A document by Bo Yang. Click on the document to view its contents.

We systematically compare Fourier Amplitude Spectra of Fault Normal (FN) and Fault Parallel (FP) seismograms at near-fault sites for 11 strike-slip earthquakes with magnitudes M≥6. For all events we find large FN/FP ratios at low frequencies consistent with the S-wave radiation patterns for strike-slip earthquakes near the faults. However, the difference diminishes with increasing frequency and FN/FP is about 1 above a transition frequency. The results may reflect small-scale isotropic source terms in the earthquake rupture zones that homogenize the high-frequency radiation in different directions at near-fault sites. The FN/FP ratios at low frequencies and transition frequencies above which FN ~ FP vary among the 11 earthquakes, and have no clear correlation with the moment magnitudes. The lack of correlation may signify a characteristic rupture length scale (e.g., process zone size) controlling the isotropic radiation and/or wave propagation and other effects that mask the source effects.

We develop an earthquake simulator to study the partitioning of seismic/aseismic slip and dynamics of Earthquakes on a Heterogeneous strike-slip Fault (HFQsim) using a generalized model of a discrete fault governed by static/dynamic friction and creep in an elastic half-space. Previous versions of the simulator were shown to produce various realistic seismicity patterns (e.g., frequency-magnitude event statistics, hypocenter and slip distributions, temporal occurrence) using friction levels and creep properties that vary in space but are fixed in time. The new simulator incorporates frictional heat generation by earthquake slip leading to temperature rises, subsequent diffusion cooling into the half space, and time-dependent creep on the fault. The model assumes a power law dependence of creep velocity on the local shear stress, with temperature-dependent coefficients based on the Arrhenius equation. Temperature rises due to seismic slip produce increased aseismic slip, which can lead to further stress concentrations, aftershocks, and heat generation in a feedback loop. The partitioning of seismic/aseismic slip and space-time evolution of seismicity are strongly affected by the temperature changes on the fault. The results are also affected significantly by the difference between the static and kinetic friction levels. The model produces realistic spatio-temporal distribution of seismicity, transient aseismic slip patterns, foreshock-mainshock-aftershock sequences, and a bimodal distribution of earthquakes with background and clustered events similar to observations. The HFQsim may be used to clarify relations between fault properties and different features of seismicity and aseismic slip, and to improve the understanding of failure patterns preceding large earthquakes.

We image the shallow seismic structure across the Southern San Andreas Fault (SSAF) using signals from freight trains and trucks recorded by a dense nodal array, with a linear component perpendicular to SSAF and two 2D subarrays centered on the Banning Fault (BF) and Mission Creek Fault (MCF). Particle motion analysis in the frequency band 2-5 Hz shows that the examined traffic sources can be approximated as moving point sources that primarily induce Rayleigh waves. Using several techniques, we resolve strong lateral variations of Rayleigh wave velocities across the SSAF, including 35% velocity reduction across MCF towards the northeast. Additionally, we derive Q-values and find strong attenuation around the BF and MCF. We further resolve 10% mass density reduction and 45% shear modulus decrease across the MCF. These findings suggest that the MCF is the main strand of the SSAF in the area with important implications for seismic hazard assessments.

An earthquake simulator is developed to study the dynamics of seismicity and seismic/aseismic slip partitioning on a heterogeneous strike-slip fault using a generalized model of a discrete fault governed by static/dynamic friction and creep in an elastic half-space. Previous versions of the simulator were shown to produce various realistic seismicity patterns (e.g., frequency-magnitude event statistics, hypocenter and slip distributions, temporal occurrence) using friction levels and creep properties that vary in space but are fixed in time. The new simulator incorporates frictional heat generation by earthquake slip leading to temperature rises, subsequent diffusion cooling into the half space, and time-dependent creep on the fault. The model assumes a power law dependence of creep velocity on the local shear stress, with temperature-dependent coefficients based on the Arrhenius equation. Temperature rises due to seismic slip produce increased aseismic slip, which can lead to further stress concentrations, aftershocks, and heat generation in a feedback loop. The partitioning of seismic/aseismic slip and space-time evolution of seismicity are strongly affected by the temperature changes on the fault. The results are also affected significantly by the difference between the static and kinetic friction levels. The model produces realistic spatio-temporal distribution of seismicity, transient aseismic slip patterns, foreshock-mainshock-aftershock sequences, and a bimodal distribution of earthquakes with background and clustered events similar to observations. The simulator (EQsim) may be used to clarify relations between fault properties and different features of seismicity and aseismic slip, and to improve the understanding of failure patterns preceding large earthquakes.

We use sparse dictionary learning to develop transformations between seismic velocity models of different resolution and spatial extent. Starting with results in the common region of both models, the method can be used to enhance a regional lower-resolution model to match the style and resolution of local higher-resolution results while preserving its regional coverage. The method is demonstrated by applying it to two-dimensional Vs and three-dimensional VP and VS local and regional velocity models in southern California. The enhanced reconstructed models exhibit clear visual improvements, especially in the reconstructed VP/VS ratios, and better correlations with geological features. We demonstrate the improvements of the reconstructed model relative to the original velocity model by comparing waveform simulation results to observations. The improved fitting to observed waveforms extends beyond the domain of the overlapping region. The developed dictionary learning approach provides physically interpretable results and offers a powerful tool for additional applications for data enhancement in earth sciences.

A document by Grzegorz Kwiatek. Click on the document to view its contents.

