1957 San Francisco earthquake


The 1957 San Francisco earthquake occurred on March 22 at with a moment magnitude of 5.7 and a maximum Mercalli Intensity of VII. It was located just off the San Francisco Peninsula near the San Andreas Fault and was felt in a limited portion of Northern and Central California. There was a non-destructive foreshock and aftershock sequence that lasted for several months. With financial losses of around US$1 million, damage was considered minimal, with one death and forty injuries.

Tectonic setting

The San Andreas Fault is a northwest-striking transform fault that accommodates motion between the Pacific and North American Plates. The Mendocino Triple Junction is an area of high seismicity, and marks its northern extremity at the Gorda Plate, which is subducting beneath the North American Plate. Three moderate events in the San Francisco Bay Area occurred on or near the San Andreas Fault in the early nineteenth-century.

Earthquake

Eight foreshocks preceded the main event. The first motion method was used to determine the focal mechanism of the mainshock. It was found to be dissimilar from the 1906 earthquake, and instead showed oblique movement on a steeply-dipping thrust fault, with the eastern side of the fault rising relative to the western side. The strike-slip component was minimal; only about half as much as the thrust component.

Damage

Damage was non-structural and was limited to content within buildings and cracked plaster and was estimated at $1 million, not including loss to building content. The most significant effects were seen in the western portion of Daly City and in the Lake Merced area of San Francisco. The minimal losses were attributed to the short duration and lack of high intensity shaking.

Strong motion

The event was felt over an area of 12,000 square miles. Its scientific value was reinforced because it was captured on 13 strong motion instruments. A maximum peak ground acceleration of 0.13 g was recorded in Golden Gate Park and.18 g was recorded on the fourteenth floor of a building in San Francisco.