JGR–Solid Earth

Syndicate content Journal of Geophysical Research: Solid Earth
Table of Contents for Journal of Geophysical Research: Solid Earth. List of articles from both the latest and EarlyView issues.
Updated: 1 day 54 min ago

Noise Characteristics of Operational Real‐Time High‐Rate GNSS Positions in a Large Aperture Network

Fri, 06/05/2020 - 12:15
Abstract

Large earthquakes are difficult to model in real‐time with traditional inertial seismic measurements. Several algorithms that leverage high‐rate RT‐GNSS positions have been proposed and it has been shown that they can supplement the earthquake monitoring effort. However, analyses of the long‐term noise behavior of high‐rate RT‐GNSS positions, which are important to understand how the data can be used operationally by monitoring agencies, have been limited to just a few sites and to short time spans. Here we show results from an analysis of the noise characteristics of one year of positions at 213 RT‐GNSS sites spanning a large geographic region from Southern California to Alaska. We characterize the behavior of noise and propose several references noise models which can be used as baselines to compare against as technological improvements allow for higher precision solutions. We also show how to use the reference noise models to generate realistic synthetic noise that can be used in simulations of HR‐GNSS waveforms. We discuss spatiotemporal variations in the noise and their potential sources and significance. We also detail how noise analysis can be used in a dynamic quality control to determine which sites should or should not contribute positions to an earthquake modeling algorithm at a particular moment in time. We posit that while there remain important improvements yet to be made, such as reducing the number of outliers in the time series, the present quality of real‐time HR‐GNSS waveforms is more than sufficient for monitoring large earthquakes.

Characterization of the 2008 phreatomagmatic eruption of Okmok from ArcticDEM and InSAR: deposition, erosion, and deformation

Fri, 06/05/2020 - 10:43
Abstract

Okmok volcano, one of the most active volcanoes in the Aleutian Islands, erupted between July 12 and August 19, 2008, and produced a new tephra cone (Ahmanilix) up to 253 m high. A co‐eruptive deposit thickness map and the post‐eruptive elevation change rate map are generated for this phreatomagmatic eruption using the new, high‐resolution (2 meter) digital elevation model (DEM) time‐series provided by ArcticDEM. The total dense rock equivalent (DRE) volume over the proximal deposit field classified from the elevation change map is estimated as 0.147 ± 0.001 km3 DRE (bulk volume of 0.453 ± 0.003 km3). Combined with the distal deposit volume derived from field measurements, the total DRE is 0.25 km3 (bulk volume of 0.76 km3). The estimated erosion rate is up to ‐15 ± 3 m/year along the flanks of Ahmanilix, and the redeposition rate is about 1 ± 0.3 m/year at the base and 6 ± 1 m/year within the center of Ahmanilix. The surface elevation change also reveals a large ground surface collapse that disrupted the old Cone D lava bench, with a total collapse area of 0.15 km2 and a volume of (7.1 ± 0.4) ×10‐3 km3. We also produce a co‐eruptive deformation map from satellite interferometric synthetic aperture radar (InSAR) using topography from both ArcticDEM and the Shuttle Radar Topography Mission (SRTM) and compare the results. Finally, based on historical eruption volumes, the magma supply rate at Okmok is estimated as (1.77 ± 0.1) × 10‐3 km3/yr.

Grand Challenge: Timescales and Processes of Methane Hydrate Formation and Breakdown, with Application to Geologic Systems

Thu, 06/04/2020 - 16:15
Abstract

Gas hydrate is an ice‐like form of water and low molecular weight gas stable at temperatures of roughly ‐10° C to 25° C and pressures of ~3 to 30 MPa in geologic systems. Natural gas hydrates sequester an estimated one‐sixth of Earth’s methane and are found primarily in deepwater marine sediments on continental margins, but also in permafrost areas and under continental ice sheets. When gas hydrate is removed from its stability field, its breakdown has implications for the global carbon cycle, ocean chemistry, marine geohazards, and interactions between the geosphere and the ocean‐atmosphere system. Gas hydrate breakdown can also be artificially driven as a component of studies assessing the resource potential of these deposits. Furthermore, geologic processes and perturbations to the ocean‐atmosphere system (e.g., warming temperatures) can cause not only dissociation, but also more widespread dissolution of hydrate or even formation of new hydrate in reservoirs. Linkages between gas hydrate and disparate aspects of Earth’s near‐surface physical, chemical, and biological systems render an assessment of the rates and processes affecting the persistence of gas hydrate an appropriate Centennial Grand Challenge. This paper reviews the thermodynamic controls on methane hydrate stability and then describes the relative importance of kinetic, mass transfer, and heat transfer processes in the formation and breakdown (dissociation and dissolution) of gas hydrate. Results from numerical modeling, laboratory, and some fields studies are used to summarize the rates of hydrate formation and breakdown, followed by an extensive treatment of hydrate dynamics in marine and cryospheric gas hydrate systems.

