EVIDENCE FOR POSSIBLE HORIZONTAL FAULTING IN SOUTHERN CALIFORNIA FROMEARTHQUAKE MECHANISMS

We find that 36 of the 505 fault-plane solutions (M greater than or eq ual to 3.0, 1981-1990) in southern California have a nodal plane dippi ng no more than 30 degrees, With the assumption of the low-angle nodal planes being the fault planes, four cross sections are constructed to show the possible horizontal faults in the middle and upper crust, Mo re than half of these low-angle faults are located within or adjacent to the Transverse Ranges, The focal depths vary from 1 km in the south ern end of the Sierra Nevada and the southwestern Mojave Desert to 20 km in the Transverse Ranges, The slip directions are also diverse, In general, east-west extensional movements are dominant in the boundary between the southern Sierra Nevada extending to the San Emigdio Mounta ins and the western Mojave Desert, whereas north-south compressional m ovements are dominant in the Transverse Ranges, In the Peninsular Rang es and the Salton Trough, both the slip directions and focal depths va ry, These features suggest that seismically active low-angle faults in southern California may exist at different depths and slip in various directions, Our data do not support the existence of a regional-scale seismically active detachment in southern California. Only in the wes tern Transverse Ranges is there some suggestion of a large detachment surface at a depth of about 13 to 14 km.