We examine locations, magnitudes, and faulting types of post‐2000 earthquakes in the trifurcation area of San Jacinto fault zone to clarify basic aspects of failure processes in the area. Most M ≥ 3.5 events have strike‐slip mechanisms, occur within 1 km of the main faults (Clark, Buck Ridge, and Coyote Creek), and have hypocenter depths of 10–13 km. In contrast, many smaller events have normal source mechanisms and hypocenters in intrafault areas deeper than 13 km. Additional small events with hypocenter depth <13 km occur in off‐fault regions and have complex geometries including lineations normal to the main faults. Five moderate earthquakes with M 4.7–5.4 have high aftershock rates (~150 M ≥ 1.5 events within 1 day from the mainshock). To obtain more details on aftershock sequences of these earthquakes, we detect and locate additional events with the matched filter method. There are almost no aftershocks within 1 km from the mainshocks, consistent with large mainshock stress drops and low residual stress. The five aftershock sequences have almost no spatial overlap. While the mainshocks are on the main faults, most aftershocks are located in intrafault and off‐fault regions. Their locations and spatial distribution reflect the mainshock rupture directions, and many also follow structures normal to the main faults. The significant diversity of observed features highlights the essential volumetric character of failure patterns in the area. The increasing rate of moderate events, productive aftershock sequences, and large inferred stress drops may reflect processes near the end of a large earthquake cycle.

We attempt to clarify processes associated with the 2019 Ridgecrest earthquake sequence by analyzing space-time variations of seismicity, potency values, and focal mechanisms of earthquakes leading to and during the sequence. Over the 20 years before the M w 7.1 mainshock, the percentage of normal faulting events decreased gradually from 25% to below 10%, indicating a long-term increase of shear stress. The M w 6.4 and M w 7.1 ruptures terminated at areas with strong changes of seismic velocity or intersections with other faults producing arresting barriers. The aftershocks are characterized by highly diverse focal mechanisms and produced volumetric brittle deformation concentrated in a 5-10 km wide zone around the main ruptures. Early aftershocks of the M w 7.1 event extended over a wide area below typical seismogenic depth, consistent with a transient deepening of the brittle-ductile transition. The Ridgecrest earthquake sequence produced considerable rock damage in the surrounding crust including below the nominal seismogenic zone. Plain Language Summary The Eastern California Shear Zone is one of the seismically most active regions in Southern California and hosted in the last few decades several large earthquakes. The most recent of these is the 5 July 2019, Ridgecrest earthquake with magnitude 7.1, which was followed by a vigorous aftershock sequence. To clarify processes associated with the Ridgecrest earthquake sequence, we analyze properties of earthquakes before, during and after the 2019 Ridgecrest mainshock. The fraction of normal faulting events was reduced gradually in the 20 years before the mainshock, indicating a long-term increase of shear stress. The Ridgecrest earthquake and a moderate event with magnitude 6.4 about 34 hr earlier terminated in areas with high seismic velocity or intersections with other faults, which might act as barriers that arrested the ruptures. The aftershocks following the Ridgecrest mainshock have highly diverse mechanisms and are widely distributed within 5-10 km wide zone around the main rupture. Many of the early aftershocks are deeper than the regular seismogenic zone. The results highlight the strongly heterogeneous volumetric nature of crustal deformation during large earthquakes off main plate-boundary faults.

We analyze space-time variations in the depth distribution of seismicity in Southern and Baja California, focusing on transients following four M ≥ 6.7 mainshocks. The regular brittle-ductile transition depth is estimated at different locations as the local bottom of 99,636 background events and is compared with the bottom of events within earthquake clusters. The four M ≥ 6.7 mainshock-aftershock sequences exhibit early aftershocks with depths up to 5 km below the regular brittle-ductile transition depth and epicentral distances up to 15 km from the mainshock ruptures. The maximum aftershock depth increases abruptly following the mainshocks and recovers to the background level after several years. The wide-spread deeper-than-usual early aftershocks favor classical brittle-ductile transition over change from unstable to stable frictional response as the mechanism governing the base of the seismogenic zone. Episodic transient deepening of the brittle-ductile transition following major earthquakes can have important long-term effects on the lower crust.Plain Language Summary Rock deformation in the Earth's crust changes from brittle failure involving localized fracturing, frictional sliding and seismicity in the upper crust to ductile deformation involving distributed flow at greater depth. Analysis of four well-located M ≥ 6.7 mainshock-aftershock sequences in Southern and Baja California indicates that many deeper-than-usual aftershocks occur after moderate to large mainshocks and are widely distributed around the mainshock rupture zones. The maximum aftershock depth extends up to 5 km below the regular bottom of the seismogenic zone following the mainshocks and decreases back to the background level after several years. The observed transient deepening of seismicity is consistent with geological observations of localized brittle failures below the regular seismogenic zone. The results have important implications for the mechanics governing the base of seismicity and the long-term properties and dynamics of the lower crust.

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