Geophysical Journal International

Syndicate content
Updated: 7 hours 49 min ago

Holocene slip rate of the frontal thrust in the western Qilian Shan, NE Tibetan Plateau

Wed, 07/17/2019 - 00:00
SummaryThe activities of frontal thrusts in the northern Qilian Shan are critical for understanding the deformation of the Qilian Shan and the northeastern Tibetan Plateau. In this study, we estimate the slip rate of the active Fodongmiao–Hongyazi thrust along the northern margin of the Qilian Shan. High-resolution satellite imagery interpretations and detailed field investigations suggest that the fault displaced late Pleistocene terraces and formed fresh prominent north-facing fault scarps. To quantify the slip rate of the fault, we measured the displacements along the fault scarps using an unmanned aerial vehicle system and dated the displaced geomorphic surfaces using optically stimulated luminescence (OSL) and 14C methods. The vertical slip rate of the fault is estimated at 1.0 ± 0.3 mm yr−1 for the western segment. The slip rates for two branches in the eastern segment are 0.3 ± 0.1 mm yr−1 and 0.6 ± 0.1 mm yr−1. Using a fault dip of 40 ± 10o, we constrain the corresponding shortening rates to 1.4 ± 0.5 mm yr−1 and 1.2 ± 0.4 mm yr−1, respectively. The rates are consistent with values over different timescales, which suggests steady rock uplift and north-eastward growth of the western Qilian Shan. Crustal shortening occurs mainly on the range-bounding frontal thrust.

Laboratory observations of frequency-dependent ultrasonic P-wave velocity and attenuation during methane hydrate formation in Berea sandstone

Wed, 07/17/2019 - 00:00
SummaryKnowledge of the effect of methane hydrate saturation and morphology on elastic wave attenuation could help reduce ambiguity in seafloor hydrate content estimates. These are needed for seafloor resource and geohazard assessment, as well as to improve predictions of greenhouse gas fluxes into the water column. At low hydrate saturations, measuring attenuation can be particularly useful as the seismic velocity of hydrate-bearing sediments is relatively insensitive to hydrate content. Here, we present laboratory ultrasonic (448 to 782 kHz) measurements of P-wave velocity and attenuation for successive cycles of methane hydrate formation (maximum hydrate saturation of 26 per cent) in Berea sandstone. We observed systematic and repeatable changes in the velocity and attenuation frequency spectra with hydrate saturation. Attenuation generally increases with hydrate saturation, and with measurement frequency at hydrate saturations below 6 per cent. For hydrate saturations greater than 6 per cent, attenuation decreases with frequency. The results support earlier experimental observations of frequency-dependent attenuation peaks at specific hydrate saturations. We used an effective medium rock physics model which considers attenuation from gas bubble resonance, inertial fluid flow, and squirt flow from both fluid inclusions in hydrate and different aspect ratio pores created during hydrate formation. Using this model, we linked the measured attenuation spectral changes to a decrease in co-existing methane gas bubble radius, and creation of different aspect ratio pores during hydrate formation.

Anisotropy of magnetic susceptibility in salt rocks from the German Zechstein Basin, Sondershausen mine

Wed, 07/17/2019 - 00:00
SummaryThe anisotropy of magnetic susceptibility (AMS) of rocks reflects the alignment of certain minerals, and therefore it can be used to investigate the deformation history of rocks. However, for salt rocks, very few studies on the AMS of salt rocks and the influence of accessory minerals exist. In this study, we analyzed the potential to use the AMS of salt rocks with low impurity content for fabric characterization. Samples of rock salt, sylvinite and carnallitite from a salt mine in Sondershausen (Germany) from the Late Permian (Zechstein 2, Stassfurt series) are investigated. The results of low-field AMS (LF-AMS) measurements show a very weak but significant magnetic anisotropy for sylvinite, carnallitite, and rock salt with an elevated content of accessory minerals. The AMS results are consistent in individual layers of the same rock type. In order to identify the magnetic minerals, which cause the magnetic anisotropy, the high-field AMS (HF-AMS) was measured using a torque magnetometer in order to separate ferrimagnetic and paramagnetic contributions to the AMS. A significant paramagnetic sub-fabrics exists, which reflects the alignment of phyllosilicates. The magnitude of the LF-AMS is considerably greater than that of the paramagnetic sub-fabric. This indicates the existence of a ferrimagnetic sub-fabric due to magnetite, which can have a different orientation than the paramagnetic sub-fabric. Differences in the orientation of the AMS in samples from two sites suggest a relationship of deformation history and AMS. At a site with dipping layers, the AMS orientation is independent of the bedding and shows large differences between individual lithological layers. In a tight fold, the AMS of all rock types has similar shape and orientation. We conclude that AMS in salt rocks can give meaningful information on the mineral fabric, which could be used in the analysis of the deformation history.

