. The obtained velocity changes vary from -1% to 3% with a dominant period of about 1 year. We calculate areal strains by using GNSS data at the volcano. It is evident that the velocity changes are well correlated with the areal strain changes whose linear trend is removed. The stress sensitivity of velocity changes, which are obtained from the observed velocity changes and areal strains, is lower at 0.5 – 1 Hz than that at higher frequencies: (7.1±1.3)×10 -8 Pa -1 at 0.5 – 1 Hz, (1.4±0.1)×10 -7 Pa -1 at 1 – 2 Hz, and (1.3±0.1)×10 -7 Pa -1 at 2 – 4 Hz. Modeling the velocity changes of Rayleigh-wave propagating in layered structures, we find the observed velocity changes are concentrated in the upper 1 km of the structure. Since similar frequency dependency is recognized among the stress sensitivity of velocity changes reported in previous studies, we conclude that the velocity changes are localized in the shallow depth because of the increase of confining pressure.

Description: In July 2013, a sequence of more than 340 earthquakes was induced by reservoir stimulations and well-control procedures following a gas kick at a deep geothermal drilling project close to the city of St. Gallen, Switzerland. The sequence culminated in an M L 3.5 earthquake, which was felt within 10-15 km from the epicenter. High-quality earthquake locations and 3D reflection seismic data acquired in the St. Gallen project provide a unique dataset, which allows high-resolution studies of earthquake triggering related to the injection of fluids into macroscopic fault zones. In this study, we present a high-precision earthquake catalog of the induced sequence. Absolute locations are constrained by a coupled hypocenter-velocity inversion, and subsequent double-difference relocations image the geometry of the M L 3.5 rupture and resolve the spatio-temporal evolution of seismicity. A joint interpretation of earthquake and seismic data shows that the majority of the seismicity occurred in the pre-Mesozoic basement, hundreds of meters below the borehole and the targeted Mesozoic sequence. We propose a hydraulic connectivity between the reactivated fault and the borehole, likely through faults mapped by seismic data. Despite the excellent quality of the seismic data, the association of seismicity with mapped faults remains ambiguous. In summary, our results document that the actual hydraulic properties of a fault system and hydraulic connections between its fault segments are complex and may not be predictable upfront. Incomplete knowledge of fault structures and stress heterogeneities within highly complex fault systems additionally challenge the degree of predictability of induced seismicity related to underground fluid injections.

Description: Hydrothermal experiments were performed at 311 °C and 3.0 kbar on natural olivine and peridotite to investigate the kinetics of serpentinization. The results show that the rates of reaction strongly depend on grain sizes of solid reactants, with smaller grain sizes resulting in faster kinetics. After 27 days of reaction, the reaction extent was 99% for peridotite with grain sizes of 〈30 μm, and the reaction extent was 28% for grain sizes of 100-177 μm. Compared to peridotite, olivine is serpentinized at much slower rates, e.g., 5.3% of reaction extent was achieved for olivine with grain sizes of 100-177 μm after 27 days, approximately five times lower than that reached during peridotite serpentinization. Such contrasting results are due to the presence of pyroxene and spinel, an interpretation which is supported by a marked increase in reaction extents for experiments with the addition of pyroxene and spinel. The reaction extent achieved in experiments with 3 wt% spinel greatly increased to 98% after 27 days, much higher than that achieved during olivine serpentinization. These results appear to be related to pyroxene and spinel releasing Al and Cr during serpentinization. As indicated by compositions of serpentine, orthopyroxene lost ~60% of Al at a reaction extent of 59%. Influence of Al and Cr is suggested by a dramatic increase in reaction extents with the addition of Al 2 O 3 and Cr 2 O 3 powders. Olivine in natural geological settings is commonly associated with pyroxene and spinel; consequently, serpentinization kinetics may be much faster than previously thought.

Description: We present and discuss JTRF2014, the Terrestrial Reference Frame (TRF) the Jet Propulsion Laboratory constructed by combining space-geodetic inputs from Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), Global Navigation Satellite Systems (GNSS) and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) submitted for the realization of ITRF2014. Determined through a Kalman filter and Rauch-Tung-Striebel smoother assimilating position observations, Earth Orientation Parameters (EOPs), and local ties, JTRF2014 is a sub-secular, time series-based TRF whose origin is at the quasi-instantaneous Center of Mass (CM) as sensed by SLR and whose scale is determined by the quasi-instantaneous VLBI and SLR scales. The dynamical evolution of the positions accounts for a secular motion term, annual, and semi-annual periodic modes. Site-dependent variances based on the analysis of loading displacements induced by mass redistributions of terrestrial fluids have been used to control the extent of random walk adopted in the combination. With differences in the amplitude of the annual signal within the range 0.5-0.8 mm, JTRF2014-derived Center of Network-to-Center of Mass (CM-CN) is in remarkable agreement with the geocenter motion obtained via spectral inversion of GNSS, Gravity Recovery and Climate Experiment (GRACE) observations and modeled Ocean Bottom Pressure (OBP) from Estimating the Circulation and Climate of the Ocean (ECCO). Comparisons of JTRF2014 to ITRF2014 suggest high-level consistency with time derivatives of the Helmert transformation parameters connecting the two frames below 0.18 mm/yr and WRMS differences of the polar motion (polar motion rate) in the order of 30  μ as (17  μ as/d).

