Search for: All records

Earthquakes may seem random, but are often concentrated in some localized areas. Thus, they are likely controlled by fault materials and stress heterogeneity, which are little understood. Here, we provide high-resolution observations of fault material and stress heterogeneity in the Japan subduction zone through an integration of material and source imaging with numerical simulations. Our results present evidence for localized, anisotropic structures with a near-zero Poisson’s ratio in the medium surrounding 1 to 2 kilometer–sized earthquake clusters, suggesting that the fault medium is damaged, foliated, and enriched with fluid. Such localized structures may cause stress perturbations on faults that in turn favor the frequent occurrence of deep interplate earthquakes at depths of 60 to 70 kilometers. Therefore, identifying the distribution and properties of fault material heterogeneity is important for more informed assessment of earthquake hazards. 

ABSTRACT Precise knowledge of earthquake magnitudes is vital for accurate characterization of seismic hazards. However, the estimation of earthquake magnitude, particularly for small events, is complicated by differences in network procedures and completeness. This produces disparate magnitude estimates for the same event and emphasizes the need for a consistent and transportable magnitude estimation procedure. Here, we investigate the use of the relative magnitude method, which measures earthquake magnitude from a least-squares inversion of interlinked waveform amplitude ratios. Our results show that that the relative magnitude method can establish both local and moment magnitudes for many events in the 2019 Ridgecrest sequence. The method also provides constraints on moment magnitude estimates for M <3 events, which are not routinely available using current methods. Although the relative magnitude method is advantageous because it can be applied uniformly in various regions and does not require empirical distance or attenuation corrections, there are several parameters that require subjective decision making and may introduce bias in the resulting magnitude estimates. These include acceptable thresholds for signal-to-noise ratios and cross correlation, filtering procedures, sampling windows, and station selection. Here, we not only calculate magnitude but also investigate how the subjective decision making affects the resulting magnitudes. Based on our analysis, we present recommendations to enhance the utility of this method for future users. 

A precise understanding of earthquake magnitudes is vital for accurate calculations of magnitude exceedance probabilities and seismic hazard assessment. However, characterization of earthquake magnitude, particularly for small events, is complicated by differences in network capabilities and procedures. Furthermore, the use of differing magnitude scales for events of various sizes introduces additional challenges and produces disparate magnitude estimates for the same events. To address the need for a consistent magnitude estimation procedure that can accurately estimate magnitude across a wide magnitude range and in diverse tectonic environments, we investigate the use of the relative magnitude method. This approach utilizes amplitude ratios of highly correlated waveforms among numerous interlinked event pairs to compute magnitude for a group of events. While the relative magnitude method is advantageous because it can be applied uniformly in various regions and does not require distance or attenuation corrections, there are several parameters that currently require human decision which may introduce bias. These include acceptable thresholds for signal-to-noise ratios and cross-correlation, filtering procedures, sampling windows, and station selection. Our research focuses on computing new relative magnitudes for events in southern California, including the 2019 Ridgecrest sequence. We investigate the uncertainty that human decision may impose on the resulting magnitudes and compare our results to other magnitude estimation methods. Finally, we present our recommendations for routine procedures that minimize uncertainty and variability in the relative magnitude method, aiming to enhance the utility of this method for future users. 

Abstract Earthquake nucleation is a crucial preparation process of the following coseismic rupture propagation. Under the framework of rate‐and‐state friction (RSF), it was found that the ratios of to parameters control whether earthquakes nucleate as an expanding crack or with a fixed length prior to the dynamic instability. However, the characteristic weakening distance controls the weakening efficiency of state variables in RSF and can influence the nucleation styles as well. Here we investigate the effects of on nucleation styles in the context of fully dynamic seismic cycles by evaluating the evolution of the nucleation zone quantitatively when it accelerates from the tectonic loading rate to seismic slip velocity. A larger (>0.75) is needed to produce expanding crack nucleation styles for relatively small , which suggests that fixed length nucleation styles may dominate on natural and laboratory faults. Furthermore, we find a more complex nucleation style when the nucleation site is not in the center of the asperity and identify a twin‐like nucleation style which includes two initial acceleration phases. We conclude that the earthquake nucleation style is strongly controlled by the value of . The possible dominance of fixed length nucleation styles suggests that the minimum size of earthquake rupture may be estimated at the early stage of the nucleation phase. 