Homogenization of fault frictional properties

Wed, 07/17/2019 - 00:00
SummaryThe frictional properties of large faults are expected to vary in space. However, fault models often assume properties are homogeneous, or nearly so. We investigate the conditions under which the details of variations may be neglected and properties homogenized. We do so by examining the behavior of nonlinear solutions for unstably accelerating fault slip under frictional heterogeneity. We consider a rate- and state-dependent fault friction, in which the characteristic wavelength for the property variations is a problem parameter. We find that homogenization is permissible only when that wavelength shows scale separation from an elasto-frictional length scale. However, fault models also often include property transitions that occur over distances comparable to the elasto-frictional length. We show that under such comparable variations, the dynamics of earthquake-nucleating instabilities are controlled by the properties’ spatial distribution.

Dynamic Rupture and Earthquake Sequence Simulations Using the Wave Equation in Second-Order Form

Wed, 07/17/2019 - 00:00
SummaryWe present a numerical method for simulating both single-event dynamic ruptures and earthquake sequences with full inertial effects in antiplane shear with rate-and-state fault friction. We use the second-order form of the wave equation, expressed in terms of displacements, discretized with high-order-accurate finite difference operators in space. Advantages of this method over other methods include reduced computational memory usage and reduced spurious high frequency oscillations. Our method handles complex geometries, such as nonplanar fault interfaces and free surface topography. Boundary conditions are imposed weakly using penalties. We prove time stability by constructing discrete energy estimates. We present numerical experiments demonstrating the stability and convergence of the method, and showcasing applications of the method, including the transition in rupture style from crack-like ruptures to slip pulses for strongly rate-weakening friction and the simulation of earthquake sequences in a viscoelastic solid with a fully dynamic coseismic phase.

The contemporary force balance in a wide accretionary wedge: numerical models of the south-central Hikurangi margin of New Zealand

Tue, 07/16/2019 - 00:00
SummaryThe south-central Hikurangi subduction margin (North Island, New Zealand) has a wide, low-taper accretionary wedge that is frontally accreting a > 3 km-thick layer of sediments, with deformation currently focused near the toe of the wedge. We use a geological model based on a depth-converted seismic section, together with physically realistic parameters for fluid pressure, and sediment and décollement friction based on laboratory experiments, to investigate the present-day force balance in the wedge. Numerical models are used to establish the range of physical parameters compatible with the present-day wedge geometry and mechanics. Our analysis shows that the accretionary wedge stability and taper angle require either high to moderate fluid pressure on the plate interface, and/or weak frictional strength along the décollement. The décollement beneath the outer wedge requires a relatively weaker effective strength than beneath the inner (consolidated) wedge. Increasing density and cohesion with depth make it easier to attain a stable taper within the inner wedge, while anything that weakens the wedge- such as high fluid pressures and weak faults—make it harder. Our results allow a near-hydrostatic wedge fluid pressure, sub-lithostatic fluid overpressure at the subduction interface, and friction coefficients compatible with measurements from laboratory experiments on weak clay minerals.