Description: A continuous active source seismic monitoring (CASSM) dataset was collected with crosswell geometry during CO 2 injection at the Frio-II brine pilot, near Liberty, TX. Previous studies have shown that spatio-temporal changes in the P-wave first arrival time reveal the movement of the injected CO 2 plume in the storage zone. To further constrain the CO 2 saturation, particularly at higher saturation levels, we investigate spatial-temporal changes in the seismic attenuation of the first arrivals. The attenuation changes over the injection period are estimated by the amount of the centroid frequency shift computed by local time-frequency analysis. We observe that: (1) at receivers above the injection zone seismic attenuation does not change in a physical trend; (2) at receivers in the injection zone attenuation sharply increases following injection and peaks at specific points varying with distributed receivers, which is consistent with observations from time delays of first arrivals; then, (3) attenuation decreases over the injection time. The attenuation change exhibits a bell-shaped pattern during CO 2 injection. Under Frio-II field reservoir conditions, White's patchy saturation model can quantitatively explain both the P-wave velocity and attenuation response observed. We have combined the velocity and attenuation change data in a cross-plot format that is useful for model-data comparison and determining patch size. Our analysis suggests that spatial-temporal attenuation change is not only an indicator of the movement and saturation of CO 2 plumes, even at large saturations, but also can quantitatively constrain CO 2 plume saturation when used jointly with seismic velocity.

Description: Crustal thickness and Poisson's ratio are important parameters to characterize regional isostasy state and material composition or state. Using the teleseismic waveform data from 141 permanent stations and 785 temporary stations in southwest China, we obtain the crustal thickness and average Poisson's ratio by the H-κ stacking of receiver functions. In the west (the SE Tibetan plateau and the Yunnan-Burma-Thailand block) and southeast (the Cathaysian block and southern Yangtze craton) of the study region, there are high correlation coefficients for the crustal thicknesses between what we obtain from the receiver functions and what we calculate from the Airy isostasy model, indicating that a state of isostasy can be achieved at the crust-mantle boundary beneath these two regions. In the northeast (northern Yangtze craton), the correlation coefficient is lower, indicating that the effect of the lithosphere needs to be considered for a regional isostasy. Intermediate Poisson's ratios (0.26≤σ≤0.28) are found beneath the northern Panzhihua-Emeishan region. Combing the high velocity features from previous study, we speculate that it may be related to the Emeishan large igneous province. High Poisson's ratios (σ〉0.28)- are found beneath the SE Tibetan plateau and the nearby strike-slip faults, such as the Anninghe-Zemuhe fault and the northern Xiaojiang fault. Combing the low velocity zones from previous study, we speculate that there may be partially melted and lower crustal flow.

Description: Characterizing scattering and absorbing properties and the power spectrum of crustal heterogeneity is a fundamental problem for informing strong ground motion estimates at high frequencies, where scattering and attenuation effects are critical. We perform a comprehensive study of local earthquake coda waves in southern California to constrain scattering and intrinsic attenuation structure. We analyze data from 1195 spatially distributed earthquakes from 1981–2013 at source depths of 10 to 15 km and epicentral distances from 0–250 km with magnitudes larger than 1.8. We stack envelope functions from 28,127 vertical-component and 27,521 transverse-component seismograms, filtered from 2 to 4 Hz. We model these observations using a particle-based Monte Carlo algorithm that includes intrinsic attenuation as well as both P- and S-wave scattering and both single and multiple scattering events. We find that spatially averaged coda-wave behavior for southern California can be explained only with models containing an increase in scattering strength and intrinsic attenuation within the uppermost crust, i.e., they are poorly fit with half-space models of constant scattering strength. A reasonable fit to our data is obtained with a two-layer model, composed of a shallow crustal layer with strong wide-angle scattering and high P and S intrinsic attenuation and a deeper layer with weaker scattering and lower intrinsic attenuation (top 5.5 km: α Q I = 250, β Q I = 125, heterogeneity correlation length a = 50 m, fractional velocity heterogeneity ε = 0.4; lower crust: α Q I = 900, β Q I = 400, a = 2 km, ε = 0.05).