Earthquake nucleation is a crucial preparation process of the following coseismic rupture propagation. Under the framework of rate-and-state friction, it was found that the ratios of a to b parameters control whether earthquakes nucleate as an expanding crack or a fixed length patch. However, as an essential parameter in earthquake physics, critical slip distance DRS controls the weakening efficiency of fault strength and can influence the nucleation styles. Here we investigate the effects of DRS on nucleation styles in the context of fully dynamic seismic cycles by evaluating the evolution of the nucleation zone quantitatively when it accelerates from the tectonic loading rate to seismic slip velocity. The inferred values of DRS from small-scale laboratory faults are 1-100 μm, several orders smaller than those obtained from geophysical observations on large natural faults. Considering the scale-dependence of widely observed DRS, the ratio of DRS to velocity weakening asperity size W is applied to substitute the absolute value of DRS in this study. We find when DRS/W is relatively large (~10-5), a/b=0.5 can separate two nucleation styles as found previously. For a relatively small DRS/W (~10-6), however, a/b larger than 0.7 is necessary to produce the typical expanding crack-like nucleation style. When DRS/W<4x10-7 and a/b<0.8, the fixed length nucleation style dominates. For some cases with a/b>0.75, the initial yielding phase accelerates to a considerable slip velocity just before the subsequent expanding fracture phase, which may explain the generation of foreshock activities. Specially, the first yielding phase is possible to trigger dynamic events without a secondary fracture phase. Furthermore, when the nucleation site is not in the middle of the asperity, large enough a/b (e.g., 0.8) could induce a complex nucleation style as well as abundant interseismic aseismic transients. We also recognize a special twin nucleation style that incorporates a failed acceleration phase. Our results reveal the critical role of DRS on earthquake nucleation styles and suggest that the fixed length nucleation style may be more common for the range of DRS/W (~10-4-~10-7) observed on natural and laboratory faults. 

Stress drop, a crucial source parameter in earthquake studies, significantly influences ground motion prediction and seismic hazard assessment. Despite several existing methods to estimate stress drops, the resulting stress drop estimates often exhibit a wide variation of up to 3-4 orders of magnitude. In this study, we address the robustness of stress drop estimation by introducing a point-wise spectral ratio stacking approach based on empirical Green’s functions (eGfs). Conventional trace-wise stacking can lead to data exclusion due to high signal-to-noise ratio requirements across a wide range of frequency. By adopting point-wise stacking, we maximize the utilization of useful recording information, leading to more accurate stress drop estimates. We applied the point-wise spectral ratio stacking method to a comprehensive dataset comprising global earthquakes from 1990 to 2020 with magnitude larger than Mw5.5 and depth shallower than 50 km. We first verified the moment magnitudes of earthquakes estimated from the resulting seismic moment ratios. We found that the moment magnitude of master events best consistent with catalog magnitudes when the magnitude difference between master and their eGfs differs by about 0.5. Our analysis indicates that stress drop of shallow earthquakes exhibits no depth dependence, while showing a slight increase with magnitude. The results obtained through our optimized stacking process shed new light on stress drop estimate of shallow earthquakes and have the potential to enhance the understanding of earthquake mechanics. 