Single Layer Recurrent Neural Network for detection of local swarm-like earthquakes - the application

Mon, 07/15/2019 - 00:00
SummaryWe present results of applying a local event detector based on artificial neural networks to two seismically active regions. The concept of artificial neural networks enables to recognize earthquake-like signals in seismograms, because well-trained neural networks are characterized by the ability to generalize to unseen examples. This means that once the artificial neural network (ANN) is trained, in our case by few tens to hundreds of examples of local event seismograms, the algorithm can then recognize similar features in unknown records. The detailed description of the single-station detection, design and training of the ANN has been described in our previous paper. Here we show the practical application of our ANN to the same seismoactive region we used for its training - West Bohemia/Vogtland (border area Czechia-Saxony, local seismic network WEBNET), and to different seismogenic area - Reykjanes Peninsula (South-West Iceland, local seismic network REYKJANET). The training process requires carefully prepared dataset which is preferably achieved by manual processing. Such data were available for the West Bohemia/Vogtland earthquake-swarm region, so we used them to train the ANN and test its performance. Due to the absence of completely manually processed activity for the Reykjanes Peninsula we use the trained ANN for swarm-like activity in such a different tectonic setting. The application of a coincidence of the single-station detections helps to reduce significantly the number of undetected events as well as the number of false alarms. Setting up the minimum number of stations which are required to confirm an event detection enables to choose the balance between minimum magnitude threshold and a number of false alarms. The ANN detection results for the Reykjanes Peninsula are compared to manual readings on the stations of the REYKJANET network, manual processing from Icelandic regional network SIL (the SIL catalogs by the Icelandic Meteorological Office) and two tested automatic location algorithms. The neural network shows persuasively better detection results in terms of completeness than the SIL catalogs and automatic location algorithms. Subsequently, we show that our ANN is capable of detecting events from various focal zones in West Bohemia/Vogtland although mainly the focal zone of Nový Kostel was used for training. The performance of our detector is comparable to an expert manual processing and we can state that no important event is missed this way even in case of complicated multiple events during the earthquake swarms.

A Computational Method for Earthquake Cycles within Anisotropic Media

Mon, 07/15/2019 - 00:00
SummaryWe present a numerical method for the simulation of earthquake cycles on a 1D fault interface embedded in a 2D homogeneous, anisotropic elastic solid. The fault is governed by an experimentally motivated friction law known as rate-and-state friction which furnishes a set of ordinary differential equations which couple the interface to the surrounding volume. Time enters the problem through the evolution of the ODEs along the fault and provide boundary conditions for the volume, which is governed by quasi-static elasticity. We develop a time-stepping method which accounts for the interface/volume coupling, and requires solving an elliptic PDE for the volume response at each time step. The 2D volume is discretized with a second order accurate finite difference method satisfying the summation-by-parts property, with boundary and fault interface conditions enforced weakly. This framework leads to a provably stable semi-discretization. To mimic slow tectonic loading, the remote side-boundaries are displaced at a slow rate, which eventually leads to earthquake nucleation at the fault. Time stepping is based on an adaptive, fourth order Runge-Kutta method and captures the highly varying time-scales present. The method is verified with convergence tests for both the orthotropic and fully anisotropic cases. An initial parameter study reveals regions of parameter space where the systems experiences a bifurcation from period one to period two behavior. Additionally, we find that anisotropy influences the recurrence interval between earthquakes, as well as the emergence of aseismic transients and the nucleation zone size and depth of earthquakes.

Anisotropic energy and ultrasonic characteristics of black shale under triaxial deformation revealed utilizing real-time ultrasonic detection and post-test CT imaging

Thu, 07/11/2019 - 00:00
SummaryFor evaluating the fracturing-related activities in a deep shale formation, it is important to investigate the effect of anisotropy on its geomechanical properties. Many effects have been performed to reveal the strength and deformation anisotropy of shale, however, the influence of bedding planes on the anisotropic energy evolution and velocity-energy dependency are still not well understood, especially under high confinement condition. In this study, triaxial compression tests with a high confining pressure of 60 MPa in combination with real-time ultrasonic detection and post-test CT scanning were performed to the shale samples cored along an angle of 0°, 30°, 60°, and 90° with respect to bedding planes. The effect of the bedding orientation on the shale geomechanical, ultrasonic, energy dissipation and energy release characteristics are explored. The experimental results show that shale structural features highly affect the total energy, elastic energy and dissipated energy. The increasing trend of elastic energy shows a slow, fast and slow mode, and the dissipate energy increases rapidly near sample failure. Good correlations have been found among the P- and S-wave velocities and the elastic and dissipated strain energy. The meso-structural changes during deformation are considered to be the primary factor controlling the energy sensitivity to the velocities. CT images further reveal the anisotropic fracture pattern which is in good agreement with energy release and dissipation analysis. The analysis of the strain energy and velocities suggests that the strain energy evolution and fracture anisotropy are bedding orientation dependent.

