JGR–Solid Earth

Syndicate content Wiley: Journal of Geophysical Research: Solid Earth: Table of Contents
Table of Contents for Journal of Geophysical Research: Solid Earth. List of articles from both the latest and EarlyView issues.
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Thermochemical Structure and Melting Distribution of the Upper Mantle Beneath Intraplate Volcanic Areas in Eastern South China Block

Thu, 12/14/2023 - 12:00
Abstract

The Eastern South China Block (SCB) has experienced complex Mesozoic-Cenozoic tectonism and intraplate volcanism. However, due to a lack of exhaustive exploration of the upper mantle's thermochemical structure, it is difficult to determine the extent of the lithospheric modification and the mechanisms by which the volcanism generate. Here, we jointly invert Rayleigh wave dispersion, surface heat flow, geoid height, and elevation data to provide a comprehensive thermal and compositional structure of the upper mantle beneath eastern SCB and infer regions of partial melting. Our model reveals widespread lithospheric thinning in the eastern SCB and large variations of lithospheric composition with a more fertile eastern Lower Yangtze lithosphere than the lithosphere elsewhere, suggesting the lithosphere of the eastern Lower Yangtze is more severely modified than the rest of the SCB. Moreover, two high-temperature anomalies are revealed: one beneath the eastern Lower Yangtze and the other beneath the Pearl River Delta region, associated with the Pacific plate subduction and Hainan plume, respectively. The high-degree partial melting (∼6%) in the asthenosphere beneath the Lower Yangtze is responsible for the strong ongoing lithospheric modification and the young intraplate volcanism in the Nvshan and Subei areas. Small-scale upper mantle convections triggered by the large mantle upwellings created a low value of ∼3% melts, possibly responsible for the intraplate volcanism in the coastal CB and less severe lithospheric modification. We demonstrate that the lithospheric thickness and its thermochemical state are the key factors that influence the composition and evolution of intraplate volcanism in the eastern SCB.

The Effect of Magma Poor and Magma Rich Rifted Margins on Continental Collision Dynamics

Thu, 12/14/2023 - 11:50
Abstract

The transition between non-rifted continental lithosphere and oceanic lithosphere in rifted margins can display a wide range of characteristics, depending on the regional tectonic evolution. The velocity and duration of the rifting process as well as the geodynamic setting influence the properties and geometry of the margins, which are often grouped into two main categories: magma-poor and magma-rich. We show how different types of rifted margins can influence the dynamics of continental collision, focusing on the time and depth of slab break-off after collision and the fate of margin material. We find that rifted margins have a noticeable impact on subduction dynamics, as we observe large variability in slab break-off times and depths. In particular, the presence of a rifted margin can delay slab break-off to up to 60 Myr after the onset of collision. Our results show that a large portion of the weak crust of magma-poor margins is likely to detach from the subducting plate and accrete to the upper plate, while the dense and strong mafic and ultramafic component of magma-rich margins causes most of the margin to subduct and be lost into the mantle, leaving only a small fraction of transitional and oceanic crust at the surface. Therefore, the volume of accreted material is much larger when the margin is magma-poor than magma-rich, which is consistent with geological observations that fossil magma-poor rifted margins are preserved in many mountain ranges, whereas remnants of magma-rich rifted margins are scarce.

Characterizing the Seafloor Sediment Layer Using Teleseismic Body Waves Recorded by Ocean Bottom Seismometers

Wed, 12/13/2023 - 11:41
Abstract

This study presents an approach to better characterize the P-wave and S-wave velocity structure of the seafloor sediment layer using ocean bottom seismometers. The presence of low-velocity seafloor sediment layers influences the observed seismic record at the seafloor over a broad frequency range, such that detailed knowledge of this sediment structure is essential to predict its effect on teleseismic records. We use the radial component of teleseismic P waves and autocorrelation functions of the radial, vertical, and pressure components of teleseismic P and S waves to obtain sediment layer models using the Markov chain Monte Carlo approach with parallel tempering. Synthetic tests show that the body waves constrain the P- and S-wave impedances and travel times and the P- to S-wave velocity ratio of the sediment layers. The proposed method resolves thin layers at a high resolution, including the uppermost thin (∼50 m to a few hundred meters) low S-wave velocity layer. Real data applications at sites across the Pacific Ocean that are coincident with previous in situ studies demonstrate the effectiveness of this method in characterizing the seafloor sediment unit. The sediment models characterized by this new approach will allow us to more accurately predict and correct the effects of sediment layers in generating P- and S-wave reverberations.

