Off-Fault Focal Mechanisms Not Representative of Interseismic Fault Loading Suggest Deep Creep on the Northern San Jacinto Fault

Abstract

Within the San Bernardino basin, some focal mechanisms show normal slip that is inconsistent with the expected interseismic strike-slip loading of the region. The discrepancy may owe to deep (>10-km depth), creep along the nearby northern San Jacinto fault. The enigmatic normal slip microseismicity occurs to the northeast of the fault and primarily below 10-km depth, consistent with off-fault deformation due to spatially nonuniform ongoing slip. Consequently, if these normal focal mechanisms are included in stress inversions from the seismic catalog, the results may provide inaccurate information about fault loading. Here we show that off-fault loading from models with deep interseismic creep on the northern San Jacinto fault match the first-order pattern of observed normal slip focal mechanisms in the basin and that this deep creep cannot be detected with GPS data due to the proximity of the San Andreas fault.