I: SEISMOTECTONICS OF THE SAN JACINTO FAULT ZONE AND THE ANZA SEISMIC GAP; II: IMAGING THE SHALLOW CRUST IN VOLCANIC AREAS WITH EARTHQUAKE SHEAR WAVES (CALIFORNIA)

In Part I we consider characteristics of the earthquake stress release along the San Jacinto fault zone. First, we locate the historic M (GREATERTHEQ) 6 earthquakes. These locations and rupture zones define patterns of large earthquake occurrence in the fault zone. An historic seismic-slip gap is located near the town of Anza.; Next, we present details of the small earthquake stress release near Anza. Earthquakes define an 18-km quiescent segment of fault which is bounded by areas of high seismicity. Moderate earthquakes with few aftershocks indicate that the fault is seismogenic and highly stressed but locked, possibly due to high normal stresses. An earthquake swarm occurred recently near the gap in an area which was previously active before two nearby M (GREATERTHEQ) 6 earthquakes. This suggests that the swarm area may signal the presence of high stresses.; Finally, we investigate variations in the depths of earthquakes in the San Jacinto-southern San Andreas fault region. The maximum depth of earthquakes in the San Jacinto fault zone progressively shallows from 20 to 10 km towards the Imperial Valley region of high heat flow. Earthquakes occur predominantly in a band along the bottom of the seismogenic zone; few earthquakes occur in the shallower fault zone. This implies that shear stresses are greater deep in the brittle fault zone, and that loading of the brittle crust occurs along aseismic extensions of the fault zones. Deeper earthquakes in the region occur principally in the major fault zones, and shallow earthquakes occur principally in the adjacent crust. This suggests that stresses in the deep brittle fault zone and in the adjacent crust are similar but that stresses in the shallow fault zone are low.; In Part II S-wave seismograms are used to image attenuation structure in the crust of the Coso geothermal region of California. Vertical-component seismograms of sixteen local earthquakes were analyzed for S(,v)-wave attenuation. Forward modeling and inversion illuminate regions in the shallow crust which severely attenuate S waves. Surprisingly, these lie south of Coso beneath Indian Wells Valley. They are coincident with the epicentral locations of migrating earthquake swarms. No attenuating effects were seen beneath Coso above 5 km depth.