BASIN STRUCTURE BENEATH THE SANTA ROSA PLAIN, NORTHERN CALIFORNIA, AND ITS POSSIBLE INFLUENCE ON DAMAGE PATTERNS FROM THE 1906 AND 1969 EARTHQUAKES

Regional gravity and magnetic data in the northern San Francisco Bay region unveil a complex basin configuration beneath the Santa Rosa Plain, which may have influenced the degree of damage in the city of Santa Rosa caused by the shaking from both the 1906 San Francisco earthquake (M8.0) and the 1969 Santa Rosa earthquake sequence (M 5.7 and 5.6). Inversion of gravity data indicates that the Santa Rosa Plain is underlain by two main basins (2-3 km deep) separated by a shallow west-striking bedrock ridge at the latitude of the city of Santa Rosa. The basement ridge, here named the Trenton ridge, coincides approximately with the location of the northwest-striking, northeast-dipping Trenton thrust fault, although the basement ridge is likely more structurally complex. The Trenton ridge is marked by a broad magnetic high that can be traced onto serpentinite that crops out west of the Plain. Surprisingly, the Healdsburg-Rodgers Creek fault does not form the northeastern margin of the northern basin (9x12 km), centered near the town of Windsor, but instead cuts across the basin without significant displacement of the gravity gradients marking the basin margin. The basin does not, therefore, appear to have been formed by the present configuration of active strike-slip faults given the current distribution of gravity stations. The southern basin, named the Cotati basin, is larger (10x18 km). The lowest gravity values in the Cotati basin (presumably the deepest part of the basin) are 3-4 km south of the Trenton ridge, and a subtle west-striking gravity high in the middle of the basin suggests that this basin may be further divided into two sub-basins.

The eastern end of the Trenton thrust fault coincides with a local magnetic and gravity high located immediately west of the Santa Rosa damage zone. The most intense damage coincides with a 5-7 mGal, 2-km-wide gravity low situated in an embayment in the northeast corner of the Cotati Basin suggesting that seismic energy may have been focused into this local depression. A similar pattern of shaking intensity is likely to recur in this area during future earthquakes because of the basement surface configuration that partitions the basins beneath the Santa Rosa Plain.