Abstract

A large magnitude earthquake (M ∼7) occurred on November 8, 1980, 60 km 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 one, six, and eleven 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) failure zone that measures 1 × 20 km and trends parallel to the shoreline on the shallow (∼60 m) and nearly flat (∼0.25°) surface of the Klamath River delta. The failure zone is characterized by a very flat terrace (∼0.02°), which is mantled by silty sand and is bounded seaward by an irregular 1-to 2-m high scarp.

Sonographs and photographs of the sea floor show that failure occurred by liquefaction and sediment flow, producing a variety of sediment patterns and relief features on the sea floor. Failure by liquefaction is inferred based on: 1. a seaward thickening of the failure deposit; 2. sand boils 5 to 25 m in diameter; 3. the presence of a prominent, nearly continuous, blocky, chaotic scarp at the seaward terminus of the failure zone; and 4. belts of small (10 m long, 0.5 m high) pressure ridges seaward of the scarp. These features indicate sand was mobilized into a flow. In addition, individual flows extended across the terminus of the failure, and overlapping flow deposits became more irregular in a seaward direction as the flows became progressively less mobile.