Data-oriented constraint on the interpretation of S receiver function and its application to observations of seismic discontinuities in the lithosphere-asthenosphere system

Thu, 07/11/2019 - 00:00
SummaryThis paper aims to improve the robustness of interpretation in the S receiver function (SRF), a technique commonly used to retrieve forward scattering of S-to-P converted waves (Sdp) originated from the lithosphere-asthenosphere system (LAS) beneath the stations. Although the SRF does not suffer interferences from backward scattering waves such as the first multiples from the Moho, one major drawback in the method is that Sdp phases can interfere with P coda waves and it is conceivable that these signal-generated noise may be misinterpreted as Sdp phase from the LAS beneath seismic stations. Through systematic analysis of full-waveform synthetics and SRFs from catalogued source parameters, we find that the strong P coda waves before the S wave in the longitudinal-component waveforms result in unwanted signal-generated noise before the S wave in the synthetic SRFs. If the mean amplitude of SRFs after the S wave is large, dubious signal-generated noise before the S arrival are strong as well. In this study, we honor the level of these unwanted signal-generated noise and devise data-oriented screening criteria to minimize the interference between P coda waves and genuine S-to-P converted waves. The first criterion is LQR, a direct measure of the amplitude ratio between longitudinal P coda waves and radial S wave in the waveform data. The second criterion is AMP, the amplitude of SRFs after the S arrival. We illustrate that these criteria effectively measure the energy level of mantle waves such as the SP wave. With synthetics and real data, we demonstrate the effectiveness of LQR and AMP criteria in minimizing these unwanted signal-generated noise in the stacked SRFs down to 1–2 per cent, improving detection threshold and interpretation of Sdp phases from seismic discontinuities in the LAS.

Upper mantle shear velocity structure beneath the equatorial East Pacific Rise from array-based teleseismic surface-wave dispersion analysis

Thu, 07/11/2019 - 00:00
SummaryThe Discovery/Gofar transform faults system is associated with a fast-spreading center on the equatorial East Pacific Rise. Most previous studies focus on its regular seismic cycle and crustal fault zone structure, but the characteristics of the upper mantle structure beneath this mid-ocean ridge system are not well known. Here we invert upper mantle shear velocity structure in this region using both teleseismic surface waves and ambient seismic noise from 24 ocean bottom seismometers (OBSs) deployed in this region in 2008. We develop an array analysis method with multi-dimensional stacking and tracing to determine the average fundamental mode Rayleigh wave phase-velocity dispersion curve (period band 20–100 s) for 94 teleseismic events distributed along the E-W array direction. Then, we combine with the previously measured Rayleigh wave phase-velocity dispersion (period band 2–25 s) from ambient seismic noise to obtain the average fundamental mode (period band 2–100 s) and the first-higher mode (period band 3–7 s) Rayleigh wave phase-velocity dispersion. The average dispersion data are inverted for the 1-D average shear wave velocity (Vs) structure from crust to 200-km depth in the upper mantle beneath our study region. The average Vs between the Moho and 200-km depth of the final model is about 4.18 km/s. There exists an ∼5-km thickness high-velocity lid (LID) beneath the Moho with the maximum Vs of 4.37 km/s. Below the LID, the Vs of a pronounced low-velocity zone (LVZ) in the uppermost mantle (15–60 km depth) is 4.03–4.23 km/s (∼10 per cent lower than the global average). This pronounced LVZ is thinner and shallower than the LVZs beneath other oceanic areas with older lithospheric ages. We infer that partial melting (0.5–5 per cent) mainly occurs in the shallow upper mantle zone beneath this young (0–2 Myr) oceanic region. In the deeper portion (60–200 km depth), the Vs of a weak LVZ is 4.15–4.27 km/s (∼5 per cent lower than the global average). Furthermore, the inferred lithosphere-asthenosphere boundary (LAB) with ∼15-km thickness can fit well with the conductive cooling model. These results are useful for understanding the depth distribution and melting characteristics of the upper mantle lithosphere and asthenosphere in this active ridge-transform fault region.

