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

Chapters 1, 2, and 3 consider the characteristics of the earthquake stress release along the San Jacinto fault zone and in the San Jacinto-southern San Andreas fault region. In Chapter 1 we locate the historic M ≥ 6 earthquakes in the San Jacinto fault zone. Intensity data are used to locate earthquakes in 1899, 1918, and 1923. Limited seismic data are also used for the 1923 location. Southern California stations and empirical station corrections obtained from recent small events are used to locate earthquakes in 1937, 1942, and 1954 and their aftershocks and preshocks. The locations and rupture zones of these earthquakes, including the 1968 Borrego Mountain earthquake, help define patterns of large earthquake occurrence in the fault zone and sections of fault which have not ruptured historically. One historic seismic-slip gap is located in the central San Jacinto fault zone near the town of Anza. We present in Chapter 2 details of the small earthquake stress release in the San Jacinto fault zone near Anza. Small earthquake epicenters near the Anza seismic gap define a 18-km quiescent segment of fault which is 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 generally locked by some mechanism. The locked nature of the fault may be due to relatively high 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 5.9) earthquakes. The occurrence of these periods of increased seismicity near Cahuilla in the years immediately before the nearby 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 an earthquake of about M 6.5 if the entire segment ruptures at once. In Chapter 3 we investigate variations in the depths of earthquakes in the San Jacinto fault zone and in the San Jacinto-southern San Andreas fault region. We observe that the maximum depths of earthquakes in the San Jacinto fault zone vary from 20 to 10 km along strike. The earthquake hypocenters are progressively shallower nearer to the Imperial Valley region of high heat flow. This observation illustrates the effect heat flow has on the maximum thickness of the seismogenic zone. In addition, earthquakes occur predominantly in a band along the bottom of the seismogenic zone; few earthquakes occur m the shallower portions of the fault zone. This implies that shear stresses must be greater in the deeper parts of the brittle fault zone than in the shallower fault zone. This implies that loading of the brittle crust occurs by aseismic displacement along deeper extensions of the brittle fault zones. Furthermore, we observe that deeper earthquakes in the region of the San Jacinto and southern San Andreas faults occur principally in the major fault zones and that shallow earthquakes occur principally in the adjacent crust. Interpretation of these observations is less clear, but they, in combination with other observations about deep and shallow earthquakes near Anza on the San Jacinto fault, seem to suggest that stresses in the deep brittle fault zone and in the adjacent crust are similar and that stresses in the shallow fault zone are low. In Chapter 4 shear-wave seismograms are used to image anomalous attenuation regions in the shallow crust beneath the Coso volcanic/geothermal region of eastern California. Vertical-component seismograms archived by CUSP (Caltech-USGS Seismic Processor) for earthquakes which occurred in the Indian Wells Valley-Coso-southern Sierra Nevada region from October 1983 to February 1984 were analyzed to determine whether attenuated Sy-wave signals were present along some raypaths. Signals of this type have previously been documented in the Long Valley magmatic area and elsewhere. We have analyzed sixteen small earthquakes with Sy signals that change considerably with azimuth and take-off angle. Forward modeling and a tomographic inversion illuminate several small regions within a 20 by 30 km area of the shallow crust (some shallower than 5 km) which severely attenuate S waves passing through them. This area is beneath the Indian Wells Valley south of the Coso Range and is coincident with the epicentral location of earthquake swarms which occurred in 1982-1983. This swarm sequence began in a centralized cluster which, with time, became two clusters that migrated several kilometers north and south. No attenuating effects were seen for rays passing beneath the Coso geothermal area above about 5 km depth.