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Science 10 January 1992:
Vol. 255. no. 5041, pp. 188 - 192
DOI: 10.1126/science.255.5041.188
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Articles

Late Holocene Tectonics and Paleoseismicity, Southern Cascadia Subduction Zone

SAMUEL H. CLARKE JR. 1 and GARY A. CARVER 2

1 Branch of Pacific Marine Geology, U.S. Geological Survey, 345 Middlefield Road, Mail Stop 999, Menlo Park, CA 94025
2 Department of Geology, Humboldt State University, Arcata, CA 95521

Holocene deformation indicative of large subduction-zone earthquakes has occurred on two large thrust fault systems in the Humboldt Bay region of northern California. Displaced stratigraphic markers record three offsets of 5 to 7 meters each on the Little Salmon fault during the past 1700 years. Smaller and less frequent Holocene displacements have occurred in the Mad River fault zone. Elsewhere, as many as five episodes of sudden subsidence of marsh peats and fossil forests and uplift of marine terraces are recorded. Carbon-14 dates suggest that the faulting, subsidence, and uplift events were synchronous. Relations between magnitude and various fault-offset parameters indicate that earthquakes accompanying displacements on the Little Salmon fault had magnitudes of at least 7.6 to 7.8. More likely this faulting accompanied rupture of the boundary between the Gorda and North American plates, and magnitudes were about 8.4 or greater.

Submitted on July 22, 1991
Accepted on November 19, 1991


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