Mapping geologic features onto subducted slabs

Thu, 07/11/2019 - 00:00
SummaryEstimating the location of geologic and tectonic features on a subducting plate is important for interpreting their spatial relationships with other observables including seismicity, seismic velocity and attenuation anomalies, and the location of ore deposits and arc volcanism in the over-riding plate. Here we present two methods for estimating the location of predictable features such as such as seamounts, ridges and fracture zones on the slab. One uses kinematic reconstructions of plate motions, and the other uses multidimensional scaling to flatten the slab onto the surface of the Earth. We demonstrate the methods using synthetic examples and also using the test case of fracture zones entering the Lesser Antilles subduction zone. The two methods produce results that are in good agreement with each other in both the synthetic and real examples. In the Lesser Antilles, the subducted fracture zones trend northwards of the surface projections. The two methods begin to diverge in regions where the multidimensional scaling method has its greatest likely error. Wider application of these methods may help to establish spatial correlations globally.

Seismic activity during the 2013–2015 inter-eruptive phase at Lascar volcano, Chile

Thu, 07/11/2019 - 00:00
SummaryIn addition to enabling the physical processes of volcanic systems to be better understood, seismology has been also used to infer the complexity of magma pathways and plumbing systems in steep-sided andesitic and stratovolcanoes. However, in these volcanic environments, the application of seismic location methods is particularly challenging and systematic comparisons of common methods are lacking. Furthermore, little is known about the characteristic seismicity and deep structure of Lascar volcano, one of the most historically active volcanoes in northern Chile known to produce VEI-4 eruptions. To better understand the inner processes and deep structure of Lascar, the local broadband seismic monitoring network was densified during a temporal installation in 2014–2015. Herein, we focus on the local seismicity during the 2014–2015 unrest episode, during which we recorded numerous seismic events mainly classified as long-period (LP) type, but also denote volcano-tectonic (VT) activity. Specifically, a long-lasting phase of LP activity is observed over a period of ∼14 months that starts in tandem with a pulse of VT activity. The LP rate and amplitude are modulated over time; they are lower in the initial phase, rise during the intermediate period from October 2014 to July 2015, and finally slowly decay while approaching the eruption time. The location of LPs is challenging due to the typical lack of clear seismic onsets. We thus encompass this problem by comparing a broad range of different standard and novel location techniques to map the source region of LPs by fitting the amplitude decay, polarization patterns, coherence of characteristic functions and cross-correlation differential times. As a result, we principally constrain LP locations within the first 5 km depth below the summit extending downward along a narrow, conduit-like path. We identify different regions of complexity: VTs dominate at depth, both VTs and LPs cluster in an intermediate depth region (down to 1.5 km), suggesting a change in the plumbing system geometry, and LPs dominate the shallowest region. Based on these results, we infer the presence of a sub-vertical conduit extending down to a depth of ∼5 km, and a region of path divergence, possibly accommodating a magma plumbing system, at a depth of ∼3 km beneath the volcano summit. Identifying the locations of complexities in the magma pathways at Lascar may help identify future unrest. The results are compared with independent observations, demonstrating the strength of the location method used herein that will be tested at volcanoes elsewhere.

Constraints on the resistivity of the oceanic lithosphere and asthenosphere from seafloor ocean tidal electromagnetic measurements

