SYSTEMATIC STRESS VARIATIONS IN THE SOUTHERN SAN-JOAQUIN VALLEY AND ALONG THE WHITE-WOLF FAULT - IMPLICATIONS FOR THE RUPTURE MECHANICS OF THE 1952 M(S)-7.8 KERN COUNTY EARTHQUAKE AND CONTEMPORARY SEISMICITY

The southern San Joaquin Valley (SSJV) stress state is characterized b y systematic variations in the regional principal stress directions an d relative magnitudes, as well as local and possibly temporal variatio ns that appear to be the result of the 1952 M(s) 7.8 Kern County earth quake, The regional maximum horizontal principal stress (S-Hmax) orien tation in the San Joaquin Valley systematically rotates from similar t o NE-SW compression along the western margins of the valley to similar to N-S compression in the SSJV, This similar to N-S S-Hmax stress dir ection in the SSJV is consistent with active development of a similar to E-W trending structural fabric of fold axes and shallow thrust, and the reverse dip-slip motion observed during the Kern County earthquak e on the south-southeast dipping White Wolf fault. Contemporary seismi city (30-40 years after the mainshock) clusters in two separate areas along the White Wolf fault: the southwest region where the 1952 earthq uake nucleated, and the northeast region near where the earthquake rup ture terminated. Earthquakes in the southwest and northeast regions sh ow a diversity of focal mechanisms that include strike-slip, reverse, oblique, and, to a lesser extent, normal slip. Inversion of earthquake focal mechanisms for in situ stress in the southwest region indicates a strike-slip/reverse stress regime with S-1 oriented approximately p erpendicular to the ruptured fault plane implying low frictional stren gth in the nucleation zone of the 1952 earthquake. Inversion of focal plane mechanisms in the northeast region indicates a strike-slip stres s regime with S-2 nearly perpendicular to secondary 1952 rupture plane s also implying low frictional strength, These results indicate near-c omplete stress drops for fault planes associated with the 1952 earthqu ake (and some of the contemporary earthquakes), implying fault surface s which are frictionally weak (i,e., slip planes subparallel to princi pal stress planes). Based on the observed stress state in the southern San Joaquin Valley, much of the seismicity may be the result of eleva ted fluid pressures within these active fault zones.