SEISMOTECTONICS OF THE SAN-ANDREAS FAULT SYSTEM BETWEEN POINT ARENA AND CAPE MENDOCINO IN NORTHERN CALIFORNIA - IMPLICATIONS FOR THE DEVELOPMENT AND EVOLUTION OF A YOUNG TRANSFORM

The northernmost and relatively youthful segment of the San Andreas fault system is situated within a 100+ km wide zone of northwest trending strike-slip faults that includes, from west to east, the San Andreas, Maacama, and Bartlett Springs faults. Although the San Andreas fault is the principal strike-slip fault in this system, it has been virtually aseismic since the 1906 earthquake. Moderate levels of seismicity locate to the east along the Maacama fault and, to a lesser extent, the Bartlett Springs fault at focal depths typical of other strike-slip faults within the San Andreas fault system in central California. North of the San Andreas fault system, within the Cape Mendocino area, earthquakes occur at depths of up to 40 km and primarily reflect internal deformation of the subducting Gorda slab, and slip along the Mendocino Fracture Zone. Seismicity along the Maacama and Bartlett Springs faults is dominated by right-lateral to oblique-reverse slip along fault planes that dip 50-degrees-75-degrees to the northeast. The northern extent of seismicity along these faults terminates near the surface projection of the southern edge of the Gorda slab. The onset of seismicity along these faults may be related to the abrupt change in the elastic thickness of the North American plate as it enters the asthenospheric window. The Maacama and Bartlett Springs faults are strike-parallel with active reverse faults within the forearc region of the Cascadia subduction zone. This preexisting structural fabric of northwest trending reverse faults in the forearc area appears to have strongly influenced the initial slip and complexity of these faults. Continuation of the moderately dipping Maacama fault to the southeast along the steeply dipping Healdsburg and Rodgers Creek fault zones and the near-vertical Hayward and Calaveras fault zones in the San Francisco Bay area suggests that these faults evolve toward a more vertical dip to minimize the shear stresses that tend to resist plate motion.