Numerical Modeling of Fracture Network Evolution in Organic‐Rich Shale with Rapid Internal Fluid Generation

Wed, 06/03/2020 - 19:00
Abstract

When low‐permeability and organic‐rich rocks such as shale experience sufficient heating, chemical reactions including shale dehydration and maturation of organic matter lead to internal fluid generation. This may cause substantial pore fluid overpressure and fracturing. In the vicinity of igneous intrusions emplaced in organic‐rich shales, temperatures of several hundred degrees accelerate these processes and lead to intense fracturing. The resulting fracture network provides hydraulic pathways, which allow fluid expulsion and affect hydrothermal fluid flow patterns. However, the evolution of these complex fracture networks and controls on geometry and connectivity are poorly understood. Here, we perform a numerical modeling study based on the Extended Finite Element Method to investigate coupled hydro‐mechanical fracture network evolution due to fast internal fluid generation. We quantify the evolution of different initial fracture networks under varying external stresses by analyzing parameters including fracture length, opening, connectivity and propagation angles. The results indicate a three‐phase process including (1) individual growth, (2) interaction and (3) expulsion phase. Magnitude of external stress anisotropy and degree of fracture alignment with the largest principal stress correlate with increased fracture opening. We additionally find that although the external stress field controls the overall fracture orientation distribution, local stress interactions may cause significant deviations of fracture paths and control the coalescence characteristics of fractures. Establishing high connectivity in cases with horizontally aligned initial fractures requires stress anisotropy with σV > σH, while the initial orientation distribution is critical for connectivity if stresses are nearly isotropic.

Uncertainty Propagation in Hierarchical Paleomagnetic Reconstructions

Wed, 06/03/2020 - 19:00
Abstract

Estimation of paleomagnetic directions plays a crucial role in magnetostratigaphy, paleogeographic reconstruction, and constraining past geomagnetic field behavior. While analysis and aggregation of paleomagnetic directional data are performed in a hierarchical fashion, the standard statistical framework employed by paleomagnetists does not consider uncertainty propagation through each level of the hierarchy. With this limitation, inferences drawn from paleomagnetic data will be affected by underestimated uncertainties. We provide here an approximate directional uncertainty propagation scheme that applies to Fisher distributions and, thus, to a number of paleomagnetic data processing tasks. The scheme is a straightforward addition to the existing paleomagnetic statistical framework and is demonstrated using case studies to show how uncertainties can be propagated through different stages of the paleomagnetic data processing chain. Furthermore, we discuss situations, such as the transformation into virtual geomagnetic poles, where this simple form of uncertainty propagation cannot be employed because the data are not Fisher distributed.

Detailed Investigation of the Foreshock Sequence of the 2010 Mw7.2 El Mayor‐Cucapah Earthquake

Wed, 06/03/2020 - 19:00
Abstract

Foreshocks can provide valuable information about possible nucleation process of a mainshock. However, their physical mechanisms are still under debate. In this study, we present a comprehensive analysis of the earthquake sequence preceding the 2010 Mw7.2 El Mayor‐Cucapah mainshock, including waveform detection of missing smaller events, relative relocation, and source parameter analysis. Based on a template matching method, we find a tenfold increase in the number of earthquakes than reported in the Southern California Seismic Network catalog. The entire sequence exhibits nearly continuous episodes of foreshocks that can be loosely separated into two active clusters. Relocated foreshocks show several seismicity streaks at depth, with a consistently active cluster at depths between 14 and 16 km where the mainshock was nucleated. Stress drop measurements from a spectral ratio approach based on empirical Green's functions show a range between 3.8 and 41.7 MPa with a median of 13.0 MPa and no clear temporal variations. The relocation results, together with the source patches estimated from earthquake corner frequencies, revealed a migration front toward the mainshock hypocenter within last 8 hr and a chain of active burst immediately 6 min prior to the mainshock. Our results support combined effects of aseismic slip and cascading failure on the evolution of foreshocks.

