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Geology; May 1983; v. 11; no. 5; p. 287-291; DOI: 10.1130/0091-7613(1983)11<287:TRFAED>2.0.CO;2
© 1983 Geological Society of America
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Triggered reverse fault and earthquake due to crustal unloading, northwest Transverse Ranges, California

R. F. Yerkes1, W. L. Ellsworth1 and J. C. Tinsley1

1 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025

A reverse-right-oblique surface rupture, associated with a ML 2.5 earthquake, formed in a diatomite quarry near Lompoc, California, in the northwesternmost Transverse Ranges on April 7, 1981. The 575-m-long narrow zone of ruptures formed in clay interbeds in diatomite and diatomaceous shale of the Neogene Monterey Formation. The ruptures parallel bedding, dip 39°–59°S, and trend about N84°E on the north limb of an open symmetrical syncline. Maximum net slip was 25 cm; maximum reverse dip slip was 23 cm, maximum right-lateral strike slip was about 9 cm, and average net slip was about 12 cm. The seismic moment of the earthquake is estimated at 1 to 2 x 1018 dyne/cm and the static stress drop at about 3 bar. The removal of an average of about 44 m of diatomite resulted in an average load reduction of about 5 bar, which decreased the normal stress by about 3.5 bar and increased the shear stress on the tilted bedding plane by about 2 bar. The April 7,1981, event was a very shallow bedding-plane rupture, apparently triggered by crustal unloading.




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