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Faulting and Effects of Associated Shaking at Pisgah Crater Volcano Caused by the 16 October 1999 Hector Mine Earthquake (M_w 7.1), Central Mojave Desert, California

Department of Geological Sciences
University of California
Santa Barbara, California 93106-9630
(A.G.S., K.C.B., W.S.W.)

A 240-m-long zone of left-stepping fractures and scarps was mapped on 23 and 24 October 1999 on the east flank of the main cinder cone at Pisgah Crater volcano about halfway between the north end of the surface rupture, as mapped by other investigators, and highway I-40. The Pisgah Crater fractures are faults, not slump features, although undoubted slumps and shallow slides are present and abundant nearby. The free faces of the scarps range from 10 to 80 cm high; evidence of strike-slip was not observed. The scarps are at the base of an erosion-scarred, 2-m-high bench in the cone slope, that suggests the cone has been faulted previously. Indicators of strong motion associated with the Hector Mine earthquake include shattered earth and tossed ground, extensive collapse of oversteepened quarry slopes, and slumping of waste dump slopes, quarry cuts, and gully banks. A 1000-kg boulder that rested on a nearly flat surface was displaced 10 cm southward from its original position within its socket. Extensive sand slips on angle-of-repose slopes, expulsion of dust and silt from compacting cinders, local collapse of lava tubes, and dislodgment of precariously balanced boulders from quarry faces were also seen. Taken together, our observations suggest that strong motion associated with the Hector Mine earthquake on Pisgah Crater was nearly thrice the 0.33g measured elsewhere by other investigators.

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