A seismotectonic analysis of the Anza Seismic Gap, San Jacinto Fault Zone, Southern California

Abstract

Small earthquake epicenters near the Anza seismic gap define a 20-km quiescent segment of fault bounded to the northwest and southeast by areas of relatively high seismicity. Recent moderate earthquakes on and near the San Jacinto fault in the gap and their relatively depressed aftershock activity indicate that the fault is seismogenic and highly stressed but locked by some mechanism. The locked nature of the fault may be due to relatively high compressive stress normal to the fault resulting from the convergent geometries of the local, active, discontinuous faults and the oblique orientation of the regional maximum compressive stress. Strain is not being relieved by aseismic fault creep. A swarm of small earthquakes in the crustal block 13 km southwest of the Anza gap beneath the Cahuilla Valley recently released stress in an area which was previously highly active before the 1918 (M 6.8) and 1937 (M_L 6.0) earthquakes. The occurrence of these periods of increased seismicity near Cahuilla in the years immediately before the nearby (closer than 35 km) large earthquakes and the recent swarm suggest that the ground beneath Cahuilla may be acting as a stress meter signaling the presence of high stresses before large local earthquakes. The length of the quiescent fault segment suggests potential for about an M 6.5 earthquake if the entire segment ruptures at once.