Thu, 07/11/2019 - 00:00
SummaryThe electromagnetic (EM) field generated by ocean tidal flow is readily detectable in both satellite magnetic field data, and in ocean-bottom measurements of electric and magnetic fields. The availability of accurate charts of tidal currents, constrained by assimilation of modern satellite altimetry data, opens the possibility of using tidal EM fields as a source to image mantle electrical resistivity beneath the ocean basins, as highlighted by the recent success in defining the globally-averaged lithosphere-asthenosphere boundary (LAB) with satellite data. In fact, seafloor EM data would be expected to provide better constraints on the structure of resistive oceanic lithosphere, since the toroidal magnetic mode, which can constrain resistive features, is a significant component of the tidal EM field within the ocean, but is absent above the surface (in particular in satellite data). Here we consider this issue in more detail, using a combination of simplified theoretical analysis and 1-D and 3-D numerical modeling to provide a thorough discussion of the sensitivity of satellite and seafloor data to subsurface electrical structure. As part of this effort, and as a step toward 3-D inversion of seafloor tidal data, we have developed a new flexible 3D spherical-coordinate finite difference scheme for both global and regional scale modeling, with higher resolution models nested in larger scale solutions. We use the new 3D model, together with Monte Carlo simulations of errors in tidal current estimates, to provide a quantitative assessment of errors in the computed tidal EM signal caused by uncertainty in the tidal source. Over the open ocean this component of error is below 0.01 nT in Bz at satellite height and 0.05 nT in Bx on the seafloor, well below typical signal levels. However, as coastlines are approached error levels can increase substantially. Both analytical and 3-D modeling demonstrate that the seafloor magnetic field is most sensitive to the lithospheric resistance (the product of resistivity and thickness), and is more weakly influenced (primarily in the phase) by resistivity of the underlying asthenosphere. Satellite data, which contain only the poloidal magnetic mode, are more sensitive to the conductive asthenosphere, but have little sensitivity to lithospheric resistance. For both seafloor and satellite data’s changes due to plausible variations in Earth parameters are well above error levels associated with source uncertainty, at least in the ocean interior. Although the 3-D modeling results are qualitatively consistent with theoretical analysis, the presence of coastlines and bathymetric variations generates a complex response, confirming that quantitative interpretation of ocean tidal EM fields will require a 3-D treatment. As an illustration of the nested 3-D scheme, seafloor data at five magnetic and seven electric stations in the northeastern Pacific (41○N, 165○W) are fit with trial-and-error forward modeling of a local domain. The simulation results indicate that the lithospheric resistance is roughly 7 × 108 Ωm2. The phase of the seafloor data in this region are inconsistent with a sharp transition between the resistive lithosphere and conductive asthenosphere.

An investigation of seismic anisotropy in the lowermost mantle beneath Iceland

Thu, 07/11/2019 - 00:00
SummaryIceland represents one of the most well-known examples of hotspot volcanism, but the details of how surface volcanism connects to geodynamic processes in the deep mantle remain poorly understood. Recent work has identified evidence for an ultra-low velocity zone (ULVZ) in the lowermost mantle beneath Iceland and argued for a cylindrically symmetric upwelling at the base of a deep mantle plume. This scenario makes a specific prediction about flow and deformation in the lowermost mantle, which can potentially be tested with observations of seismic anisotropy. Here we present an investigation of seismic anisotropy in the lowermost mantle beneath Iceland, using differential shear wave splitting measurements of S-ScS and SKS-SKKS phases. We apply our techniques to waves propagating at multiple azimuths, with the goal of gaining good geographical and azimuthal coverage of the region. Practical limitations imposed by the suboptimal distribution of global seismicity at the relevant distance ranges resulted in a relatively small dataset, particularly for S-ScS. Despite this, however, our measurements of ScS splitting due to lowermost mantle anisotropy clearly show a rotation of the fast splitting direction from nearly horizontal for two sets of paths that sample away from the low velocity region (implying VSH > VSV) to nearly vertical for a set of paths that sample directly beneath Iceland (implying VSV > VSH). We also find evidence for sporadic SKS-SKKS discrepancies beneath our study region; while the geographic distribution of discrepant pairs is scattered, those pairs that sample closest to the base of the Iceland plume tend to be discrepant. Our measurements do not uniquely constrain the pattern of mantle flow. However, we carried out simple ray-theoretical forward modeling for a suite of plausible anisotropy mechanisms, including those based on single-crystal elastic tensors, those obtained via effective medium modeling for partial melt scenarios, and those derived from global or regional models of flow and texture development in the deep mantle. These simplified models do not take into account details such as possible transitions in anisotropy mechanism or deformation regime, and test a simplified flow field (vertical flow beneath the plume and horizontal flow outside it) rather than more detailed flow scenarios. Nevertheless, our modeling results demonstrate that our ScS splitting observations are generally consistent with a flow scenario that invokes nearly vertical flow directly beneath the Iceland hotspot, with horizontal flow just outside this region.

