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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. 

Fault damage zones can influence various aspects of the earthquake cycle, such as the recurrence intervals and magnitudes of large earthquakes. The properties and structure of fault damage zones are often characterized using dense arrays of seismic stations located directly above the faults. However, such arrays may not always be available. Hence, our research aims to develop a novel method to image fault damage zones using broadband stations at relatively larger distances. Previous kinematic simulations and a case study of the 2003 Big Bear earthquake sequence demonstrated that fault damage zones can act as effective waveguides, amplifying high-frequency waves along directions close to fault strike via multiple reflections within the fault damage zone. The amplified high-frequency energy can be observed by stacking P-wave spectra of earthquake clusters with highly-similar waveforms (Huang et al., 2016), and the frequency band which is amplified may be used to estimate the width and velocity contrast of the fault damage zone. We attempt to identify the high-frequency peak associated with fault zone waves in stacked spectra by conducting a large-scale study of small earthquakes (M1.5–3). We use high quality broadband data from seismic stations at hypocentral distances of 20-80 km in the 2019 Ridgecrest earthquake regions. First, we group the Ridgecrest earthquakes in clusters by their locations and their waveform similarity, and then stack their velocity spectra to average the source effects of individual earthquakes. Our results show that the stations close to the fault strike record more high-frequency energies around the characteristic frequency of fault zone reflections. We find that the increase in the amount of high-frequencies is consistent across clusters with average magnitudes ranging from 1.6-2.4, which suggests that the azimuthal variation in spectra is caused by fault zone amplification rather than rupture directivity. We will apply our method to other fault zones in California, in order to search for fault damage zone structures and estimate their material properties. 

The Salton Sea Geothermal Field (SSGF) is one of the most seismically active and geothermally productive fields in California. Here we present a detailed analysis of short-term seismicity change in SSGF from 2008 to 2013 during and right following large distant earthquakes, as well as long-term seismicity change due to geothermal productions. We first apply a GPU-based waveform matched-filter technique (WMFT) to the continuous data recorded by the Calenergy Borehole (EN) Network and detect more than 70 000 new micro-earthquakes than listed in the standard Southern California Seismic Network catalogue. We then analyse the seismicity rate changes in the SSGF associated with transient stress fluctuations triggered by regional and large teleseismic earthquakes from 1999 to 2019. We find triggered seismicity in the SSGF following seven regional M > 5.5 earthquakes. In comparison, most teleseismic earthquakes with M > 8.0 did not trigger significant seismicity rate change in the SSGF, likely indicating a frequency dependence in remote dynamic triggering. We further characterize the correlation between the long-term seismicity rate and geothermal production rates, and the temporal and spatial distribution of Guttenberg–Richter b-values inside and outside the SSGF with the newly detected catalogue. The long-term seismicity shows that events with M > 1.5 are likely correlated with net production rates, while smaller events do not show any correlation. The b-values inside the SSGF are higher than those outside the SSGF, and the locations of dynamically triggered events are close to locations with high b-values.

 

Fault damage zones can influence various aspects of the earthquake cycle, such as the recurrence intervals and magnitudes of large earthquakes. The properties and structure of fault damage zones are often characterized using dense arrays of seismic stations located directly above the faults. However, such arrays may not always be available. Hence, our research aims to develop a novel method to image fault damage zones using broadband stations at relatively larger distances. Previous kinematic simulations and a case study of the 2003 Big Bear earthquake sequence demonstrated that fault damage zones can act as effective waveguides, amplifying high-frequency waves along directions close to fault strike via multiple reflections within the fault damage zone. The amplified high-frequency energy can be observed using the stacked P-wave spectra of earthquake clusters with highly-similar waveforms (Huang et al., 2016). We attempt to identify the high-frequency peak associated with fault zone waves in stacked spectra by conducting a large-scale study of small earthquakes (M1.5–3). We use high quality broadband data from seismic stations at hypocentral distances of 20-100km in the 2004 Parkfield and 2019 Ridgecrest earthquake regions. First, we group earthquakes in clusters by their locations and their waveform similarity, and then stack their velocity spectra to average the source effects of individual earthquakes. We applied our method to the 2019 Ridgecrest earthquake sequence, and our preliminary results show that stations close to the fault strike tend to record more high-frequency energies around the characteristic frequency of fault zone reflections. The frequency bands in which amplified high-frequency energies are observed may be used to estimate the width and velocity contrast of the fault damage zone. We aim to develop a robust and versatile method that can be used to search for fault damage zone structures and estimate their material properties, in order to shed light on earthquake source processes. 

Abstract The border between Georgia and South Carolina has a moderate amount of seismicity typical of the Piedmont Province of the eastern United States and greater than most other intraplate regions. Historical records suggest on average a Mw 4.5 earthquake every 50 yr in the region of the J. Strom Thurmond Reservoir, which is located on the border between Georgia and South Carolina. The Mw 4.1 earthquake on 15 February 2014 near Edgefield, South Carolina, was one of the largest events in this region recorded by nearby modern seismometers, providing an opportunity to study its source properties and aftershock productivity. Using the waveforms of the Mw 4.1 mainshock and the only cataloged Mw 3.0 aftershock as templates, we apply a matched‐filter technique to search for additional events between 8 and 22 February 2014. The resulting six new detections are further employed as new templates to scan for more events. Repeating the waveform‐matching method with new templates yields 13 additional events, for a total of 19 previously unidentified events with magnitude 0.06 and larger. The low number of events suggests that this sequence is deficient in aftershock production, as compared with expected aftershock productivities for other mainshocks of similar magnitudes. Hypocentral depths of the Mw 4.1 mainshock and Mw 3.0 aftershock are estimated by examining the differential time between a depth phase called sPL and P‐wave arrivals, as well as by modeling the depth phase of body waves at shorter periods. The best‐fitting depths for both events are around 3–4 km. The obtained stress drops for the Mw 4.1 mainshock and Mw 3.0 aftershock are 3.75 and 4.44 MPa, respectively. The corresponding updated moment magnitude for the aftershock is 2.91. 

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 M_w7.2 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.