Description: Geochemical models of secondary mineral precipitation on Mars generally assume semi-open systems (open to the atmosphere but closed at the water-sediment interface) and equilibrium conditions. However, in natural multicomponent systems, the reactive surface area of primary minerals controls the dissolution rate and affects the precipitation sequences of secondary phases; and simultaneously the transport of dissolved species may occur through the atmosphere-water and water-sediment interfaces. Here we present a suite of geochemical models designed to analyze the formation of secondary minerals in basaltic sediments on Mars, evaluating the role of (i) reactive surface areas and (ii) the transport of ions through a basalt sediment column. We consider fully open conditions, both to the atmosphere and to the sediment, and a kinetic approach for mineral dissolution and precipitation. Our models consider a geochemical scenario constituted by a basin (i.e., a shallow lake) where supersaturation is generated by evaporation/cooling, and the starting point is a solution in equilibrium with basaltic sediments. Our results show that cation removal by diffusion, along with the input of atmospheric volatiles and the influence of the reactive surface area of primary minerals, play a central role in the evolution of the secondary mineral sequences formed. We conclude that precipitation of evaporites finds more restrictions in basaltic sediments of small grain size than in basaltic sediments of greater grain size.

Description: Quasi-static electromagnetic (EM) approximation was frequently used in numerical modeling of the seismoelectric wavefields, but the computational error it brings is unclear. In this study, we investigate the error caused by the quasi-static EM approximation based on a horizontally layered model. With such an approximation we obtain a simplified set of Pride's equations and present an analytically-based algorithm to solve the seismoelectric responses to an explosive source. First, we solve the seismic wavefields by ignoring the influence of the converted EM fields on the propagation of the seismic waves. Second, we simplify Maxwell equations to a Poisson equation of the electric potential, from which the EM signals are solved. The solved EM signals are compared with the solutions solved from the full Pride's equations to investigate the error caused by the quasi-static EM approximation. The result shows that the quasi-static EM approximation causes the loss of the electric field accompanying the S waves and yields errors in modeling the coseismic magnetic fields accompanying the S waves. The errors tend to become smaller when increasing the frequency and decreasing the salinity, implying that the quasi-static EM approximation seems to be more suitable for simulating the coseismic EM signals under high-frequency and low-salinity conditions. The quasi-static EM approximation also affects the simulation of the interfacial EM waves. Only under the condition that the wavelength of the EM wave is much larger than the source-receiver distance, the quasi-static method is valid in simulating the EM wave.

Description: We explore the impact of phyllosilicate (weak but velocity-strengthening) in a majority tectosilicate (strong but velocity-weakening) matrix in bulk shear strength and slip stability of faults. Numerical simple-shear experiments using a Distinct Element Model (DEM) are conducted on both uniform mixtures of quartz and talc analogs and on textured mixtures consisting of a talc layer embedded in a quartz matrix. The mechanical response of particles is represented by a linear-elastic contact model with a slip weakening constitutive relation representing the essence of rate-state friction. The weight percentage of the talc in the uniform mixtures and the relative thickness of the talc layer in the textured mixtures are varied to investigate the transitional behavior of shear strength and slip stability. Specifically, for uniform mixtures, ~50% reduction on bulk shear strength is observed with 25% talc present, and a dominant influence of talc occurs at 50%; for textured mixtures, a noticeable weakening effect is shown at a relative layer thickness of 1-particle, ~50% shear strength reduction is observed with 3-particles, and a dominant influence occurs at 5-particles. In terms of slip stability, a transition from velocity-weakening to velocity-strengthening is observed with 10% to 25% talc present in the uniform mixtures or with 3-particles to 5-particles in the textured mixtures. In addition, further analysis suggest that quartz has a high tendency towards dilation, potentially promoting permeability; while talc dilates with increased slip rate, but compacts rapidly when slip rate is reduced, potentially destroying permeability. The simulation results match well with previous laboratory observations.

Description: Earthquake clustering along plate boundaries suggests that earthquakes may interact, and static Coulomb stress change (CSC) is commonly invoked as one possible mechanism for stress transfer between earthquakes and nearby faults. Previous work has addressed the precision of CSC predictions that are influenced by observational noise, inversion regularization, and simplified modeling assumptions. Here, we address the accuracy of CSC predictions informed by geodetic observations in subduction zones where inversion model resolution is poor. We conduct synthetic tests to quantify the degree to which the sign and magnitude of CSC can be reliably inferred from slip distributions inverted from various geodetic observations (InSAR, GPS, seafloor observations). We find that in an idealized subduction zone, CSC can only be confidently inferred for receiver faults far (10s of km) from the earthquake source; though this distance shortens with the addition of synthetic seafloor observations. We apply this methodology to the 2010 M w 8.8 Maule earthquake and identify 13 aftershocks from a population of 475 documented events for which we can confidently resolve co-seismic stress changes. These results demonstrate that the low model resolution of fault slip inversions in subduction zones limits our ability to address fundamental questions about earthquake sources and stress interactions. Nonetheless, our results highlight that stress change predictions are considerably more accurate after the introduction of seafloor geodetic observations. Additionally, we show that InSAR observations are not required to substantially improve stress change approximations in regions where GPS may be the only viable observation, such as in island arcs settings.

