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The structure of the continental margin, between Monterey Bay on the south and Point Reyes on the north, is dominated by a northwest-trending belt of rocks composed of continental crust (Salinian block) that is separated from oceanic crust on the east by the active San Andreas fault system, and on the west by the Sur-Nacimiento fault zone. Recent marine geophysical
investigation have shown extensive faulting within this crustal block--some of which appears to have locally offset Holocene deposits. Most of the faults within the Salinian block in the Monterey Bay region occur in two major intersecting fault zones; the northward-trending Carmel Canyon fault zone, extending offshore from Point Sur (oriented N25°W), and the northwest-trending Monterey Bay fault zone, extending offshore from the town of Monterey (oriented N50°W). The Carmel Canyon fault zone appears to connect the Palo Colorado fault in the south with the San Gregorio fault in the north. The Monterey Bay fault zone appears to be the offshore continuation of the Sur-Nacimiento fault zone.
Epicenters of many recent earthquakes are concentrated at the intersection of the Carmel Canyon and Monterey Bay fault zones, in the central part of Monterey Bay. First-motion studies of 8 earthquakes indicate right-lateral strike-slip displacement on these offshore faults. The cessation of a 10-day period of rapid tectonic creep along the adjacent San Andreas fault in 1970 coincided with a 4.3-magnitude earthquake in the Monterey Bay fault zone. This, as well as first-motion studies of the earthquakes and mapping of the offshore faults and seismicity, suggests a direct coupling between the San Andreas fault and the adjacent fault zones.
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