Ultramafic Rock Carbonation: Constraints From Listvenite Core BT1B, Oman Drilling Project

Wed, 06/03/2020 - 19:00
Abstract

The occurrence of the quartz‐carbonate alteration assemblage (listvenite) in ophiolites indicates that ultramafic rock represents an effective sink for dissolved CO2. However, the majority of earlier studies of ultramafic rock carbonation had to rely on the surface exposure of reaction textures and field relationships. Here we present the first observations on ultramafic rock alteration obtained from the 300 m deep BT1B drill hole, ICDP Oman Drilling Project, allowing for a continuous and high‐resolution investigation. Hole BT1B recovered continuous drill core intersecting surface alluvium, 200 m of altered ultramafic rock comprising mainly listvenite and minor serpentinite bands at 90 and 180 m depth, and 100 m of the underlying metamorphic sole. Textural evidence suggests that the carbonation of fully serpentinized harzburgite commenced by non‐equilibrium growth of spheroidal carbonate characterized by sectorial zoning resulting from radially oriented low‐angle boundaries. In the serpentinite, carbonate spheroids are composed of alternating magnesite cores and dolomite rims, whereas texturally similar carbonate in the listvenite is composed of Fe‐rich magnesite cores and Ca‐Fe‐rich magnesite rims. The distinct compositions and mineral inclusions indicate that the carbonation extent was controlled by fluid accessibility resulting in the simultaneous formation of limited carbonate in the serpentinite bands and complete carbonation in the listvenite parts of BT1B. The presence of euhedral magnesite overgrowing spheroidal carbonate in the listvenite suggests near‐equilibrium conditions during the final stage of carbonation. The carbonate clumped isotope thermometry constrains carbonate crystallization between 50 °C and 250 °C, implying repeated infiltration of reactive fluids during ophiolite uplift and cooling.

Nature of low‐frequency, atmosphere‐generated seismic noise

Wed, 06/03/2020 - 17:34
abstract

Characteristics of atmosphere‐generated seismic noise below 0.05 Hz are investigated when surface pressure is large. In this paper, large pressure means pressure power spectral density exceeding 100 Pa 2 /Hz (at 0.01 Hz). We discuss three main points. The first point is existence of two frequency ranges that show high coherence between co‐located pressure and vertical seismic data. The lower frequency (LF) range is broad and its upper bound is about 0.002 Hz. The higher frequency (HF) range is bounded between about 0.01 Hz and 0.05 Hz. Phase difference between pressure and vertical displacement is different for the two ranges. The LF range shows phase difference of zero and the HF range shows phase difference of 180 degrees. The second point is on the excitation mechanism in the HF range. Using theory and data, we show that seismic noise in the HF range is primarily excited by wind‐related pressure. When pressure is high, wind speeds become high and wind directions become uni‐directional. In such a case, a deterministic, moving pressure‐source by Sorrells (1971) captures the characteristics of data better than stochastic source models. The third point is on the cause of phase differences between the LF range and the HF range. The root cause is that, even after removing the instrument response, vertical seismic data contain effects from gravity and Earth rotation. Gravity effects become significant for frequencies below 0.005 Hz and create discrepancies between deconvolved vertical displacements and true vertical ground displacements. Phase‐difference results are naturally explained by it.

The frequency‐Bessel spectrograms of multi‐component cross‐correlation functions from seismic ambient noise

Wed, 06/03/2020 - 16:22
abstract

The recently developed frequency‐Bessel transformation (F‐J) method is effective to extract multimode surface wave dispersion curves from ambient noise cross‐correlation functions (CCFs). However, this method is currently limited to the vertical‐vertical component CCFs, and only Rayleigh wave dispersion curves can be obtained. In this study, we first relate the F‐J spectrogram to the spatial autocorrelation coefficients; we then extend the F‐J method to the full multi‐component CCFs tensor, including the radial‐radial, transverse‐transverse, and the mixed‐component CCFs. Using the newly derived formulation, not only the signal of higher‐mode Rayleigh wave phase velocity dispersion can be enhanced, but also the multimode Love wave phase velocity dispersion curves and the higher‐mode Rayleigh wave ellipticity can be extracted. The formulation is tested in several numerical examples, and is applied to field data in North America. Our derivation and formulation provide an extension to the current F‐J method, and help to take usage of multi‐component CCFs. The resulting higher‐mode surface wave dispersions and Rayleigh wave ellipticity provide complementary constraint on the Earth structures.

