Seismotectonics of the easternmost transverse ranges, California: Relevance for seismic potential of the southern San Andreas Fault

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Abstract
Earthquake locations, depths and focal mechanisms from the Southern California Regional Network (1977–1985) are used to identify the orientation and sense of slip of active subsurface faults in the Easternmost Transverse Ranges (ETR). The ETR are separated from the Salton Trough province by the southernmost strands of the San Andreas fault (SAF). Much of the seismicity in the ETR is concentrated well northeast of the SAF at relatively shallow depths under the Little San Bernardino Mountains. Many of these earthquakes reflect slip on steeply dipping, left‐lateral faults striking northeast to east, at relatively high angles to the adjacent SAF. Focal mechanisms in the ETR show predominantly strike‐slip, normal, or oblique‐normal faulting, and share common near‐horizontal T axes striking WNW. P axes range from near vertical to near horizontal and strike mostly NNE. In contrast, reverse and strike‐slip focal mechanisms that exhibit persistent north trending, near‐horizontal P axes characterize the San Gorgonio Pass area immediately to the west. These different patterns of strain geometries are inferred to represent changes in local stress regime and clearly establish a boundary between contrasting tectonic styles of contemporary secondary deformation along the SAF. This boundary, which in the Coachella Valley may be the Mission Creek fault, is also distinguished by abrupt changes in (1) rate and depth of seismic activity; (2) topography; (3) Quaternary vertical deformation; (4) strikes and dips of major branches of the SAF; and (5) seismic velocities in the crust and upper mantle. The preponderance of secondary normal faulting in the ETR versus secondary reverse faulting in the San Gorgonio Pass region suggests that fault‐normal stress is much less across the SAF adjacent to the ETR. If a friction law where strength is proportional to normal stress applies to the SAF, then a smaller tectonic shear stress would be required tor slip in large earthquakes along the Salton Trough segment. In this case, the southernmost SAF may have shorter repeat times, smaller average displacements, and lower moment release rates than earthquakes that rupture through the major restraining bend in San Gorgonio Pass.