New Insights Into the Active Tectonics of the Northern Canadian Cordillera From an Enhanced Earthquake Catalog

Tue, 12/12/2023 - 12:09
Abstract

Seismic activity in the Northern Canadian Cordillera is characterized by diffuse earthquakes that extend hundreds of km northwest from the Yakutat collision zone. We use 25 months of broadband seismic data from Mackenzie Mountain Earthscope Project (MMEP), USArray Transportable Array (TA), and permanent Canadian National Seismic Network stations to present a local earthquake catalog with high sensitivity to small regional events. Deep learning techniques are adopted for both seismic phase detection and association. Event relocations are performed to provide well constrained estimates of earthquake depth distributions. Clusters of seismicity spanning the upper crust are located in the central Richardson Mountains, along the Tintina fault, and in the northeast Selwyn Basin. These clusters suggest that the core of the Richardson Anticlinorium is tectonically active and that the Tintina fault is a locus for low levels of active deformation. We interpret seismicity in the northeast Selwyn Basin as primarily occurring in the hanging wall of the Plateau thrust fault and suggest that some combination of localized duplex structures and lithological strength contrasts both within the Selwyn Basin and between abutting Paleozoic shelf sequences may be responsible for seismicity in the Mackenzie Mountain foreland.

The Crustal Magmatic Structure Beneath the Denali Volcanic Gap Imaged by a Dense Linear Seismic Array

Mon, 12/11/2023 - 16:44
Abstract

The crustal structure in south-central Alaska has been influenced by terrane accretion, flat slab subduction, and a modern strike-slip fault system. Within the active subduction system, the presence of the Denali Volcanic Gap (DVG), a ∼400 km region separating the active volcanism of the Aleutian Arc to the west and the Wrangell volcanoes to the east, remains enigmatic. To better understand the regional tectonics and the nature of the volcanic gap, we deployed a month-long north-south linear geophone array of 306 stations with an interstation distance of 1 km across the Alaska Range. By calculating multi-component noise cross-correlation and jointly inverting Rayleigh wave phase velocity and ellipticity across the array, we construct a 2-D shear wave velocity model along the transect down to ∼16 km depth. In the shallow crust, we observe low-velocity structures associated with sedimentary basins and image the Denali fault as a narrow localized low-velocity anomaly extending to at least 12 km depth. About 12 km, below the fold and thrust fault system in the northern flank of the Alaska Range, we observe a prominent low-velocity zone with more than 15% velocity reduction. Our velocity model is consistent with known geological features and reveals a previously unknown low-velocity zone that we interpret as a magmatic feature. Based on this feature's spatial relationship to the Buzzard Creek and Jumbo Dome volcanoes and the location above the subducting Pacific Plate, we interpret the low-velocity zone as a previously unknown subduction-related crustal magma reservoir located beneath the DVG.

Lateral Variations in Teleseismic Attenuation of the Conterminous U.S. and New Insights Derived From Its Relationship to Mantle Seismic Velocity

Mon, 12/11/2023 - 16:40
Abstract

Much of our knowledge of the North American lithosphere comes from imaging seismic velocities. Additional constraints on the subsurface can be gained by studying seismic attenuation, which has different sensitivity to physical properties. We produce a model of lateral variations in attenuation across the conterminous U.S. by analyzing data recorded by the EarthScope Transportable Array. We divide the study area into 12 overlapping tiles and differential attenuation is measured in each tile independently; and twice for four of the tiles. Measurements are combined into a smooth map using a set of linear inversions. Comparing results for adjacent tiles and for repeated tiles shows that the imaged features are robust. The final map shows generally higher attenuation west of the Rocky Mountain Front than east of it, with significant small length scale variations superimposed on that broad pattern. In general, there is a strong anticorrelation between differential attenuation and shear wave velocities at depths of 80–250 km. However, a given change in velocity may correspond to a large or small change in attenuation, depending on the area; suggesting that different physical mechanisms are operating. In the western and south-central U.S., as well as the Appalachians, velocity variations are large compared to attenuation changes, while the opposite is true in the north-central and southeastern U.S. Calculations with the Very Broadband Rheology calculator show that these results are consistent with the main source of heterogeneity being temperature and melt fraction in the former regions and grain size variability in the latter ones.

