San Gregorio Fault

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U.S. Geological Survey map showing the trace of the San Gregorio Fault in yellow, lower center.

The San Gregorio Fault is an active, 209 km (130 mi) long fault located off the coast of Northern California. The southern end of the fault is in the Pacific Ocean just south of Monterey Bay, and the northern end is about 20 km northwest of San Francisco, near Bolinas Bay, where the San Gregorio intersects the San Andreas Fault. Most of the San Gregorio fault trace is located offshore beneath the waters of Monterey Bay, Half Moon Bay, and the Pacific Ocean, though it cuts across land near Point Año Nuevo and Pillar Point. The San Gregorio Fault is part of a system of coastal faults which run roughly parallel to the San Andreas.

The movement of the San Gregorio is right-lateral strike-slip, and the slip rate is estimated to be 4–10 mm/year (0.2–0.4 inch/year). The most recent major earthquake along the fault had an estimated magnitude of 7 to 7.25 and occurred after 1270 AD—the earliest calibrated radiocarbon date for a native Californian cooking hearth at the Seal Cove site near Moss Beach that shows signs of displacement—but before 1775, when Spanish missionaries arrived in northern California and recorded history began for the region.

Portions of the fault that come ashore near the cliffs between Pillar Point and Moss Beach are sometimes referred to as "Seal Cove Fault" in the geological literature.