Repeated seismic-reflection surveys of the northern California continental margin off Eureka delineate a large slump zone measuring 10 × 15 km in area. The zone lies on a structural plateau of low slope (1 to 2°) in water depths of 450 to 650 m. The area is immediately west of the Eel River, which has an annual suspended-sediment load of nearly 24 million T/year.

High-resolution (3.5-hKz, 1-kJ, uniboom) acoustic-reflection records show a rhythmic, hummocky surface topography and back-rotated broken beds within the upper 80 m of the sediment mass. Numerous west-dipping failure planes with a spacing of approximately 400 m occur within the slump mass. A history of repeated failure within the region is indicated on deep-penetration (160 kJ sparker) seismic-reflection records that show additional failures bounded by flat-lying, undisturbed acoustic reflectors to depths of 500 m below the sea floor.

Analyses of 3-m-long gravity cores in the slump zone show that the sediment is overconsolidated, composed dominantly of silt, is gassy, and enriched in plant and wood debris. Radiographs of split cores show contorted bedding in some areas of the slump zone that suggests a degree of plastic deformation.

The Eureka area is a seismically active region lying immediately north of the Mendocino Ridge and has an expected earthquake frequency of one event greater than magnitude 6 per decade. In addition, some areas of the sea floor are undergoing local uplift; there is evidence that this activity,

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which may not be accompanied by measurable earthquakes, also causes some failures. These tectonic factors appear to be important in triggering repeated failures in the thick sedimentary unit that is forming offshore of the Eel River.

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