Full‐Field Numerical Simulation of Halite Dynamic Recrystallization From Subgrain Rotation to Grain Boundary Migration

Mon, 12/11/2023 - 15:59
Abstract

Full-field numerical modeling is a useful method to gain understanding of rock salt deformation at multiple scales, but it is quite challenging due to the anisotropic and complex plastic behavior of halite, together with dynamic recrystallization processes. This contribution presents novel results of full-field numerical simulations of coupled dislocation glide and dynamic recrystallization of halite polycrystalline aggregates during simple shear deformation, including both subgrain rotation and grain boundary migration (GBM) recrystallization. The results demonstrate that the numerical approach successfully replicates the evolution of pure halite microstructures from laboratory torsion deformation experiments at 100–300°C. Temperature determines the competition between (a) grain size reduction controlled by dislocation glide and subgrain rotation recrystallization (at low temperature) and (b) grain growth associated with GBM (at higher temperature), while the resulting crystallographic preferred orientations are similar for all cases. The relationship between subgrain misorientation and strain follows a power law relationship with a universal exponent of 2/3 at low strain. However, dynamic recrystallization causes a progressive deviation from this relationship when strain increases, as revealed by the skewness of the subgrain misorientation distribution. A systematic investigation of the subgrain misorientation evolution shows that strain or temperature prediction from microstructures requires careful calibration.

Phase Transformation Under High Pressure Radiates as a Double Couple Deep Earthquake

Mon, 12/11/2023 - 15:54
Abstract

Deep-focus earthquakes (DFEs) originating at the Mantle-Transition-Zone (MTZ) (400–700 km) have a Double Couple (DC) radiation pattern similar to crustal earthquakes; however, their mechanism is different and governed by high pressures (15–25 GPa) at nucleation depths. We present a model of nucleation and growth of regions of phase transformation, undergoing a sudden reduction in volume (5%–10%), “volume collapse.” Successive symmetry-breaking instabilities minimize the energy spent to move the boundary of phase discontinuity and a collapsing volume expands as a flattened pancake-like self-similarly expanding Eshelby ellipsoidal inclusion. At the vanishing of the M integral, expressing the balance of flows of energy across the inclusion boundary, at a critical value of the pressure, an arbitrarily small inclusion nucleates and grows at constant potential energy driven by the pressure acting on the change in volume. The inclusion develops shear eigenstrains that decompose into two DC, placing one on the basal plane to radiate without energy losses. The symmetric volume collapse radiates out as an anti-symmetric DC, and the radiated energy is obtained as the “excess energy,” of the ambient pressure acting on the “volume collapse,” reduced by the energy consumed for the growth of the pancake surfaces, with a “pressure drop” (p 0 − p cr) driving the expansion emitting a DC, even under full isotropy. The solution explains some features of the DFEs, (a) the DC radiation, (b) their large energies (the Mw 8.3 Okhotsk earthquake [2013]), (c) the absence of volumetric radiation, and (d) why they can originate in the MTZ, a long-standing open problem.

