Abstract

On November 8, 1980, a major earthquake (magnitude 6.5 to 7.2) occurred 60 km off the coast of northern California. A survey of the area using high-resolution seismic-reflection and side-scan sonar equipment revealed the presence of extensive sediment failure and flows in a zone about 1 km wide and 20 km long that trends parallel to the shelf on the very gently sloping (< 0.25°) Klamath River delta. The failure zone is marked by a well-defined terrace that slopes seaward at about 0.02c and a prominent 1- to 2-m high, seaward-facing toe scarp on the outer margin of the terrace. This toe scarp is sinuous, appears to be nearly continuous for a distance of 20 km, and closely parallels the 60-m isobath. Evidence indicates that the toe scarp is the terminus of lateral spreads and lobes of sediment flows. Side-scan sonar records show evidence of gas vents and small (10 × 3 × 0.5 m high) pressure ridges formed seaward of and parallel to the sediment flow scarp. Indicators of liquefaction (sand boils and collapse craters) are present on the sediment flow terrace. Extensive sediment failure occurred on a sea-floor slope of less than 0.25°, and we can unequivocally pinpoint the cause as an earthquake of known location, magnitude, and time.

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