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Dozens of researchers deployed nearly 2,000 seismic stations—and a T-Rex—to better illuminate subsurface structure and magma storage below the summit of the highly active volcano. 

Abstract The 2019 Mw 7.1 Ridgecrest earthquake was the largest event in California over the past 20 years. The earthquake was preceded by a sequence of foreshocks. However, the physical processes leading to the mainshock remain unclear. Here, we image the ratios of compressional (P)‐ to shear (S)‐wave velocity (V_p/V_s) in the fault zones and examine the spatial and temporal evolution of near‐source material properties during the Ridgecrest earthquake sequence. We find that theV_p/V_sratios are spatially homogeneous in the rupture zones, indicating a lack of fault‐zone material difference along strike. We identify an anomalously lowV_p/V_sratio fault patch near the mainshock hypocenter before its occurrence, which returned to the background value after the earthquake. This lowV_p/V_sratio suggests fluid overpressure, which may have facilitated the nucleation of the 2019 Ridgecrest mainshock. 

Abstract We present the high-resolution Parkfield matched filter relocated earthquake (PKD-MR) catalog for the 2004 Mw 6 Parkfield earthquake sequence in central California. We use high-quality seismic data recorded by the borehole High Resolution Seismic Network combined with matched filter detection and relocations from cross-correlation derived differential travel times. We determine the magnitudes of newly detected events by computing the amplitude ratio between the detections and templates using a principal component fit. The relocated catalog spans from 6 November 2003 to 28 March 2005 and contains 13,914 earthquakes, which is about three times the number of events listed in the Northern California Seismic Network catalog. Our results on the seismicity rate changes before the 2004 mainshock do not show clear precursory signals, although we find an increase in the seismic activity in the creeping section of the San Andreas fault (SAF) (about ∼30 km northwest of the mainshock epicenter) in the weeks prior to the mainshock. We also observe a decrease in the b-value parameter in the Gutenberg–Richter relationship in the creeping section in the weeks prior to the mainshock. Our results suggest stress is increasingly released seismically in the creeping section, accompanied by a decreasing aseismic creeping rate before the mainshock occurrence. However, b-value and seismicity rates remain stable in the Parkfield section where the 2004 mainshock ruptured. This updated catalog can be used to study the evolution of aftershocks and their relations to afterslip following the 2004 Parkfield mainshock, seismicity before the mainshock, and how external stresses interact with the Parkfield section of the SAF and the 2004 sequence. 

SUMMARY We present our estimations and comparisons of the in situ Vp/Vs ratios and seismicity characteristics for the Parkfield segment of the San Andreas fault in northern California and the San Jacinto Fault Zone and its adjacent regions in southern California. Our results show that the high-resolution in situ Vp/Vs ratios are much more complex than the tomographic Vp/Vs models. They show similar variation patterns to those in the tomographic Vp models, indicating that Vp/Vs ratios are controlled by material properties but are also strongly influenced by fluid contents. In Parkfield, we observe velocity contrasts between the creeping and locked sections. In southern California, we see small-scale anomalous Vp/Vs variation patterns, especially where fault segments intersect, terminate and change orientations. In addition, our investigation confirms that the seismicity in Parkfield is more repeatable than in southern California. However, the earthquakes in the southernmost portion of the San Andreas fault, the trifurcation area of the San Jacinto Fault Zone and the Imperial fault are as much likely falling into clusters as those in Parkfield. The correlation of highly similar events with anomalous in situ Vp/Vs ratios supports the important role of fluids in the occurrence of repeating earthquakes. The high-resolution Vp/Vs ratio estimation method and the corresponding results are helpful for revealing roles of fluids in driving earthquake, fault interaction and stress distribution in fault zones. 

Abstract Puerto Rico is a highly seismically active island, where several damaging historical earthquakes have occurred and frequent small events persist. It situates at the boundary between the Caribbean and North American plates, featuring a complex fault system. Here, we investigate the seismotectonic crustal structure of the island by interpreting the 3D compressional-wave velocity VP and compressional- to shear-wave velocity ratio VP/VS models and by analyzing the distribution of the relocated earthquakes. The 3D velocity models are obtained by applying the simul2000 tomographic inversion algorithm based on the phase arrivals recorded by the Puerto Rico seismic network. We find high-VP and low-VP/VS anomalies in the eastern and central province between the Great Northern Puerto Rico fault zone and the Great Southern Puerto Rico fault zone, correlating with the Utuado pluton. Further, there are low-VP anomalies beneath both the Great Southern Puerto Rico fault zone and the South Lajas fault, indicating northerly dipping structures from the southwest to the northwest of the island. We relocate 19,095 earthquakes from May 2017 to April 2021 using the new 3D velocity model and waveform cross-correlation data. The relocated seismicity shows trends along the Investigator fault, the Ponce faults, the Guayanilla rift, and the Punta Montalva fault. The majority of the 2019–2021 Southwestern Puerto Rico earthquakes are associated with the Punta Montalva fault. Earthquakes forming 17° northward-dipping structures at various depths possibly manifest continuation of the Muertos trough, along which the Caribbean plate is being subducted beneath the Puerto Rico microplate. Our results show complex fault geometries of a diffuse fault network, suggesting possible subduction process accommodated by faults within a low-velocity zone. 