Accretion Cycles, Structural Evolution, and Thrust Activity in Accretionary Wedges With Various Décollement Configurations: Insights From Sandbox Analog Modeling

Fri, 12/08/2023 - 11:34
Abstract

The architecture (geometry, fault network, and stacking pattern of accreted thrust sheets) of accretionary wedges influences subduction zone processes. However, it remains challenging to constrain the architectural evolution in natural accretionary wedges over geological timescales. In this study, we undertook sandbox analog modeling, with quantitative analysis of the wedge geometry and digital image correlation-based kinematics, to delineate the wedge growth history with four décollement settings (single or double and continuous or discontinuous). The results show that the wedge is formed by repeated episodic frontal accretion with a constant cycle (i.e., the accretion cycle), and the degree of coupling between the base of the wedge and subducting plate interface appears to depend on the relative strengths of the wedge and detachment. An interbedded décollement layer in the incoming sediment facilitated wedge segmentation and rearrangement of the internal fault network, which weakened the wedge strength. A combination of a detachable high-friction patch in the basal décollement and a continuous interbedded weak layer enabled underplating of underthrusted sediment beneath the inner wedge, which involved a low-angle, long-lived forethrust and multiple cycles of frontal accretion on short-lived forethrusts at the deformation front. Our findings suggest that décollement configuration is a key factor in controlling the accretion cycle, strain distribution, fault network, and wedge strength on timescales of ∼105 yr in natural accretionary systems. This result should be considered when investigating modern subduction zones.

Real‐Time Fault Tracking and Ground Motion Prediction for Large Earthquakes With HR‐GNSS and Deep Learning

Fri, 12/08/2023 - 11:28
Abstract

Earthquake early warning (EEW) systems aim to forecast the shaking intensity rapidly after an earthquake occurs and send warnings to affected areas before the onset of strong shaking. The system relies on rapid and accurate estimation of earthquake source parameters. However, it is known that source estimation for large ruptures in real-time is challenging, and it often leads to magnitude underestimation. In a previous study, we showed that machine learning, HR-GNSS, and realistic rupture synthetics can be used to reliably predict earthquake magnitude. This model, called Machine-Learning Assessed Rapid Geodetic Earthquake model (M-LARGE), can rapidly forecast large earthquake magnitudes with an accuracy of 99%. Here, we expand M-LARGE to predict centroid location and fault size, enabling the construction of the fault rupture extent for forecasting shaking intensity using existing ground motion models. We test our model in the Chilean Subduction Zone with thousands of simulated and five real large earthquakes. The result achieves an average warning time of 40.5 s for shaking intensity MMI4+, surpassing the 34 s obtained by a similar GNSS EEW model. Our approach addresses a critical gap in existing EEW systems for large earthquakes by demonstrating real-time fault tracking feasibility without saturation issues. This capability leads to timely and accurate ground motion forecasts and can support other methods, enhancing the overall effectiveness of EEW systems. Additionally, the ability to predict source parameters for real Chilean earthquakes implies that synthetic data, governed by our understanding of earthquake scaling, is consistent with the actual rupture processes.

Physics‐Informed Neural Networks for Fault Slip Monitoring: Simulation, Frictional Parameter Estimation, and Prediction on Slow Slip Events in a Spring‐Slider System

Thu, 12/07/2023 - 07:10
Abstract

The episodic transient fault slips called slow slip events (SSEs) have been observed in many subduction zones. These slips often occur in regions adjacent to the seismogenic zone during the interseismic period, making monitoring SSEs significant for understanding large earthquakes. Various fault slip behaviors, including SSEs and earthquakes, can be explained by the spatial heterogeneity of frictional properties on the fault. Therefore, estimating frictional properties from geodetic observations and physics-based models is crucial for fault slip monitoring. In this study, we propose a Physics-Informed Neural Network (PINN)-based new approach to simulate fault slip evolutions, estimate frictional parameters from observation data, and predict subsequent fault slips. PINNs, which integrate physical laws and observation data, represent the solution of physics-based differential equations. As a first step, we validate the effectiveness of the PINN-based approach using a simple single-degree-of-freedom spring-slider system to model SSEs. As a forward problem, we successfully reproduced the temporal evolution of SSEs using PINNs and obtained implications on how to choose the appropriate collocation points by analyzing the residuals of physics-based differential equations. As an inverse problem, we estimated the frictional parameters from synthetic observation data and demonstrated the ability to obtain accurate values regardless of the choice of first-guess values. Furthermore, we discussed the potential of the predictability of the subsequent fault slips using limited observation data, taking into account uncertainties. Our results indicate the significant potential of PINNs for fault slip monitoring.

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