Systematic stress variations in the southern San Joaquin Valley and along the White Wolf fault: Implications for the rupture mechanics of the 1952 Ms 7.8 Kern County earthquake and contemporary seismicity

Abstract

The southern San Joaquin Valley (SSJV) stress state is characterized by systematic variations in the regional principal stress directions and relative magnitudes, as well as local and possibly temporal variations that appear to be the result of the 1952 Ms 7.8 Kern County earthquake. The regional maximum horizontal principal stress (SHmax) orientation in the San Joaquin Valley systematically rotates from ∼NE‐SW compression along the western margins of the valley to ∼N‐S compression in the SSJV. This ∼N‐S SHmax stress direction in the SSJV is consistent with active development of a ∼E‐W trending structural fabric of fold axes and shallow thrust, and the reverse dip‐slip motion observed during the Kern County earthquake on the south‐southeast dipping White Wolf fault. Contemporary seismicity (30–40 years after the mainshock) clusters in two separate areas along the White Wolf fault: the southwest region where the 1952 earthquake nucleated, and the northeast region near where the earthquake rupture terminated. Earthquakes in the southwest and northeast regions show 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 S1 oriented approximately perpendicular to the ruptured fault plane implying low frictional strength in the nucleation zone of the 1952 earthquake. Inversion of focal plane mechanisms in the northeast region indicates a strike‐slip stress regime with S2 nearly perpendicular to secondary 1952 rupture planes also implying low frictional strength. These results indicate near‐complete stress drops for fault planes associated with the 1952 earthquake (and some of the contemporary earthquakes), implying fault surfaces which are frictionally weak (i.e., slip planes subparallel to principal stress planes). Based on the observed stress state in the southern San Joaquin Valley, much of the seismicity may be the result of elevated fluid pressures within these active fault zones.