Effusion Rate Evolution During Small‐Volume Basaltic Eruptions: Insights From Numerical Modeling

Wed, 06/03/2020 - 16:21
Abstract

The temporal evolution of effusion rate is the main controlling factor of lava spreading and emplacement conditions. Therefore, it represents the most relevant parameter for characterizing the dynamics of effusive eruptions and thus for assessing the volcanic hazard associated with this type of volcanism. Since the effusion rate curves can provide important insights into the properties of the magma feeding system, several efforts have been performed for their classification and interpretation. Here, a recently published numerical model is employed for studying the effects of magma source and feeding dike properties on the main characteristics (e.g., duration, erupted mass, and effusion rate trend) of small‐volume effusive eruptions, in the absence of syn‐eruptive magma injection from deeper storages. We show that the total erupted mass is mainly controlled by magma reservoir conditions (i.e., dimensions and overpressure) prior to the eruption, whereas conduit processes along with reservoir properties can significantly affect mean effusion rate, and thus, they dramatically influence eruption duration. Simulations reproduce a wide variety of effusion rate trends, whose occurrence is controlled by the complex competition between conduit enlargement and overpressure decrease due to magma withdrawal. These effusion rate curves were classified in four groups, which were associated with the different types described in the literature. Results agree with the traditional explanation of effusion rate curves and provide new insights for interpreting them, highlighting the importance of magma reservoir size, initial overpressure, and initial width of the feeding dike in controlling the nature of the resulting effusion rate curve.

Late Paleo‐ to early Mesoproterozoic mafic magmatism in the SW Yangtze Block: Mantle plumes associated with Nuna breakup?

Wed, 06/03/2020 - 12:09
Abstract

Numerous late Paleoproterozoic to early Mesoproterozoic mafic magmatic rocks are exposed in the southwestern Yangtze Block, corresponding to early breakup of the Nuna supercontinent. Geochronological data reveal three episodes of mafic magmatism at ~1740 Ma, 1700 Ma and 1500 Ma. The two older generations have MORB‐like Nb/Ta ratios and superchondritic Nd‐Hf isotopes, indicating that they were likely derived from partial melting of asthenospheric mantle. In contrast, the ~1500 Ma mafic rocks possess OIB‐like Nb/Ta (16.7‐19.1) and Zr/Hf (41.0‐45.1) ratios, εNd (t) values (‐0.8 to +0.1) and εHf (t) values (‐3.3 to +8.4), suggesting that they originated from a mantle plume. These three pulses of mafic magmatic activity in the Yangtze Block have geochemical features (such as TiO2 contents, Nb/Y and La/Yb values), similar to coeval mafic rocks in Siberia and Laurentia, consistent with detrital zircon provenance studies which show a spatial linkage of the Yangtze Block with northern Laurentia in the Nuna configuration. Taking all synchronous mafic magmatism into account, a mantle plume beneath southern Siberia was considered to be initiated at ~1750 Ma forming a broad region of mafic magmatism that extended into the Yangtze Block as well as beneath northern Laurentia and resulted in lithospheric extension. The Yangtze Block was possibly the nearest neighbor to northwestern Siberia at ~1500 Ma and subsequently drifted from the Nuna supercontinent induced by the ~1500 Ma Kuonamka mantle plume, heralding a period of limited magmatic activity and tectonic quiescence in the Yangtze Block that extended throughout the remainder of the Mesoproterozoic.