Fault damage zones can influence various aspects of the earthquake cycle, such as the recurrence intervals and magnitudes of large earthquakes. Hence, our research aims to develop a novel method to image fault damage zones using high-frequency P-waves reflected within them. Previous studies have demonstrated that fault damage zones can amplify high-frequency waves along directions close to fault strike. The associated frequency band of the amplified secondary peak may be used to estimate the width and velocity contrast of the fault damage zone. Here we use the stacked P-wave velocity spectra of M1.5–3 earthquakes in the Parkfield region to identify the azimuthal variation in high-frequency energy. Our preliminary results show that for 62% of the Parkfield clusters, stations close to the fault strike record more high-frequency energies around 10–20 Hz. The frequency band is lower than what we observed for the 2019 Ridgecrest earthquakes region, and corresponds to a fault zone velocity reduction of ~50% assuming a fault zone width of 200m. We also observe along-strike differences in our results, where clusters along some fault sections show greater azimuthal variation than clusters in other sections. Moreover, to account for the possible effects of site conditions underneath the stations, we will quantify their effects using the spectra of regional earthquakes. We will compute the root-mean-square spectra at different frequency bands for each event, and calculate the average deviation in spectra at each station. We can then generate an empirical correction term for each station as a function of frequency. By applying these corrections to the stacked P-wave velocity spectra of our earthquake clusters, we can separate the contribution of site effects from fault zone structures. Our results demonstrate that the new method can be applied to search for fault damage zone structures in different tectonic regions with broadband stations in order to enhance our understanding of the co-evolution of fault zones and earthquake cycle. 

Accurate estimates of earthquake magnitude are necessary to improve our understanding of seismic hazard. Unbiased magnitudes for small earthquakes are especially important because magnitude exceedance probabilities for large earthquakes are derived from the behavior of small earthquakes. Also, accurate characterization of small events is becoming increasingly important for ground motion models. However, catalog magnitudes may vary for the same event depending on network procedures and capabilities. In addition, different magnitude scales are often used for events of varying sizes. For example, moment magnitude (Mw) is the widely preferred estimate for earthquake size but it is often not available for small earthquakes (M < 3.5). As a result, statistical measures such as magnitude frequency distribution (MFD) and b-value can be biased depending on magnitude type and uncertainties that arise during the measurement process. In this research we demonstrate the capability of the relative magnitude method to provide a uniform and accurate estimate of earthquake magnitude in a variety of regions, while only requiring the use of waveform data. The study regions include the Permian Basin in Texas, central Oklahoma, and southern California. We present results in which only relative magnitudes are used to estimate MFD and b-value as well as relative magnitudes that are benchmarked to an absolute scale using a coda-envelope derived Mw calibration for small events. We also discuss potential sources of uncertainty in the relative magnitude method such as acceptable signal-to-noise ratios, cross-correlation thresholds, and choice of scaling constant, as well as our attempts to mitigate those uncertainties. 

ABSTRACT We present initial findings from the ongoing Community Stress Drop Validation Study to compare spectral stress-drop estimates for earthquakes in the 2019 Ridgecrest, California, sequence. This study uses a unified dataset to independently estimate earthquake source parameters through various methods. Stress drop, which denotes the change in average shear stress along a fault during earthquake rupture, is a critical parameter in earthquake science, impacting ground motion, rupture simulation, and source physics. Spectral stress drop is commonly derived by fitting the amplitude-spectrum shape, but estimates can vary substantially across studies for individual earthquakes. Sponsored jointly by the U.S. Geological Survey and the Statewide (previously, Southern) California Earthquake Center our community study aims to elucidate sources of variability and uncertainty in earthquake spectral stress-drop estimates through quantitative comparison of submitted results from independent analyses. The dataset includes nearly 13,000 earthquakes ranging from M 1 to 7 during a two-week period of the 2019 Ridgecrest sequence, recorded within a 1° radius. In this article, we report on 56 unique submissions received from 20 different groups, detailing spectral corner frequencies (or source durations), moment magnitudes, and estimated spectral stress drops. Methods employed encompass spectral ratio analysis, spectral decomposition and inversion, finite-fault modeling, ground-motion-based approaches, and combined methods. Initial analysis reveals significant scatter across submitted spectral stress drops spanning over six orders of magnitude. However, we can identify between-method trends and offsets within the data to mitigate this variability. Averaging submissions for a prioritized subset of 56 events shows reduced variability of spectral stress drop, indicating overall consistency in recovered spectral stress-drop values.