The Cape Mendocino, California, Earthquakes of April 1992: Subduction at the Triple Junction

Abstract

The 25 April 1992 magnitude 7.1 Cape Mendocino thrust earthquake demonstrated that the North America-Gorda plate boundary is seismogenic and illustrated hazards that could result from much larger earthquakes forecast for the Cascadia region. The shock occurred just north of the Mendocino Triple Junction and caused strong ground motion and moderate damage in the immediate area. Rupture initiated onshore at a depth of 10.5 kilometers and propagated up-dip and seaward. Slip on steep faults in the Gorda plate generated two magnitude 6.6 aftershocks on 26 April. The main shock did not produce surface rupture on land but caused coastal uplift and a tsunami. The emerging picture of seismicity and faulting at the triple junction suggests that the region is likely to continue experiencing significant seismicity.

Rapid detection and characterization of large earthquakes using quasi-finite-source Green's functions in continuous moment tensor inversion

“…Depending on the distance of the area of uplift from the coast, the open-ocean transit times for tsunami waves may be about 6 to 20 minutes. However, the actual transit time is likely to be closer to 20 minutes due to decreasing water depth, and as reported after an Mw7.1 earthquake in 1992 that occurred at Cape Mendocino [Oppenheimer et al, 1993]. Its largest amplitudes, from an edge wave, arrived 3 hours after the event [Gonzalez et al, 1995].…”

The Cascadia Initiative: A Sea Change In Seismological Studies of Subduction Zones

“…Despite the accumulating strain and the history of great earthquakes, the current seismicity is largely confined to intermediate depth, in-slab events in the downgoing Juan de Fuca Plate, largely beneath the Puget Lowland and around the Mendocino Triple Junction, and eerily is almost nonexistent on the plate boundary (Wells et al, 1998;McCrory et al, 2012). Confirmed low angle thrust earthquakes on or near the plate boundary are restricted to the M7.2 Petrolia earthquake of 1992 (Oppenheimer et al, 1992;Velasco et al, 1994;Hagerty and Schwartz, 1996) The occurrence of large earthquakes in the past along this boundary was first documented by Atwater (1987), and the date of the most recent event was determined by Satake et al (1996) to be January 26, 1700, based on historic tsunami records in Japan. Onshore and offshore paleoseismic work over the past two decades has refined the history of earthquakes along this boundary.…”

National Earthquake Hazards Reduction Program, summaries of technical reports; Volume XXXV

“…Moreover, this earthquake has raised questions concerning the location of the recently mapped McLaughlin and others, in press) Gorda-North America plate boundary in this area. The earthquake was a thrust event in the lower part of the North America plate (Oppenheimer and others, 1993), and related uplift should have been mostly or wholly within the North America plate. However, measurements from the onland studies noted above suggest that uplift extended across this plate boundary and affected an area that is ostensibly underlain by both plates ( fig.…”