Significant imbalances in terrestrial water storage (TWS) and severe drought have been observed around the world as a consequence of climate changes. Improving our ability to monitor TWS and drought is critical for water-resource management and water-deficit estimation. We use continuous seismic ambient noise to monitor temporal evolution of near-surface seismic velocity, dv/v, in central Oklahoma from 2013 to 2022. The derived dv/v is found to be negatively correlated with gravitational measurements and groundwater depths, showing the impact of groundwater storage on seismic velocities. Seasonal cycling of dv/v follows atmospheric temperature changes with a phase shift, which can be explained by thermo-elastic strain in the uppermost crust and sedimentary cover. The occurrences of droughts appear simultaneously with the local peaks of dv/v, demonstrating the sensitivity of near-surface seismic velocities to droughts. The results illustrate the potential of using seismic data for monitoring TWS and drought at regional to local scales.

The spatial organization of deformation may provide key information about the timing of catastrophic failure in the brittle regime. In an ideal homogenous system, deformation may continually localize toward macroscopic failure, and so increasing localization unambiguously signals approaching failure. However, recent analyses demonstrate that deformation, including low magnitude seismicity, and fractures and strain in triaxial compression experiments, experience temporary phases of delocalization superposed on an overall trend of localization toward large failure events. To constrain the conditions that promote delocalization, we perform a series of X-ray tomography experiments at varying confining stresses (5-20 MPa) and fluid pressures (zero to 10 MPa) on Westerly granite cores with varying amounts of preexisting damage. We track the spatial distribution of the strain events with the highest magnitudes of the population within a given time step. The results show that larger confining stress promotes more dilation, and promotes greater localization of the high strain events approaching macroscopic failure. In contrast, greater amounts of preexisting damage promote delocalization. Importantly, the dilative strain experiences more systematic localization than the shear strain, and so may provide more reliable information about the timing of catastrophic failure than the shear strain.

We perform 3D simulations of seismic wavefields to clarify effects of strong reductions of shallow velocities on long-period seismic waves. The simulations use a reference community velocity model of Southern California and a modified version with strong velocity reductions in the top 500 m of the Los Angeles basin. Differences between wavefields generated by ten earthquakes in the reference and perturbed models are analyzed. Velocity changes are estimated by measuring relative time shifts between reference and perturbed seismograms using wavelet cross-correlation spectra. The results indicate that strong localized temporal velocity drops near the surface, such as those observed during strong ground motions, may generate regional perturbations of wavefields at periods up to 20 s. These perturbations may be misinterpreted as generated by temporal changes at seismogenic depths. The results also have important implications for waveform tomography studies.

We quantify the evolving spatial distribution of fracture networks throughout six in situ X-ray tomography triaxial compression experiments on monzonite and granite at confining stresses of 5-35 MPa. We first assess whether one dominant fracture continually grows at the expense of others by tracking the proportion of the maximum fracture volume to the total fracture volume. This metric does not increase monotonically. We next examine if the set of the largest fractures continually dominates deformation by tracking the proportion of the cumulative volume of fractures with volumes >90th percentile to the total fracture volume. This metric indicates that the fracture networks tend to increase in localization toward the largest set of fractures for up to 80% of the experimental time (differential stress), consistent with observations from southern California of localizing and delocalizing seismicity. Experiments with higher confining stress tend to have greater localization. To further assess the fracture networks localization, we compare the geometry of the set of the largest fractures to a plane. We find the best fit plane through the fractures with volumes >90th percentile immediately preceding failure, and calculate the distance between these fractures and the plane, and the r2 score of the fractures and the plane throughout each experiment. The r2 scores and the distance indicate greater localization in the monzonite experiments than in the granite experiments. The smaller mean grain size of the minerals in the granite may produce more sites of fracture nucleation and termination, leading to more delocalized fracture networks that deviate further from a plane. The higher applied confining stress in the monzonite experiments (25-35 MPa) relative to the granite experiments (5-10 MPa) may also contribute to the more localized fracture networks in the monzonite experiments. The evolution of the clustering the fractures toward the plane and the Gini coefficient, which measures the deviation of a population from uniformity, closely match each other. Tracking these metrics of localization also reveals that macroscopic yielding appears to occur when the rate of fracture network localization increases.

Plate motions in Southern California have undergone a transition from compressional and extensional regimes to a dominantly strike-slip regime in the Miocene. Strike-slip motion is most easily accommodated on vertical faults, and major transform fault strands in the region are typically mapped as near-vertical on the surface. However, some previous work suggests these faults have a dipping or listric geometry at depth. We analyze receiver function arrivals that vary harmonically with backazimuth at all available broadband stations in the region. The results show a dominant signal from contrasts in dipping foliation as well as dipping isotropic contrasts from all crustal depths, including from the ductile middle to lower crust. We interpret these receiver function observations as a dipping fault-parallel structural fabric that is pervasive throughout the region. The strike of these structures and fabrics is parallel to that of nearby fault surface traces. We also plot microseismicity on depth profiles perpendicular to major strike-slip faults and find consistently NE-dipping lineations in seismicity shallowing in dip from near vertical (80-85) on the Elsinore Fault near the coastal ranges to 60-65 slightly further inland on the San Jacinto Fault to 50-55 on the San Andreas Fault. Taken together, the dipping features in seismicity and in rock fabric suggest that preexisting fabrics and faults likely act as strain guides in the modern slip regime, with reactivation-like mechanisms operating both above and below the brittle-ductile transition.