Description: On 15 June 2010, a M w 5.7 earthquake occurred near Ocotillo, California in the Yuha Desert. This event was the largest aftershock of the 4 April 2010 M w 7.2 El Mayor-Cucapah (EMC) earthquake in this region. The EMC mainshock and subsequent Ocotillo aftershock provide an opportunity to test the Coulomb failure hypothesis (CFS). We explore the spatiotemporal correlation between seismicity rate changes and regions of positive and negative CFS change imparted by the Ocotillo event. Based on simple CFS calculations we divide the Yuha Desert into three subregions, one triggering zone and two stress shadow zones. We find the nominal triggering zone displays immediate triggering, one stress shadowed region experiences immediate quiescence, and the other nominal stress shadow undergoes an immediate rate increase followed by a delayed shutdown. We quantitatively model the spatiotemporal variation of earthquake rates by combining calculations of CFS change with the rate-state earthquake rate formulation of [?Dieterich1994], assuming that each subregion contains a mixture of nucleating sources that experienced a CFS change of differing signs. Our modelling reproduces the observations, including the observed delay in the stress shadow effect in the third region following the Ocotillo aftershock. The delayed shadow effect occurs because of intrinsic differences in the amplitude of the rate response to positive and negative stress changes and the time constants for return to background rates for the two populations. We find that rate-state models of time-dependent earthquake rates are in good agreement with the observed rates and thus explain the complex spatiotemporal patterns of seismicity.

Description: We analyse the orientation of the stress field in the southern Hikurangi subduction zone, New Zealand, using focal mechanism inversions, S-wave splitting fast directions, and gravitational stresses. Here, the oceanic Pacific plate is being obliquely subducted beneath the continental Australian Plate in the New Zealand plate boundary zone. The study makes use of 399 earthquakes for focal mechanism inversion and 425 earthquakes for shear wave splitting analysis, located with a network of seismic stations spanning southern North Island. We distinguish between stresses in the Pacific Plate (from focal mechanism inversion) and Australian Plate (from S-wave fast directions) and gravitational stresses, in three regions: Western, Central Basin, and Eastern. In the Western region, the principal axis of horizontal compression (SHmax) is oriented NE-SW, parallel to the margin, in the upper Australian and lower Pacific Plate. In the Central Basin, SHmax in the Australian Plate is oriented NW-SE, perpendicular to the margin; in the lower subducting Pacific Plate SHmax is oriented NE-SW. In the Eastern region, SHmax is oriented NE-SW in the upper plate, while in the lower plate there is a change in orientation to NNW-SSE. We interpret the stress orientations of the lower plate in the Western and Central Basin regions as a consequence of bending of the subducting plate. Sources of upper Australian plate stresses are likely to be bending stresses, gravitational stresses, and tectonic loading, with differing relative magnitudes across the study area.

Description: Interactions between dislocations potentially provide a control on strain rates produced by dislocation motion during creep of rocks at high temperatures. However, it has been difficult to establish the dominant types of interactions and their influence on the rheological properties of creeping rocks due to a lack of suitable observational techniques. We apply high-angular resolution electron backscatter diffraction (HR-EBSD) to map geometrically necessary dislocation (GND) density, elastic strain, and residual stress in experimentally deformed single crystals of olivine. Short-range interactions are revealed by cross-correlation of GND density maps. Spatial correlations between dislocation types indicate that non-collinear interactions may impede motion of proximal dislocations at temperatures of 1000°C and 1200°C. Long-range interactions are revealed by autocorrelation of GND density maps. These analyses reveal periodic variations in GND density and sign, with characteristic length-scales on the order of 1–10 μm. These structures are spatially associated with variations in elastic strain and residual stress on the order of 10 -3 and 100 MPa, respectively. Therefore, short-range interactions generate local accumulations of dislocations, leading to heterogeneous internal stress fields that influence dislocation motion over longer length-scales. The impacts of these short- and/or long-range interactions on dislocation velocities may therefore influence the strain rate of the bulk material, and are an important consideration for future models of dislocation-mediated deformation mechanisms in olivine. Establishing the types and impacts of dislocation interactions that occur across a range of laboratory and natural deformation conditions will help to establish the reliability of extrapolating laboratory-derived flow laws to real Earth conditions.

Description: The (de)hydration process and the amount of hydrated sediment carried by the downgoing oceanic plate play a key role in the subduction dynamics. A high-resolution shear velocity model from the crust down to the uppermost mantle, extending from trench to forearc, is constructed in the northern Cascadia subduction zone to investigate seismic characteristics related to slab deformation and (de)hydration at the plate boundary. A total of 220 seismic stations are used, including the Cascadia Initiative Amphibious Array and inland broadband and short-period stations. The empirical Green's functions extracted from continuous ambient noise data from 2006-2014 provide high-quality Rayleigh-wave signals at periods of 4-50 s. We simulate wave propagation using finite-difference method to generate station Strain Green's Tensors and synthetic waveforms. The phase delays of Rayleigh waves between the observed and synthetic data are measured at multiple period ranges. We then invert for the velocity perturbations from the reference model and progressively improve the model resolution. Our tomographic imaging shows many regional- and local-scale low-velocity features, which are possibly related to slab (de)hydration from the oceanic plate to the overriding plate. Specifically, we observe (1) NW-SE oriented linear low-velocity features across the trench, indicating hydration of the oceanic plate induced by bending-related faultings; (2) W-E oriented fingerlike low-velocity structures off the continental margins due to dehydration of the Juan de Fuca plate; and (3) Seismic lows atop the plate interface beneath the Washington forearc, indicating fluid-rich sediments subducted and overthrusted at the accretionary wedge.

