The San Andreas Fault system through the Transverse Ranges as illuminated by earthquakes

The pattern of seismogenic faulting in the San Gorgonio Pass‐San Bernardino basin area of the San Andreas fault (SAF) zone is mapped from about 7000 quality‐selected focal mechanisms (1981‐Nov '92). They are derived from phase data of the southern California network by a relocation procedure based on location‐dependent station‐corrections and by a grid‐search procedure. About 3/4 of the mechanisms have been interpreted as planes of rupture, or “slip planes”, that delineate many distinct faults. One of these faults is right‐lateral, dips steeply northeast and is continuous trough the San Gorgonio Pass area; we interpret it as the main branch of the SAF. A very large earthquake seems possible on this portion of the SAF because of its continuity and because of the unusually deep reach of the seismicity (23 km). Southwest of this fault, a volume of diffuse and persistent seismicity has a sharp downward cut‐off which may reflect a basal detachment dipping 20° northeast and intersecting the SAF along the deepest seismicity. Northeast of the SAF, the floor of the seismicity is shallower by as much as 10 km. Stress characteristics are derived from slip planes in 10 selected subregions. The regime is transpressional near the constricting bend of the SAF at San Gorgonio Pass; it is dominated by horizontal extension in the San Bernardino Basin area; and it is again transpressional in the eastern San Gorgonio Mountains north of the Cucamonga thrust. The broad features of this zonation may be related to the intersection of the SAF with the Pinto Mountain fault and with the San Jacinto fault. A change of fault kinematics in the Yucaipa cluster coincides with the 1992 Landers and Big Bear main shocks and may be a manifestation of static stress change.