Velocity-porosity-mineralogy trends in chalk and consolidated carbonate rocks

Wed, 07/10/2019 - 00:00
SummaryPublished laboratory elastic-wave velocity versus porosity data in carbonate rocks exhibit significant scatter even at a fixed mineralogy. This scatter is usually attributed to the strong variability in the rock-frame or pore-space geometry, which, in turn, is driven by the richness and complexity of diagenetic alteration in these very reactive sediments. Yet, by examining wireline data from oil-bearing high-to-medium porosity chalk deposits, we find surprisingly tight velocity-porosity trends. Moreover, these trends are continued into the low-porosity domain by data from a location thousands of miles away from the chalk field. This congruence implies a universality of diagenetic trends, at least in the massive deposits under examination. We also find that the elastic bulk and shear moduli of the pure-calcite end member are somewhat smaller than such values reported in the literature. Using the end-member elastic constants relevant to the data under examination, we establish a theoretical rock physics model to match and generalize these data.

A reduced stochastic model of core surface dynamics based on geodynamo simulations

Wed, 07/10/2019 - 00:00
SummaryWe make use of recent geodynamo simulations to propose a reduced stochastic model of the dynamics at the surface of Earth’s core. On decadal and longer periods, this model replicates the most energetic eigen directions of the geodynamo computation. Towards shorter time-scales, it proposes a compensation for weaknesses of these simulations. This model furthermore accounts for the signature, in the geomagnetic secular variation, of errors of representativeness associated with unresolved processes. We incorporate the reduced stochastic model into a geomagnetic data assimilation algorithm – an augmented state ensemble Kalman filter – and apply it to re-analyze magnetic field changes over the period 1880–2015. Errors of representativeness appear to be responsible for an important fraction of the observed changes in the secular variation, as it is the case in the dynamo simulation. Recovered core surface motions are primarily symmetric with respect to the equator. We observe the persistence of the eccentric westward gyre over the whole studied era, and vortices that partly follow isocontours of the radial magnetic field at the core surface. Our flow models provide a good fit to decadal changes in the length-of-day, and predict its interannual variations over 1940–2005. The largest core flow acceleration patterns are found in an equatorial belt below 10○ in latitude, and are associated with non-axisymmetric features. No systematic longitudinal drift of acceleration patterns is found, even over the past decades where satellite data are available. The acceleration of the high latitude westward jet in the Pacific hemisphere is, during the satellite era, a factor 5 smaller than previously reported, and its structure shows some evidence for equatorial asymmetry. The era of continuous satellite records provides enhanced contrast on the rapid core flow variations. The proposed assimilation algorithm offers the prospect of evaluating Earth-likeness of geodynamo simulations.

Bayesian joint muographic and gravimetric inversion applied to volcanoes

Wed, 07/10/2019 - 00:00
SummaryGravimetry is a technique widely used to image the structure of the Earth. However, inversions are ill-posed and the imaging power of the technique rapidly decreases with depth. To overcome this limitation, muography, a new imaging technique relying on high energy atmospheric muons, has recently been developed. Because muography only provides integrated densities above the detector from a limited number of observation points, inversions are also ill-posed. Previous studies have shown that joint muographic and gravimetric inversions better reconstruct the 3D density structure of volcanic edifices than independent density inversions. These studies address the ill-posedness of the joint problem by regularizing the solution with respect to a prior density model. However, the obtained solutions depend on some hyperparameters, which are either determined relative to a single test case or rely on ad-hoc parameters. This can lead to inaccurate retrieved models, sometimes associated with artefacts linked to the muon data acquisition. In this study, we use a synthetic example based on the Puy de Dôme volcano to determine a robust method to obtain the resulting model closest to the synthetic model and devoid of acquisition artefacts. We choose a Bayesian approach to include an a priori density model and a smoothing by a Gaussian spatial correlation function relying on two hyperparameters: an a priori density standard-deviation and an isotropic spatial correlation length. This approach has the advantage to provide a posteriori standard-deviations on the resulting densities. Using our synthetic volcano, we investigate the most reliable criterion to determine the hyperparameters. Our results suggest that k-fold Cross-Validation Sum of Squares and the Leave One Out methods are more robust criteria than the classically used L-curves. The determined hyperparameters allow to overcome the artefacts linked to the data acquisition geometry, even when only a limited number of muon telescope is available. We also illustrate the behaviour of the inversion in case of offsets in the a priori density or in the data and show that they lead to recognizable structures that help identify them.