Description: No abstract is available for this article.

Description: We use a large number of high-quality P-wave arrival-time data recorded by the Hi-CLIMB project to determine a 3-D model of azimuthal anisotropy tomography beneath central Tibet. In the Himalayan block, variations of fast velocity orientation (FVO) are revealed from the crust to the upper mantle. In contrast, the FVO in the Lhasa block exhibits only a slight difference between the lower crust and upper mantle, reflecting a coherent deformation there. Different FVOs are revealed near the Bangong-Nujiang suture, which may reflect anisotropies in different parts of the underthrusting Indian plate. In the upper mantle beneath the Qiangtang block, a strong anisotropy is revealed in the shallower part, whereas a weak anisotropy appears in the deeper part, suggesting that a two-layer anisotropy model is applicable there. A layered lithosphere is detected in the eastern part of the Lhasa block, whereas a consistent FVO is revealed in its western part. Our results indicate that strong deformation has occurred in both the Indian and Eurasian lithospheres.

Description: Non-heating paleointensity methods utilize an anhysteretic remanent magnetization (ARM) or a saturation isothermal remanent magnetization (SIRM) to model the natural thermal remanent magnetization (TRM) to avoid heating-induced alteration. We report the results of paleointensity experiments using the ARM, pseudo-Thellier, and ratio of equivalent magnetization (REM) methods conducted to investigate their relative efficiency in recovering the true paleofield strength and to provide additional estimates of their calibration factors. The experiments on synthetic magnetite-bearing samples representing single-domain (SD) and pseudo-single-domain (PSD) magnetic states indicated that the correction factors for the ARM-based methods depend on the magnetic grain size/domain state changing from ~6.3 (for SD grains) to ~4.1 (for ~1.5 µm PSD grains). The pseudo-Thellier method yielded correct absolute paleointensity values when normalization by the TRM/ARM demagnetization slope was used. When applied to samples of lava flows and dikes from a ~32 kyr Lemptégy volcano (France), both the ARM and pseudo-Thellier methods produced similar paleointensity estimates (28.0 ± 5.1 μT and 26.9 ± 4.7 μT, respectively) consistent with the available Thellier data for the 31-33 kyr time interval. The correction factors estimated from our synthetic samples for the REM (~3000) and for REMc (~1500) and REM’ (~1500) variants are consistent with the previously published estimates. However, all REM variants yielded unrealistically high estimates (〉110 μT) of the paleofield strength from our natural samples. Our experimental results support ARM as a better proxy of TRM and suggest that the ARM-based methods currently represent the best alternative to heating-based absolute paleointensity method.

Description: We examine recent Caspian Sea level change using both satellite radar altimetry and satellite gravity data. The altimetry record for 2002-2015 shows a declining level at a rate that is approximately 20 times greater than the rate of global sea level rise. Seasonal fluctuations are also much larger than in the world oceans. With a clearly defined geographic region and dominant signal magnitude, variations in the sea level and associated mass changes provide an excellent way to compare various approaches for processing satellite gravity data. An altimeter time series derived from several successive satellite missions is compared with mass measurements inferred from Gravity Recovery and Climate Experiment (GRACE) data in the form of both spherical harmonic (SH) and mascon solutions. After correcting for spatial leakage in GRACE SH estimates by constrained forward modeling, and accounting for steric and terrestrial water processes, GRACE and altimeter observations are in complete agreement at seasonal and longer time scales, including linear trends. This demonstrates that removal of spatial leakage error in GRACE SH estimates is both possible and critical to improving their accuracy and spatial resolution. Excellent agreement between GRACE and altimeter estimates also provides confirmation of steric Caspian Sea level change estimates. GRACE mascon estimates (both the JPL CRIv02 solution and Center for Space Research (CSR) regularized) are also affected by leakage error. After leakage corrections, both JPL and CSR mascon solutions also agree well with altimeter observations. However, accurate quantification of leakage bias in GRACE mascon solutions is a more challenging problem.

