Seismicity and fault kinematics through the Eastern Transverse Ranges, California: Block rotation, strike‐slip faulting and low‐angle thrusts

Abstract

Hypocentral locations and first‐motion data from the southern California seismic network are used to infer the present pattern of deformation along the southern San Andreas fault. The study area lies between the San Jacinto fault and Desert Hot Springs and includes the San Bernardino Mountains and San Gorgonio Pass. This region has some of the deepest earthquakes observed anywhere along the entire San Andreas system, exhibits a complex surface geology characterized by both right‐ and left‐lateral faults, has high topographic relief as a result of recent uplift, and is a potential site for the nucleation of a great earthquake. Although this area is unusually seismogenic, little activity can be directly associated with major throughgoing faults. Seismicity is also generally absent in the upper 5 km. The predominant style of faulting above 10–12 km is oblique slip with a large reverse component. The spatial distribution of relocated hypocenters and first‐motion data suggests the presence of a system of left‐slip faults striking northeast. This pattern of faulting, in conjunction with an unusual set of normal and reverse focal mechanisms, is interpreted as the clockwise rotation of a small set of crustal blocks subject to regional right‐lateral shear. At depths greater than about 10 km, seismicity defines a wedge‐shaped volume undergoing pervasive internal deformation on a combination of strike‐slip and low‐angle thrust faults. Velocity structures determined from earthquake arrival times suggest a low‐velocity zone at about 10 km below the San Bernardino Mountains but not below the San Jacinto Mountains. This is nearly the same depth as the transition between the two layers of different kinematic behavior seen south of the Mill Creek—Mission Creek branch of the San Andreas fault and the maximum depth of seismicity seen north of that fault. The low‐velocity zone and the transition between block rotations and the deeper deformation may thus correspond to a detachment under much of this region and would imply that the overthrust San Bernardino Mountains are allochthonous. The present pattern of seismic deformation in shocks of small to moderate size may only characterize the interval between large earthquakes and may change systematically as the region prepares to accommodate large right‐lateral displacements.