Notable Earthquake of the Week

Gilroy, California M6.2 Earthquake  

On June 20, 1897, an earthquake measuring M6.2 occurred along the Calaveras fault in the south San Francisco Bay Area. The quake was centered in Santa Clara Valley south of San Jose near Gilroy.

The earthquake shook the epicentral area quite hard and was felt strongly in San Francisco and as far away as Woodland to the north and San Luis Obispo tot he south. Numerous unreinforced brick buildings collapsed throughout Santa Clara Valley, including at Gilroy, Hollister and Morgan Hill. Brick chimneys toppled in Salinas and San Jose. Although there were no reports and given the significant damage, no injuries or facilities were reported. Fissures along the trace of the Calaveras fault were observed southeast of Gilroy near Pacheco Pass Road and in San Felipe Valley north of Morgan Hill.

The fault is part of the much larger Calaveras-Concord-Greenville fault system which is an extensive fault system that extends from San Benito in Central California to the Vaca Mountains north of Fairfield. It parallels the San Andreas fault in the southern Diablo Range, cuts through downtown Hollister and extends along the eastern side of Santa Clara Valley. It cuts through the San Felipe Creek drainage and Halls Valley at the base of Mt. Hamilton, crosses through Calaveras Reservoir, bends slightly northeast-ward as it extends through the Sunol Valley, then cuts across the western Pleasanton and San Ramon Valleys. The fault right-steps to the east and to the Concord fault, crosses the Carquinez Straits and forms the Green Valley fault system along the western Cordelia Slough, and extends into the southern Vaca Mountains north of Fairfield. Substantial active fault creep is observed in Santa Clara Valley and Hollister, as well as along the southern Green Valley fault.

Three quakes have registered in the M6.0 range and each along the Calaveras fault in Santa Clara Valley: Gilroy (M6.2) 1897; and two near Morgan Hill 1911 (M6.5) and 1984 (M6.2). The Santa Clara Valley segment shows abundant microseismicity and stunning examples of fault creep in Hollister where recent man made structures such as curbs, fences and roads, have been offset along the fault between a few inches to a couple feet. Unfortunately, the Northern segment of the Calaveras fault shows low microseismic levels and no fault creep, suggesting to scientists that it is locked and accumulating strain which could be releases in a strong earthquake.

Seismo-Watch Earthquake Reports

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