Many unknowns exist regarding the energy radiation processes of the inland low-frequency earthquakes (LFEs) often observed beneath volcanoes. To evaluate their energy radiation characteristics, we estimated the scaled energy for LFEs and regular earthquakes in and around the focal area of the 2008 Mw 6.9 Iwate-Miyagi earthquake. We computed the source spectra for regular earthquakes, deep LFEs, and shallow LFEs by correcting for the site and path effects from direct S-waves. We computed the radiated energy and seismic moments, and obtained the scaled energy (eR) for 1464 regular earthquakes, 169 deep LFEs, and 52 shallow LFEs. The eR for regular earthquakes is in the order of 10-5 to 10-4, typical for crustal earthquakes, and tends to become smaller near volcanoes and shallow LFEs. In contrast, eR is in the order of 10-7 and 10-6 for deep and shallow LFEs, respectively, one to three orders of magnitude smaller than that for regular earthquakes. This result suggests that LFEs are associated with a much lower stress drop and/or slower rupture and deformation rates than regular earthquakes. Although the energy magnitudes derived from radiated energy generally show good agreement with the local magnitudes for the three types of earthquakes, the moment and local magnitudes show a large discrepancy for the LFEs. This suggests that the local magnitude based only on the maximum amplitude of the observed seismic records may not provide good information on the static sizes of LFEs whose eR values are substantially different from those of regular earthquakes.

Stress accumulation and release in the crust remains poorly understood compared to that at the plate boundaries. Spatiotemporal variations in foreshock and aftershock activities can provide key constraints on time-dependent stress and deformation processes in the crust. The 2017 M5.2 Akita-Daisen intraplate earthquake in NE Japan was preceded by intense foreshock activity and triggered a strong sequence of aftershocks. We examine the spatiotemporal distributions of foreshocks and aftershocks and determine the coseismic slip distribution of the mainshock. Our results indicate that seismicity both before and after the mainshock was concentrated on a planar structure with N-S strike that dips steeply eastward. We observe a migration of foreshocks towards the mainshock rupture area, suggesting that foreshocks were triggered by aseismic phenomena preceding the mainshock. The mainshock rupture propagated toward the north, showing less slip beneath foreshock regions. The stress drop of the mainshock was 1.4 MPa and the radiation efficiency was 0.72. Aftershocks were intensely triggered near the edge of large coseismic slip regions where shear stress increased. The aftershock region expanded along the fault strike, which is attributed to the post-seismic aseismic slip of the mainshock. The postseismic slip possibly triggered repeating earthquakes with M ~3. We find that the foreshocks, mainshock, aftershocks, and post-seismic slip released stress at different segments along the fault, which may reflect differences in frictional properties. Obtained results were similar to those observed for interplate earthquakes, which supports the hypothesis that the deformation processes along plate boundaries and crustal faults are fundamentally the same.

Monitoring of in-situ, stress-induced, seismic velocity change provides an increasingly important contribution to the study of the earthquake nucleation process. Continuous Active-Source Seismic Monitoring (CASSM) with borehole sources and sensors has proven to be a very effective tool to monitor seismic velocity and to identify its temporal variations at depth. Since June 2017, we have been operating a crosswell CASSM field experiment at the San Andreas Fault Observatory at Depth (SAFOD) where a previous CASSM experiment identified the two seismic velocity reductions approximately 10 and 2 hours before micro-earthquakes. The ultimate goal of our experiment is to continuously monitor tectonic stress for the San Andreas Fault near seismogenic depth. Our active-source experiment makes use of two boreholes drilled at the SAFOD project site. A piezoelectric source and a three-component accelerometer have been installed in the SAFOD pilot and main holes, respectively, at about 1 km depth. A seismic pulse is generated by the piezoelectric source four times per second, and waveforms are recorded with a 48 kHz sample rate, with recordings summed for 1 to 10 minutes to capture seismic velocity changes at a high-temporal resolution. Since deployment in June 2017, and as of July, 2019, local seismicity has not been above our current threshold of detection. However, we have identified a velocity reduction at the SAFOD site (0.5 microsecond change in crosswell travel time, measured in a coda window) possibly induced by dynamic stress changes from the distant 6 July 2019 M 7.1 Ridgecrest earthquake, California. We will characterize and report the co-seismic change and post-seismic recovery process for this remotely triggered velocity change. We will also report on the overall status of this unique CASSM experiment.

The Calaveras Fault (CF) branches from the San Andreas Fault (SAF) near San Benito, extending sub-parallel to the SAF for about 50 km with only 2-6 km separation and diverging northeastward. Both the SAF and CF are partially coupled, exhibit spatially variable aseismic creep and have hosted moderate to large earthquakes in recent decades. Understanding how slip partitions among the main fault strands of the SAF system and establishing their degree of coupling is crucial for seismic hazard evaluation. We perform a timeseries analysis using more than 5 years of Sentinel-1 data covering the Bay Area (May 2015-October 2020), specifically targeting the spatiotemporal variations of creep rates around the SAF-CF junction. We derive the surface creep rates from cross-fault InSAR timeseries differences along the SAF and CF including adjacent Sargent and Quien Sabe Faults. We show that the variable creep rates (0-20 mm/yr) at the SAF-CF junction are to first order controlled by the angle between the fault strike and the background stress orientation. We further examine the spatiotemporal variation of creep rates along the SAF and CF and find a multi-annual coupling increase during 2016-2018 the subparallel sections of both faults, with the CF coupling change lagging behind the SAF by 3 to 6 months. Similar temporal variations are also observed in both b-values inferred from declustered seismicity and aseismic slip rates inferred from characteristic repeating earthquakes. The high correlation of b-value and slip-rate changes may indicate that the SAF is extremely sensitive to small stress perturbations.

On 20 March 2012, a Mw 7.5 thrust earthquake started a series of large events up to magnitude Mw 8.2 (2017) that struck central Mexico during a period of nine years. Before this event, the Mexican subduction zone did not experience subduction earthquakes (Mw > 7.0) for at least 12 years. Most of the events during this highly active period (2012-) occurred in the plate interface, resulting in a significantly larger interplate slip rate in the states of Oaxaca and Guerrero. In this study, we explore how the aseismic slip transient caused by the 2012 Mw 7.5 earthquake affected the region and whether this earthquake had a causal relationship with a nearby similar magnitude event that occurred in 2018 (Mw 7.2). To this end, we identified and analyzed characteristic repeating earthquakes along the Mexican subduction zone for assessing the plate interface slip history and found a notably increase in the aseismic slip rate inferred from repeating earthquake activity following the 2012 mainshock, which suggests a long-standing slip perturbation in Oaxaca near the trench that continued until the 2018 Mw 7.2 Pinotepa Nacional earthquake.