The intensity of the July 21, 1986, Chalfant Valley, California, earthquake

Abstract

On 21 July 1986 at 14:42 UTC Chalfant Valley, California was rocked by a strong earthquake with a magnitude of 6.5ML (BRK). The hypocenter (37.537°N., 118.447 e W., with a depth of 9 km) is about 8 km northwest of the town of Chalfant, a small desert community about 15 km north of Bishop, California. Considerable foreshock activity began approximately 3 weeks prior to the mainshock. The largest foreshock occurred on 20 July 1986, at 14:29 UTC, with a magnitude of 5.9AfL (BRK) (Cockerham and Corbett, 1987). The Chalfant Valley earthquake had a maximum Modified Mercalli (MM) Intensity of VI. The strongest acceleration recorded was .46 g about 14 km north of the epicenter. The most severe damage was sustained at Chalfant. The earthquake was felt over approximately 202,000 km2 of land area in California, Nevada, Utah, and Arizona (see fig. 1). High rise buildings were swayed in Los Angeles, San Francisco, and Salt Lake City. Official estimates of damage exceed $2.7 million (Southern California Earthquake Preparedness Project, 1986). Surface fracturing that occurred along 15.5 km of the north-trending White Mountain fault zone (east of the epicenter and Chalfant) indicate displacement along the fault is right-lateral oblique. General Geology The Chalfant area is located in a seismically active part of the western Basin and Range. Chalfant Valley, between the White Mountains and the Volcanic Tablelands, (Cockerham and Corbett, 1987) is located in a graben between the eastern Sierra Nevada Mountains and the White Mountains to the east. Bishop, the Volcanic Tableland, and the Long Valley caldera are ail located in this graben (DePolo and Ramelli, 1987). Chalfant