Small‐scale stress heterogeneity in the Anza seismic gap, southern California

Abstract

Focal mechanism inversions reveal significant lateral variations in stress orientations along the Anza segment of the San Jacinto fault zone. The most notable stress anomaly is within the 20‐km aseismic (seismic gap) portion of the fault zone, where , the maximum compressive stress, is nearly horizontal and is oriented at 74°±13° relative to the fault strike. This contrasts with orientations ranging from 62°±11° to 49°±7° along the more seismically active portions of the fault zone immediately to the northwest and southeast of the seismic gap. Regional stress results, found by inverting all focal mechanisms simultaneously, indicate that is horizontal and trends north‐south, while is horizontal and trends east‐west. Approximately, 15 km west of the seismic gap, in the off‐fault Cahuilla swarm area, and solutions are rotated clockwise by about 25° relative to the regional model. Roughly, 10 km southeast of the seismic gap near the Buck Ridge fault, and are rotated counterclockwise by about 10° relative to the regional solution. Northwest of the seismic gap along the fault zone, plunges about 30° from the horizontal, correlating with a local increase in reverse faulting between the Hot Springs and San Jacinto faults. Southeast of the seismic gap, plunges about 45° from the horizontal, correlating with a local increase in normal faulting in the trifurcation region of the Buck Ridge, Clark, and Coyote Creek faults. We propose a simple mechanical model in which a block rotation superimposed on the dominant right‐lateral strikeslip motion of the fault zone satisfies the first‐order observations of stress orientation, faulting, and horizontal surface strain. Under this model the Anza seismic gap is the region of zero convergence between the northeast and southwest sides of the fault, and the fault zone strength within the seismic gap is either comparable to or exceeds the fault zone strength adjacent to the gap.