Decarbonation of stagnant slab in the mantle transition zone

Wed, 06/03/2020 - 10:44
Abstract

Phase relations of carbonated basalts have been investigated at 13–20 GPa and 1200–1600 oC, to model the decarbonation process of stagnant slab in the mantle transition zone (MTZ). Two synthetic mixes with CO2 contents of 2.5 wt% (PC‐a) and 5.0 wt% (PC‐b) were used as the starting materials. The estimated solidus was ~1350 oC at the top of the MTZ (~13–15 GPa), which declined to ~1250 oC for pressures of above ~15–16 GPa for both mixes. The average slab geotherms are lower than the obtained solidus, creating a carbonate‐bearing stagnant slab, followed by decarbonation of stagnant slab with increased residence time. Dehydration of stagnant oceanic lithosphere could induce decarbonation of the upper oceanic crust for temperatures below the solidus of carbonated basalts. The resulting carbonate melt is highly reactive with the ambient mantle, producing a carbonated‐domain in the MTZ or at the top of the 410‐km seismic discontinuity. A portion of carbonate melt could ascend to shallow depth because of low density and low viscosity, and bring oceanic crust signatures into the source of some volcanoes after complex interactions with the surrounding mantle. On the example of Eastern Asia, decarbonation of stagnant slab is inferred to be a possible prerequisite for the formation of the big mantle wedge (BMW), and triggered the intraplate volcanoes of Eastern Asia (Xu et al., 2018).

Anhydrite‐assisted hydrothermal metal transport to the ocean floor – insights from thermo‐hydro‐chemical modeling

Tue, 06/02/2020 - 19:00
Abstract

High‐temperature hydrothermal venting has been discovered on all modern mid‐ocean ridges at all spreading rates. Although significant strides have been made in understanding the underlying processes that shape such systems, several first order discrepancies between model predictions and observations remain. One key paradox is that numerical experiments consistently show entrainment of cold ambient seawater in shallow high permeability ocean crust causing a temperature drop that is difficult to reconcile with high vent temperatures. We investigate this conundrum using a thermo‐hydro‐chemical model that couples hydrothermal fluid flow with anhydrite‐ and pyrite‐forming reactions in the shallow sub‐seafloor. The models show that precipitation of anhydrite in warming seawater and in cooling hydrothermal fluids during mixing results in the formation of a chimney‐like sub‐seafloor structure around the upwelling, high‐temperature plume. The establishment of such anhydrite‐sealed zones reduces mixing between the hydrothermal fluid and seawater and results in an increase in vent temperature. Pyrite subsequently precipitates close to the seafloor within the anhydrite chimney. Although anhydrite thus formed may be dissolved when colder seawater circulates through the crust away from the spreading axis, the inside pyrite walls would be preserved as veins in present‐day metal deposits, thereby preserving the history of hydrothermal circulation through shallow oceanic crust.

Stress Sensitivity of Instantaneous Dynamic Triggering of Shallow Slow Slip Events

Tue, 06/02/2020 - 19:00
Abstract

Dynamic triggering of large, detectable slow slip events (SSEs) is rarely observed, even though regional earthquakes often trigger tectonic tremors and very low frequency earthquakes. In this study, we investigate stress sensitivity of dynamic triggering of shallow SSEs in the Nankai Trough offshore of Kii Peninsula, Japan. We first identify additional shallow SSEs that have not been reported in previous studies and obtain a 15‐year‐long catalog of SSEs, some of which are triggered by passing seismic waves originating from large regional earthquakes. We then quantify dynamic and static stress perturbations on the plate interface induced by 19 candidate regional earthquakes using numerical simulations of seismic wave propagation. We find that SSE propensity to dynamic triggering depends mainly on the maximum Coulomb stress change and that relatively large dynamic stresses (>10–20 kPa) are needed to trigger a shallow SSE in the Nankai Trough. Regional earthquakes that can induce such large amplitude of dynamic stresses on the plate interface are relatively rare, which might explain the scarcity of dynamic triggering of large, detectable SSEs along the Nankai as well as other subduction zones. In addition, our analysis suggests that intraslab earthquakes can efficiently trigger SSEs in subduction zones via less‐attenuated, slab‐guided waves. Moreover, our results support the idea that an accretionary wedge in subduction zones promotes the dynamic triggering of shallow SSEs.