Description: The key to defining the termination of accretion in an accretionary orogen is to recognize the initial magmatic processes that are generated at the time of ocean closure. We present new age, geochemical and isotopic data for magmatic rocks related to terminal collision along the Solonker-Xar Moron suture zone in the southern Central Asian Orogenic Belt (CAOB) that record such processes following closure of the Paleo-Asian Ocean. These magmatic rocks were emplaced in the Early-Middle Triassic (251-245 Ma) and show high Sr/Y signatures. Their low MgO, Cr and Ni contents and variable whole-rock ε Nd (t) values (+5.8 to -5.3), together with the range in zircon ε Hf (t) (+15.6 to -9.8) and δ 18 O values (5.1 to 7.9 ‰), indicate an origin from partial melting of juvenile lower crustal rocks with some old components, including supracrustal recycling under garnet amphibolite facies conditions. Our data, along with available geological and geophysical evidence, lead us to propose a model of final oceanic contraction in the southern CAOB, resulting in sub-linear distribution of high Sr/Y melts along the resultant collision zone, thus defining the onset of post-accretionary processes in the southern CAOB. The identification of collision-related high Sr/Y granitoids from the southern CAOB not only reveal the magmatic process in response to the final episode of orogenic evolution in the CAOB accretionary collision zone, but also constrain how and when an archipelago-type accretionary orogen terminated.

Description: The Mars Science Laboratory Curiosity rover undertook comprehensive exploration of the Kimberley waypoint within Gale crater, Mars in order to understand its context within the larger geologic picture of Gale crater and its evidence for past Martian habitability. Coordinated observations from Curiosity's rich science payload revealed important insights into new Martian crustal compositions, the prevalence and diversity of sedimentary processes within Gale crater, and surface erosion rates. Exploration at the Kimberley, in part informed by a decade of orbital observations of Gale crater, underscored the critical synergy between landed and orbital observations and furthered understanding of complex geological processes on Mars.

Description: Paleoseismic data on the timing of ground-rupturing earthquakes constrain the recurrence behavior of active faults and can provide insight on the rupture history of a fault if earthquakes dated at neighboring sites overlap in age and are considered correlative. This study presents the evidence and ages for 11 earthquakes that occurred along the Big Bend section of the southern San Andreas Fault at the Frazier Mountain paleoseismic site. The most recent earthquake to rupture the site was the Mw7.7-7.9 Fort Tejon earthquake of 1857. We use over 30 trench excavations to document the structural and sedimentological evolution of a small pull-apart basin that has been repeatedly faulted and folded by ground-rupturing earthquakes. A sedimentation rate of 0.4 cm/yr and abundant organic material for radiocarbon dating contribute to a record that is considered complete since 800 A.D. and includes 10 paleoearthquakes. Earthquakes have ruptured this location on average every ~100 years over the last 1200 years but individual intervals range from ~22 to 186 years. The coefficient of variation of the length of time between earthquakes (0.7) indicates quasi-periodic behavior, similar to other sites along the southern San Andreas Fault. Comparison with the earthquake chronology at neighboring sites along the fault indicates that only one other 1857-size earthquake could have occurred since 1350 A.D., and since 800 A.D., the Big Bend and Mojave sections have ruptured together at most 50% of the time in Mw ≥ 7.3 earthquakes.

Description: The North China Craton (NCC) is believed to be the best example of cratonic destruction. However, the processes leading to cratonic destruction remain unclear, largely due to a lack of knowledge of the nature of the Mesozoic NCC lithospheric mantle. Here we report new petrological and geochemical data for Early Cretaceous NCC basalts, which provide insights into the nature of the underlying lithospheric mantle. The Early Cretaceous basalts (all tholeiites) show a limited variation in geochemical composition. In contrast, olivine-hosted melt inclusions from these basalts display a wide range in compositional variation, and include both alkalic and tholeiitic basaltic compositions. This result provides the direct evidence of the contribution of silica-undersaturated alkali basaltic melts in the petrogenesis of the Early Cretaceous NCC basalts. In addition, the compositions of olivine phenocrysts and reconstructed primary melts indicate that the Early Cretaceous basalts are derived from a mixed peridotite and refertilized peridotite source. The Pb isotopic compositions of melt inclusions in high-Fo olivines combined with trace element characteristics of these basalts reveal that heterogeneous lithospheric mantle sources for Early Cretaceous basalts were metasomatized by carbonate-bearing eclogite-derived melts. The Pb isotopic variations of the melt inclusions, and clinopyroxene and plagioclase phenocrysts demonstrate that the mantle-derived magmas were variably contaminated by lower continental crust. We propose that multiple subduction events during the Phanerozoic, combined with mantle-plume activity, likely play a vital role in the generation of the Early Cretaceous voluminous magmatism and cratonic destruction.

