San Andreas Fault


San Andreas Fault
The San Andreas Fault is a dextral strike-slip fault. The fault marks the boundary between the North American Plate on the east and the Pacific Plate on the west. A lesser proportion of the motion between these two plates is accommodated along the Walker Lane Shear Zone and in the Basin and Range Province. The San Andreas Fault connects the Cascadia Subduction Zone in the north (at Cape Mendocino) to the Gulf of California Rift in the south. The San Andreas Fault is composed of a zone several miles wide which incorporates multiple strands. Activity and displacement on the various strands migrates in time and space. The main active strand currently runs on and off-shore between Cape Mendocino and the Sea of Cortez. From Cape Mendocino, it runs offshore to Tomales Bay, southward through Bolinas Lagoon, just west of the San Francisco Peninsula, to come onshore again at Daly City, through the hills of the Peninsula (Crystal Springs reservoir is formed by the fault itself). In the Santa Cruz mountains, it bends slightly eastward, forming a restraining bend. This is the site of the transpressional (blind reverse) 1989 Loma Prieta Earthquake. The fault continues south through the historic mission at San Juan Bautista and through the town of Hollister (where active creep can be seen to offset sidewalks and even houses). The Transverse Ranges north of Santa Barbara are formed by compression across a major restraining bend to the east. Strands of the fault snake under the LA Basin. South of LA the fault expresses an extensional bend to the west -causing the Salton Trough. From there it connects into the active spreading under the Sea of Cortez.
The San Andreas fault was discovered by professor Andrew Lawson in 1895, who famously climbed into the faulted serpentinite well where the south tower of the Golden Gate Bridge was being poured. In spite of the extreme deformation of the serpentinite, Lawson declared the bridge perfectly safe. The events of the 1906 San Francisco earthquake would seem to substantiate Lawson's conclusion as the ruptured strand was not the one he observed in the footprint of the bridge.


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