Near‐Field Effects of Earthquake Rupture Velocity Into Tsunami Runup Heights

Tue, 06/02/2020 - 19:00
Abstract

Tsunamis have often been treated as an isolated phenomena from the earthquake itself. In tsunami modeling, seafloor deformation is generated from an earthquake. That deformation is copied into the sea surface, and then, the tsunami is propagated over the ocean. On the other hand, rupture velocities from earthquakes are in the span of 1.5–2.5 km/s; therefore, it is safe to approximate the earthquake rupture propagation as an instantaneous phenomena relative to the tsunami propagation. However, this is not necessarily true for all earthquakes. Several types of large slow earthquakes or nonregular earthquakes, such as low frequency earthquakes and very low frequency earthquakes, and tsunami earthquakes have been detected and observed in certain zones around the world. A key question is: Do giant thrust tsunamigenic earthquakes produce slow rupture (0.1–0.5 km/s) velocities? In this study, we model heterogeneous earthquakes sources using very slow rupture velocities (0.1– 2.5 km/s) with the aim of understanding how this parameter affects the tsunami propagation and runup. We compute the amplification due to a very slow moment release in megathrust earthquakes. Our research shows that rupture velocity plays a key role on runup amplification, and the classic instantaneous case might not work as expected for every case.

Grain size control on detrital zircon cycloprovenance in the late Paleozoic Paradox and Eagle basins, USA

Tue, 06/02/2020 - 18:12
Abstract

Detrital zircon U‐Pb data are routinely used to assess provenance in sedimentary systems. However, recent studies have suggested that zircon provenance interpretation may be biased by grain‐size dependent selective transport and deposition. In this study, we present zircon grain‐size and U‐Pb data from 11 sandstone and 11 mudstone samples from the late Paleozoic Eagle and Paradox basins to test whether apparent provenance changes between sandstone and mudstone samples can be explained by changes in zircon grain‐size alone. The Eagle and Paradox basins formed in association with the Ancestral Rocky Mountains (ARM). These basins provide an excellent system to test grain‐size/provenance relationships because they are filled with cyclic stratigraphy that records, via grain size and facies changes, flooding/exposure driven by interglacial/glacial cycles. These cycles are interpreted to have substantially altered sediment transport pathways and provenance of ARM‐related sedimentary rocks. Data presented here suggest that although zircon grain‐size does correlate to sample grain‐size in most samples, age spectra appear to be independent of grain size. As such, differences in age spectra between sandstone and mudstone samples appear to be the result of changes in relative inputs of two endmember sources: (1) grains eroded from adjacent basement‐cored ARM uplifts, and (2) far‐travelled sediment sourced from outside the ARM system. The data presented in this study also provide a successful proof‐of‐concept test for a manual method of determining zircon grain‐size. This method is simple, straightforward, and can be applied to any sample analyzed using the Chromium offline targeting software.

Causes and consequences of wehrlitization beneath a trans‐lithospheric fault: Evidence from Mesozoic basalt‐borne wehrlite xenoliths from the Tan‐Lu fault belt, North China Craton

Tue, 06/02/2020 - 17:35
Abstract

Trans‐lithospheric faults represent weak zones where some of the most intense interaction between the lithospheric mantle and melts occurs. We carried out detailed petrological observations and mineral chemical analyses of feldspar‐bearing wehrlites entrained in the Mesozoic basalts from Liaoyuan, northeastern North China Craton (NCC) to obtain new insights into the nature and consequences of wehrlitization. Wehrlites (n = 12) show textural evidence for replacement of orthopyroxene by clinopyroxene, combined with low Ti/Eu (692–4425), and variable, though generally high Ca/Al, (La/Yb)N and Zr/Hf in clinopyroxenes. This is ascribed to interaction with a silica‐undersaturated, carbonated silicate melt. Feldspar with variable K2O (0.17–9.84 wt.%) and CaO (0.03–12.8 wt.%) and some clinopyroxene likely formed by decomposition of amphibole and mica during heating prior to entrainment. Integrated with data from peridotite xenoliths in Cenozoic basalts along the Tan‐Lu fault belt (TLFB), these observations indicate deep volatile‐rich melt metasomatism. This may have occurred during a known Late Cretaceous to Early Tertiary extensional stage and was precursory to basaltic magmatism in the north‐south direction along the TLFB. We suggest that wehrlitization beneath the TLFB – and elsewhere – monitors the passage and mobility of carbonated melts linked to lithospheric thinning up to decratonization. Upward transport of the wehrlitizing carbonated melts through the massive TLFB and ultimate degassing of CO2 at the surface during its extensional stage may have contributed to the mid‐Cretaceous greenhouse climate.

