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Abstract Dynamic triggering of earthquakes is when seismic waves from earthquakes induce seismic activity at a distance. The observability of the seismic wave stresses and their results presents a unique opportunity to understand earthquake interactions and associated hazard implications. The extent and timing of dynamic triggering at given specific stress changes still remain inadequately predicted due to limited studies and data sets. In particular, the requirement for complete, well‐characterized catalogs to detect triggering systematically seriously limits the types of studies possible. To address this, we utilized 7‐year continuous waveform data from 239 stations in southern California and used PhaseNet for phase picking to identify local earthquakes and measure triggering without constructing any earthquake catalog. We map the triggering intensity over the region and find that overall, the Mojave segment of the San Andreas is the most easily triggered region. However, the spatial pattern changes after the Ridgecrest earthquake and the area appears to become much less prone to triggering, likely due to an exhaustion of the faults near failure in the immediate aftermath of the Ridgecrest sequence. We further observe a slow decay rate of dynamic triggering and conclude that low‐frequency waves (0.04–0.1 Hz) may be more effective in dynamic triggering than high‐frequency waves (1–3 Hz) which is consistent with a rate‐state assisted aseismic creep or hydrological triggering mechanism. 

Abstract Earthquakes rarely occur in isolation but rather as complex sequences of fore, main and aftershocks. Assessing the associated seismic hazard requires a holistic view of event interactions. We conduct frictional sliding experiments on faulted Westerly Granite samples at mid‐crustal stresses to investigate fault damage and roughness effects on aftershock generation. Abrupt laboratory fault slip is followed by periods of extended stress relaxation and aftershocks. Large roughness promotes less co‐seismic slip and high aftershock activity whereas smooth faults promote high co‐seismic slip with few aftershocks. Conditions close to slip instability generate lab‐quake sequences that exhibit similar statistical distributions to natural earthquakes. Aftershock productivity in the lab is linearly related to the residual strain energy on the fault which, in turn, is controlled by the level of surface heterogeneity. We conclude that roughness and damage govern slip stability and seismic energy partitioning between fore, main and aftershocks in lab and nature. 

Abstract Seismic moment and rupture length can be combined to infer stress drop, a key parameter for assessing earthquakes. In natural earthquakes, stress drops are largely depth‐independent, which is surprising given the expected dependence of frictional stress on normal stresses and hence overburden. We have developed a transparent experimental fault that allows direct observation of thousands of slip events, with ruptures that are fully contained within the fault. Surprisingly, the observed stress drops are largely independent of both the magnitude of normal stress and its heterogeneity, capturing the independence seen in nature. However, we observe larger, normal stress‐dependent stress drops when the fault area is reduced, which allows slip events to frequently reach the edge of the interface. We conclude that confined ruptures have normal stress independent stress drops, and thus the depth‐independent stress drops of tectonic earthquakes may be a consequence of their confined nature. 

Abstract Seismology is witnessing explosive growth in the diversity and scale of earthquake catalogs. A key motivation for this community effort is that more data should translate into better earthquake forecasts. Such improvements are yet to be seen. Here, we introduce the Recurrent Earthquake foreCAST (RECAST), a deep‐learning model based on recent developments in neural temporal point processes. The model enables access to a greater volume and diversity of earthquake observations, overcoming the theoretical and computational limitations of traditional approaches. We benchmark against a temporal Epidemic Type Aftershock Sequence model. Tests on synthetic data suggest that with a modest‐sized data set, RECAST accurately models earthquake‐like point processes directly from cataloged data. Tests on earthquake catalogs in Southern California indicate improved fit and forecast accuracy compared to our benchmark when the training set is sufficiently long (>10⁴events). The basic components in RECAST add flexibility and scalability for earthquake forecasting without sacrificing performance. 

Abstract The number of aftershocks increases with mainshock size following a well‐defined scaling law. However, excursions from the average behavior are common. This variability is particularly concerning for large earthquakes where the number of aftershocks varies by factors of 100 for mainshocks of comparable magnitude. Do observable factors lead to differences in aftershock behavior? We examine aftershock productivity relative to the global average for all mainshocks () from 1990 to 2019. A global map of earthquake productivity highlights the influence of tectonic regimes. Earthquake depth, lithosphere age, and plate boundary type correspond well with earthquake productivity. We investigate the role of mainshock attributes by compiling source dimensions, radiated seismic energy, stress drop, and a measure of slip heterogeneity based on finite‐fault source inversions for the largest earthquakes from 1990 to 2017. On an individual basis, stress drop, normalized rupture width, and aspect ratio most strongly correlate with aftershock productivity. A multivariate analysis shows that a particular set of parameters (dip, lithospheric age, and normalized rupture area) combines well to improve predictions of aftershock productivity on a cross‐validated data set. Our overall analysis is consistent with a model in which the volumetric abundance of nearby stressed faults controls the aftershock productivity rather than variations in source stress. Thus, we suggest a complementary approach to aftershock forecasts based on geological and rupture properties rather than local calibration alone. 

Abstract On 4 and 6 July 2019, two large strike‐slip earthquakes withW‐phase moment magnitudesM_WW6.5 (foreshock) andM_WW7.1 (mainshock) struck the Eastern California Shear Zone, northeast of Ridgecrest. The faulting geometry and kinematic coseismic slip distribution of both events are determined by jointly inverting seismological and geodetic observations guided by aftershock and surface rupture locations. The foreshock ruptured two orthogonal faults with a prominent L‐shaped geometry with maximum slip of ~1.1 m on the NE‐SW segment. The mainshock faulting extended NW‐SE along several primary fault segments that straddle the foreshock slip. The surface rupture and slip model indicate mostly near‐horizontal strike‐slip motion with maximum slip of ~3.7 m, but there is a localized vertical dip‐slip motion. Both the foreshock and mainshock ruptures terminate in regions of complex surface offsets. High aftershock productivity and low rupture velocity may be the result of rupture of a relatively immature fault system.