CONVERGENCE NORMAL TO THE SAN ANDREAS FAULT SYSTEM [c5, p145-147]

Focal mechanisms of earthquakes occurring off the San Andreas fault system suggest that the component of the San Andreas discrepancy normal to the fault system may, indeed, be accommodated by distributed brittle deformation on either side of the fault system. These mechanisms range from dextral strike slip on planes subparallel to the San Andreas fault, through oblique-reverse slip, to nearly pure reverse slip with a slip direction perpendicular to the San Andreas fault.

The Coalinga-North Kettleman Hills earthquake sequence provides clear evidence for crustal convergence perpendicular to the San Andreas fault system in the Coast Ranges. The several smaller events with similar mechanisms to the north along both the eastern and western (coastal) margins of the Coast Ranges (Figure 5.11) suggest that the convergence responsible for the Coalinga earthquake may be active the length of the Coast Ranges (Wong and others, 1988; Eaton and Rymer, 1990). The subparallelism of fold axes within the Coast Ranges with the San Andreas fault indicates that fault-normal convergence has been important for the past 3 Ma in central California ( Figure 5.12); Page and Engebretson, 1984). Namson and Davis (1988) proposed that the entire system of Coast Range folds may be genetically related to Coalinga-like earthquake sequences and low-angle (blind) thrust faults that are rooted in a decollement near the base of the seismogenic crust. The reverse focal mechanisms for earthquakes associated with offshore faults along the western margin of the Coast Ranges suggest that, here, convergence involves westward thrusting of the Coast Ranges over oceanic crust of the Pacific plate.

The pronounced discrepancy in the strike of the San Andreas fault through the Transverse Ranges with respect to the Pacific-North American plate slip direction provides an obvious source of local crustal convergence (Hill and Dibblee, 1953; Atwater, 1970), and the associated structural complexities serve to distribute brittle deformation (seismicity) much more widely about the San Andreas fault system in southern California than about the relatively straight sections of the fault system in central and northern California. The largest earthquake in California since the great 1906 San Francisco earthquake occurred near the northern margin of this convergent regime; this M=7.7 Kern County earthquake ruptured some 35 km of the southeast-dipping White Wolf fault with left-oblique reverse slip on July 21, 1952.

The focal mechanisms of larger Transverse Range earthquakes, together with the mapped attitudes of major faults with Holocene offsets, show that much of this convergence occurs with slip on north-dipping thrust faults within and along the southern margin of the central Transverse Ranges (Figure 5.11A). For earthquakes in the western Transverse Ranges, the direction of reverse slip is more southwestward, consistent with thrusting of the western Transverse Ranges over the Pacific plate similar to that in the Coast Ranges to the north.