ABSTRACT The 1989 Mw 6.9 Loma Prieta earthquake is the first major event to occur along the San Andreas fault (SAF) zone in central California since the 1906 M 7.9 San Francisco earthquake. Given the complexity of this event, uncertainty has persisted as to whether this earthquake ruptured the SAF itself or a secondary fault. Recent work on the SAF in the Coachella Valley in southern California has revealed similar complexity, arising from a nonplanar, nonvertical fault geometry, and has led us to reexamine the Loma Prieta event. We have compiled data sets and data analyses in the vicinity of the Loma Prieta earthquake, including the 3D seismic velocity model and aftershock relocations of Lin and Thurber (2012), potential field data collected by the U.S. Geological Survey following the earthquake, and seismic refraction and reflection data from the 1991 profile of Catchings et al. (2004). The velocity model and aftershock relocations of Lin and Thurber (2012) reveal a geometry for the SAF that appears similar to that in the Coachella Valley (although rotated 180°): at Loma Prieta the fault dips steeply near the surface and curves with depth to join the moderately southwest-dipping main rupture below 6 km depth, itself also nonplanar. The SAF is a clear velocity boundary, with higher velocities on the northeast, attributable to Mesozoic accretionary and other rocks, and lower velocities on the southwest, attributable to Cenozoic sedimentary and volcanic rocks of the La Honda block. Rocks of the La Honda block have been offset right-laterally hundreds of kilometers from similar rocks in the southern San Joaquin Valley and vicinity, providing evidence that the curved northeast fault boundary of this block is the plate boundary. Thus, we interpret that the Loma Prieta earthquake occurred on the SAF and not on a secondary fault. 

SUMMARY In the Gulf of California, Mexico, the relative motion across the North America–Pacific boundary is accommodated by a series of marine transform faults and spreading centres. About 40 M> 6 earthquakes have occurred in the region since 1960. On 2009 August 3, an Mw 6.9 earthquake occurred near Canal de Ballenas in the region. The earthquake was a strike-slip event with a shallow hypocentre that is likely close to the seafloor. In contrast to an adjacent M7 earthquake, this earthquake triggered a ground-motion-based earthquake early warning algorithm being tested in southern California (∼600 km away). This observation suggests that the abnormally large ground motions and dynamic strains observed for this earthquake relate to its rupture properties. To investigate this possibility, we image the rupture process and resolve the slip distribution of the event using a P-wave backprojection approach and a teleseismic, finite-fault inversion method. Results from these two independent analyses indicate a relatively simple, unilateral rupture propagation directed along-strike in the northward direction. However, the average rupture speed is estimated around 4 km s−1, suggesting a possible supershear rupture. The supershear speed is also supported by a Rayleigh wave Mach cone analysis, although uncertainties in local velocity structure preclude a definitive conclusion. The Canal de Ballenas earthquake dynamically triggered seismicity at multiple sites in California, with triggering response characteristics varying from location-to-location. For instance, some of the triggered earthquakes in California occurred up to 24 hr later, suggesting that nonlinear triggering mechanisms likely have modulated their occurrence. 

Abstract I present a high-precision earthquake relocation catalog and first-motion focal mechanisms before and during the 2019 Ridgecrest earthquake sequence in eastern California. I obtain phase arrivals, first-motion polarities, and waveform data from the Southern California Earthquake Data Center for more than 24,000 earthquakes with the magnitudes varying between −0.7 and 7.1 from 1 January to 31 July 2019. I first relocate all the earthquakes using phase arrivals through a previously developed 3D seismic-velocity model and then improve relative location accuracies using differential times from waveform cross correlation. The majority of the relocated seismicity is distributed above 12 km depth. The seismicity migration along the northwest–southeast direction can be clearly seen with an aseismic zone near the Coso volcanic field. Focal mechanisms are solved for all the relocated events based on the first-motion polarity data with dominant strike-slip fault solutions. The Mw 6.4 and 7.1 earthquakes are positioned at 12.45 and 4.16 km depths after the 3D relocation, respectively, with strike-slip focal solutions. These results can help our understanding of the 2019 Ridgecrest earthquake sequence and can be used in other seismological and geophysical studies.