Abstract

Published reports attribute the 1978 Santa Barbara earthquake to a north-dipping reverse fault supported in part by first-motion studies. Surface geology augmented by oil-well and trench logs shows a predominance of late Quaternary reverse faults which dip steeply south, suggesting that the alternate nodal plane determined from first-motion studies is preferred. A steep fault would better explain the narrow rectangular map pattern of aftershocks. The Santa Barbara region has a high historical seismicity compared to the onshore Ventura basin to the east, where late Quaternary displacements on seismically active north-dipping faults are much higher, a paradox if the 1978 event were also produced by a north-dipping fault. On the other hand, the tectonic subprovince characterized by steeply south-dipping faults is much better developed at Santa Barbara, where the historical seismicity is higher, than in the onshore Ventura basin.

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