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On November 8, 1980, a large magnitude earthquake (M ~ 7) occurred 60 km (37 mi) off the coast of northern California. Damage was minimal onshore, but extensive changes to the sea floor were reported from the area of the Klamath River delta. Data from three successive surveys conducted in the area at intervals of 1, 6, and 11 months after the shock demonstrate the extent and type of sea floor failure. Side-scan sonar and high-resolution seismic reflection profiles, together with sea floor photographs and video images, define a thin (< 15 m, 49 ft) failure zone that measures 1 × 20 km (0.6 × 12.5 mi) and trends parallel to the shoreline on the shallow (~ 60 m, 200 ft) and nearly flat (~ 0.25°) surface of the Klamath River delta. The failure zone is charac erized by a very flat (~ 0.02°) terrace that is mantled by silty sand and is bounded to seaward by an irregular 1 to 2 m (3 to 6.5 ft) high scarp.
Sonographs and bottom photographs provide evidence that failure occurred by liquefaction, lateral spreading, and sediment flow, producing various sediment patterns and relief features on the sea floor. The modes of failure with their corresponding features are: (1) liquefaction--identified from side-scan sonographs showing sand boils 5 to 25 m (16 to 82 ft) in diameter; (2) lateral spreading--identified from photographs and sonographs showing a prominent, nearly continuous, blocky, chaotic scarp at the seaward terminus of the failure zone and belts of small (10 m long, 0.5 m high; 33 ft long, 20 in. high) pressure ridges seaward of the scarp; and (3) sediment flow--identified from sonographs showing both (a) overlapping rhythmic flow deposits that become more irregular in a seaward di ection as flow became progressively less mobile, and (b) flow "windows," or voids, left by highly viscous, dewatered flows.
In addition to these large- and small-scale changes to the morphology of the sea floor on the Klamath River delta, the sediment failure resulted in several distinctive phenomena. These phenomena include a net seaward translation of sand, a reported temporary decrease in the abundance of Dungeness crabs, and plumes of gas venting into the water column that were still evident 11 months after the earthquake.
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