Seismicity off the coast of Northern California determined from ocean bottom seismic measurements

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Abstract
Approximately 190 local earthquakes (Δ < 4°) with suboceanic epicenters have been recorded over a 14-month period by the Ocean Bottom Geophysical Station of the Lamont-Doherty Geological Observatory. The station is located 200 km west-northwest of San Francisco and 220 km south of the Mendocino Fracture Zone at a water depth of 3.9 km. Most of the observed earthquakes are spatially related to the Mendocino fracture system. Only 20 per cent of the earthquakes recorded at the ocean bottom station were observed at an equidistant land station at Point Arena, California. P and S phases detected at the ocean bottom bottom have greater amplitudes and higher frequencies than the same phases recorded at the nearby land station. This is ascribed to a combination of near-receiver crustal resonance effects and attenuation on transmission across the continental/oceanic interface. Observations of Sn and epicentral locations support the hypothesis that the Mendocino fracture system is a transform fault. Phases converted from P to S at the base of the sediment layer beneath the ocean bottom station are used to compute the average shear-wave velocity and thickness of the sediment layer. The resulting values are 0.34-0.40 km/sec for the average shear velocity and a sediment thickness of 0.78-0.88 km. A phase tentatively identified as the False S of Byerly is observed on records from a coastal land station, but is not observed on the ocean bottom records.

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