The Cape Mendocino Tsunami, 25 April 1992

Abstract

On 25 April 1992, a magnitude 7.1 Ms earthquake occurred near Cape Mendocino, California, at the southern end of the Cascadia Subduction Zone. Seismological analyses suggest an almost pure dip-slip reverse fault, with subduction of the Gorda Plate beneath the North American Plate. Modeling of crustal deformation due to this fault mechanism produces uplift of the ocean bottom in relatively shallow water just offshore and along the coast, consistent with coastal observations.

A tsunami was generated that was measured by the National Oceanic and Atmospheric Administration sea level gauges along the California, southern Oregon, and Hawaii coastlines and in the deep ocean in the Gulf of Alaska. Several records are characterized by two distinct packets of tsunami energy, the first representing propagation along a minimum time path traversing deep offshore water, the second an apparent edge wave mode trapped along the coast. The largest peak-to-trough amplitude of 1.1 m was recorded at Crescent City in the second packet, approximately 3–4 hours after arrival of the first tsunami wave.