Analysis of geomagnetic secular variation for the last 1.5 Ma recorded by volcanic rocks of the Trans Mexican Volcanic Belt: New data from Sierra de Chichinautzin, Mexico

Wed, 07/10/2019 - 00:00
SummaryThe great wealth of volcanism along the Trans Mexican Volcanic Belt (TMVB) and the need to improve the secular variation curve of the Earth magnetic field of the region is the aim of this research. 300 oriented cores from 33 sites and 21 individual cooling units were acquired from Sierra de Chichinautzin volcanic field (ChVF) and Sierra de Santa Catarina (SSC). Directional analysis and rock magnetic experiments were performed (e.g. thermal demagnetization, hysteresis loop, susceptibility vs temperature), achieving 21 new averaged paleomagnetic directions. New results are consistent with the previous studies on the same cooling unit. We compiled all the paleomagnetic studies performed on the ChVF, updating age and calculating an average direction per cooling unit and estimating an overall mean direction for the ChVF (Dec = 359.1°, Inc = 35.3°, N = 33, k = 21.6, α95 = 5.5°, Plat = 87.7° N, Plong = 227.4° E, K = 31.8, A95 = 4.5°).Afterwards, we compiled all the previous paleomagnetic studies along the whole TMVB with age ranging from 0 to 1.5 Ma, and constrained the directional analyses by specific quality criteria such as well-defined age, number of samples and quality of kappa) on the cooling unit consistency.The mean direction and virtual geomagnetic pole (VGP) estimated for the TMVB, during the periods 0–40 ka and 0–1.5 Ma, are close to the geographic pole, supporting the validity of the geocentric axial dipole hypothesis. The directional results of this study also fit well with the predictions at Mexico City of the models SHA.DIF.14k and CALS10k2 calculated for the last 14 ka. The dispersion of the VGP's on the TMVB are also consistent with the expected values proposed by different models of paleosecular variation (e.g. Opdyke et al., 2015; Cromwell et al., 2018). However, large gaps in the temporal record remain that should be filled by further paleomagnetic studies.

The influence of inherited continental margin structures on the stress and strain fields of the south-central Taiwan fold-and-thrust belt

Wed, 07/10/2019 - 00:00
SummaryIn this paper we test whether or not structural and morphological features inherited from the Eurasian continental margin are affecting the contemporary stress and strain fields in south-central Taiwan. Principal stress directions (σ1, σ2, and σ3) are estimated from the inversion of clustered earthquake focal mechanisms and the direction of maximum compressive horizontal stress (SH) is calculated throughout the study area. From these data the most likely fault plane orientations and their kinematics are inferred. The results of the stress inversion are then discussed together with the directions of displacement, compressional strain rate, and maximum shear strain rate derived from GPS data. These data show that there is a marked contrast in the direction of SH from north to south across the study area, with the direction of SH remaining roughly sub-parallel to the relative plate motion vector in the north, whereas in the south it rotates nearly 45° counterclockwise. The direction of horizontal maximum compression strain rate (εH) and associated maximum shear planes, together with the displacement field display an overall similar pattern between them, although undergoing a less marked rotation. We interpret the southward change in the SH, εH, and the dextral maximum shear planes directions, together with that of the horizontal displacement field to be related to the reactivation of east-northeast striking faults inherited from the rifted Eurasian margin and to the shelf/slope break. Inherited faults in the basement are typically reactivated as strike-slip faults, whereas newly formed faults in the fold-and-thrust belt are commonly thrusts or oblique thrusts. Eastward, the stress inversions and strain data show that the western flank of the Central Range is undergoing extension in the upper crust. SH in the Central Range is roughly parallel to the relative plate convergence vector, but in southwestern Taiwan it undergoes a marked counterclockwise rotation westward across the Chaochou fault. Farther north, however, there is no significant change across the Lishan fault. This north to south difference is likely due to different margin structures, although local topographic effects may also play a role.

Theme by Danetsoft and Danang Probo Sayekti inspired by Maksimer