Description: Carbon contents in reduced Martian basalts at graphite saturation were experimentally studied at 1400-1550 °C, 1-2 GPa, and log f O 2 of IW-0.4 to IW+1.5 (IW denotes the Fe-FeO buffer). The results show that carbon solubility in Martian basalts, determined by SIMS, is 20 to 1400 ppm, increasing with increasing f O 2 . Raman and FTIR measurements on the quenched silicate glasses show that the dominant carbon species in Martian basalts is carbonate (CO 3 2- ). The experimental data generated here were combined with literature data on similar graphite-saturated carbon solubility for mafic-ultramafic compositions to develop an empirical model that can be used to predict carbon content of graphite-saturated reduced basalts at vapor-absent conditions: At IW+1.7 ≥ log f O 2 ≥ IW-1: log ( C ,  ppm ) = − 3702(±534)/ T  − 194(±49) P / T  − 0.0034(±0.043) logX H 2 O  + 0.61(±0.07) NBO / T  + 0.55(±0.02) ΔIW  + 3.5(±0.3) ( R 2  = 0.89)At IW-5.3 ≤ log f O 2 ≤ IW-1: log  ? ( C ,  ppm ) = 0.96(±0.19) logX H 2 O  − 0.25(±0.04) ?  IW  + 2.83(±0.34)( R 2  = 0.6)in which T is temperature in K, P is pressure in GPa, X H 2 O is mole fraction of water in basalts, ΔIW is the oxygen fugacity relative to the IW buffer, and NBO / T  = 2  total O / T  − 4 ( T  =  Si  +  Ti  +  Al  +  Cr  +  P ). This model was applied to predict carbon content in graphite-saturated mantle melts of the Mercury, Mars, and the Moon. The results show that graphite may be consumed during the production and extraction of some Martian basalts, and CO 2 released by volcanism on Mars cannot be an efficient greenhouse gas in the early Mars. The lunar mantle carbon may be one of the main propellant driving the fire-fountain eruption on the Moon; however, the Mercurian mantle carbon may not be an important propellant for the explosive eruption on Mercury.

Description: We present a comparative study of crack damage evolution in dry sandstone under both conventional (σ 1  〈 σ 2  = σ 3 ), and true triaxial (σ 1  〈 σ 2  〈 σ 3 ) stress conditions using results from measurements made on cubic samples deformed in three orthogonal directions with independently controlled stress paths. To characterize crack damage we measured the changes in ultrasonic compressional and shear wave velocities in the three principal directions, together with the bulk acoustic emission (AE) output contemporaneously with stress and strain. We use acoustic wave velocities to model comparative crack densities and orientations. In essence, we create two end-member crack distributions; one displaying cylindrical transverse isotropy (conventional triaxial) and the other planar transverse isotropy (true triaxial). Under the stress conditions in our experiments we observed an approximately fivefold decrease in the number of AE events between the conventional and true triaxial cases. When taken together, the AE data, the velocities and the crack density data indicate that the intermediate principal stress suppresses the total number of cracks and restricts their growth to orientations sub-normal to the minimum principal stress. However, the size of individual cracks remains essentially constant, controlled by the material grain size. Crack damage is only generated when the differential stress exceeds some threshold value. Cyclic loading experiments show that further damage commences only when that previous maximum differential stress is exceeded, regardless of the mean stress, whether this is achieved by increasing the maximum principal stress or by decreasing the minimum principal stress.

Description: We develop a two-dimensional plane-strain large-deformation coupled poroelastoplastic finite-element model to simulate initiation and rise of a salt wall from a flat salt body during sedimentation. We run transient analyses with high-permeability and low-permeability sediments in the model to simulate drained conditions and overpressure (or pore fluid overpressure). We investigate deformation, stress, and overpressure in the evolving supra-salt basin. Model results show that horizontal stress increases even higher than vertical stress at the flank of the salt wall and in the minibasin due to horizontal pushing out of the rising salt wall, and that orientations of principal stresses rotate in the minibasin relative to far-field stress state. Model predicts that compared with far-field overpressure, the overpressure near the salt wall and within the minibasin is largely perturbed by the salt body. We find that perturbations of pore pressure (or pore fluid pressure) near the salt wall and within the minibasin cannot be resolved by traditional pore pressure prediction methods such as normal compaction trend approach and mean stress model approach. In order to predict pore pressure more accurately, especially in the regions near salt bodies or where stresses are perturbed, we need to apply a method that includes both effects of mean effective stress and deviatoric stress on pore pressure and compaction, for example, the Modified Cam Clay model approach, to pore pressure prediction. Our results provide geoscientists insights into evolution of salt basins, near-salt deformation, stress, overpressure and overpressure prediction methods, and have implications for near-salt wellbore drilling programs.

Description: The current study concerns fundamental controls on fluid flow in tight carbonate rocks undergoing CO 2 -injection. Tight carbonates exposed to weak carbonic acid exhibit order of magnitude changes in permeability while maintaining a nearly constant porosity with respect to the porosity of the unreacted sample. This study aims to determine – if not porosity – what are the microstructural changes that control permeability evolution in these rocks? Given the pore-scale nature of chemical reactions, we took a digital rock physics approach. Tight carbonate mudstone was imaged using X-ray micro-computed tomography. We simulated calcite dissolution using a phenomenological numerical model that stands from experimental and microstructural observations under transport-limited reaction conditions. Fluid flow was simulated using the lattice-Boltzmann method, and the pore wall was adaptively eroded at a rate determined by the local surface area and velocity magnitude, which we use in place of solvent flux. We identified preexisting, high-conductivity fluid pathways imprinted in the initial microstructure. Though these pathways comprise a subset of the total connected porosity, they accommodated 80 to 99.4% of the volumetric flux through the digital sample and localized dissolution. Porosity-permeability evolution exhibited two stages: selective widening of narrow pore throats that comprised preferential pathways and development and widening of channels. We quantitatively monitored attributes of the pore geometry, namely porosity, specific surface area, tortuosity, and average hydraulic diameter, which we qualitatively linked to permeability. This study gives a pore-scale perspective on the microstructural origins of laboratory permeability-porosity trends of tight carbonates undergoing transport-limited reaction with CO 2 -rich fluid.