Low friction coefficient of phyllosilicate fault gouges and the effect of humidity: Insights from a new microphysical model

Sun, 05/31/2020 - 15:53
Abstract

Fault slip is often localized in phyllosilicate‐rich fault gouges in a manner consistent with the relatively low friction coefficients measured for dry and especially wet phyllosilicates in laboratory experiments. However, the microphysics controlling these low friction coefficients remains unclear. Here, we propose a microphysical model, inspired by microstructural observations, for the prediction of the absolute value of the friction coefficient of pure dry and wet phyllosilicates. Experimentally produced phyllosilicate gouges suggest that shearing is controlled by sliding along (001) grain/platelet interfaces operating in series with removal of overlapping grain edge barriers by basal cleavage. We derive a model incorporating a subcritical crack propagation equation for the latter, constrained by subcritical crack growth data for muscovite. Model predictions for muscovite show similar trends regarding the effects of humidity and slip velocity on friction coefficient as do experiments at room temperature. The absolute value predicted for the friction coefficient is difficult to compare with experimental values as it critically depends on atomic scale (001) sliding resistance, which is poorly constrained by available experimental data. Further discrepancies with experimental data can be explained by effects of varying grain size, grain aspect ratio and porosity on the friction coefficient. While numerous qualitative explanations have been proposed previously for the low friction coefficient exhibited by phyllosilicates, especially in the presence of water, our study provides a new step towards a quantitative, physically based model.

Eikonal Tomography Using Coherent Surface Waves Extracted From Ambient Noise by Iterative Matched Filtering—Application to the Large‐N Maupasacq Array

Sat, 05/30/2020 - 19:00
Abstract

Standard ambient noise tomography relies on cross‐correlation of noise records between pairs of sensors to estimate empirical Green's functions. This approach is challenging if the distribution of noise sources is heterogeneous and can get computationally intensive for large‐N seismic arrays. Here, we propose an iterative matched filtering method to isolate and extract coherent wave fronts that travel across a dense array of seismic sensors. The method can separate interfering wave trains coming from different directions, to provide amplitude and travel time fields for each detected wave front. We use the eikonal equation to derive phase velocity maps from the gradient of these travel time fields. Artifacts originating from scattered waves are removed by azimuthal averaging and spatial smoothing. The method is validated on a synthetic test and then applied to the data of the Maupasacq experiment. Rayleigh wave phase velocity maps are obtained for periods between 2 and 9 s. These maps correlate with surface geology at short period (T <3 s) and reveal the deep architecture of the Arzacq and Mauleon basins at longer periods (T >4 s).

Spatial Variations in Crustal and Mantle Anisotropy Across the North American‐Caribbean Boundary on Haiti

Sat, 05/30/2020 - 19:00
Abstract

Haiti, on the island of Hispaniola, is situated across the North American‐Caribbean plate boundary at the transition point between oblique subduction in the east and a transform plate boundary in the west. Here we use shear wave splitting measurements from S waves of local (0–50 km) and intermediate depth (50–150 km) earthquakes as well as SK(K)S phases from teleseismic earthquakes to ascertain good spatial and vertical resolution of the azimuthal anisotropic structure. This allows us to place new constraints on the pattern of deformation in the crust and mantle beneath this transitional region. SK(K)S results are dominated by plate boundary parallel (E‐W) fast directions with ~1.9 s delay times, indicating subslab trench parallel mantle flow is continuing westward along the plate boundary. Intermediate depth earthquakes originating within the subducting North American plate show a mean fast polarization direction of 065° and delay time of 0.46 s, subparallel to the relative plate motion between the Caribbean and North American plates (070°). We suggest a basal shear zone within the lower ductile crust and upper lithospheric mantle as being a potential major source of anisotropy above the subducting slab. Upper crustal anisotropy is isolated using shear wave splitting measurements on local seismicity, which show consistent delay times on the order of 0.2 s. The fast polarization directions indicate that the crustal anisotropy is controlled by the fault networks in close proximity to the major strike‐slip faults, which bisect the north and south of Haiti, and by the regional stress field where faulting is less pervasive.

Theme by Danetsoft and Danang Probo Sayekti inspired by Maksimer