Description: On 16 September 2015, a Mw 8.3 earthquake ruptured the subduction zone offshore of Illapel, Chile, generating an aftershock sequence with 14 Mw 6.0-7.0 events. A double source W-phase moment tensor inversion consists of a Mw 7.2 sub-event and the main Mw 8.2 phase. We determine two slip models for the mainshock, one using teleseismic broadband waveforms and the other using static GPS and InSAR surface displacements, which indicate high slip north of the epicenter and west-northwest of the epicenter near the oceanic trench. These models and slip distributions published in other studies suggest spatial slip uncertainties of ~25 km and have peak slip values that vary by a factor of 2. We relocate aftershock hypocenters using a Bayesian multiple event relocation algorithm, revealing a cluster of aftershocks under the Chilean coast associated with deep (20-45 km depth) mainshock slip. Less vigorous aftershock activity also occurred near the trench and along strike of the main aftershock region. Most aftershocks are thrust faulting events, except for normal faulting events near the trench. Coulomb failure stress change amplitudes and signs are uncertain for aftershocks collocated with deeper mainshock slip; other aftershocks are more clearly associated with loading from the mainshock. These observations reveal a frictionally heterogeneous interface that ruptured in patches at seismogenic depths (associated with many aftershocks) and with homogeneous slip (and few aftershocks) up to the trench. This event likely triggered seismicity separate from the main slip region, including along-strike events on the megathrust and intraplate extensional events.

Description: Cratonic basins cover more than 10% of Earth's continental surface area, yet their origin remains enigmatic. Here, we use a 1400 km long, deep (20 s TWTT) seismic reflection profile, five wide-angle split-spread receiver gathers, gravity anomaly and well data to constrain the origin of the Parnaíba basin, a cratonic basin in Northeast Brazil. In the centre of the basin, the depth to pre-Paleozoic basement is ~3.3 km, a zone of Mid-Crustal Reflectivity (MCR) can be traced laterally for ~250 km at depths between 17-25 km and Moho depth is ~42±2 km. Gravity and P-wave modelling suggests that the MCR represents the upper surface of a high density (2985 kg m -3 ) and V p (6.75–7.0 km s -1 ) lower crustal body, likely of magmatic origin. Backstripping of well data show a concave up decreasing tectonic subsidence, similar in form to that observed in rift-type basins. We show, however, that our seismic and gravity data are inconsistent with an extensional origin. Other basin forming mechanisms are therefore required. We show that an intrusive body in the lower crust that has loaded and flexed the surface of the crust, combined with sediment loading, provides a satisfactory fit to the observed gravity anomaly. A buried load model is consistent with seismic data, which suggest that Moho is as deep or deeper beneath the basin centre than its flanks and accounts for at least part of the exponentially decreasing tectonic subsidence through a viscoelastic stress relaxation that occurs in the lithosphere following load emplacement.

Description: The Mars Science Laboratory Curiosity rover performed coordinated measurements to examine the textures and compositions of aeolian sands in the active Bagnold dune field. The Bagnold sands are rounded to subrounded, very fine- to medium- sized (~45-500 µm) with ≥6 distinct grain colors. In contrast to sands examined by Curiosity in a dust-covered, inactive bedform called Rocknest and soils at other landing sites, Bagnold sands are darker, less red, better sorted, have fewer silt-sized or smaller grains, and show no evidence for cohesion. Nevertheless, Bagnold mineralogy and Rocknest mineralogy are similar with plagioclase, olivine, and pyroxenes in similar proportions comprising 〉90% of crystalline phases, along with a substantial amorphous component (35% ± 15%). Yet, Bagnold and Rocknest bulk chemistry differ. Bagnold sands are Si-enriched relative to other soils at Gale crater, and H 2 O, S, and Cl are lower relative to all previously measured martian soils and most Gale crater rocks. Mg, Ni, Fe, and Mn are enriched in the coarse-sieved fraction of Bagnold sands, corroborated by VNIR spectra that suggest enrichment of olivine. Collectively, patterns in major element chemistry and volatile release data indicate two distinctive volatile reservoirs in martian soils: (1) amorphous components in the sand-sized fraction (represented by Bagnold) that are Si-enriched, hydroxylated alteration products and/or impact or volcanic glasses; and (2) amorphous components in the fine fraction (〈40 µm; represented by Rocknest and other bright soils) that are Fe-, S-, and Cl-enriched with low Si and adsorbed and structural H 2 O.