Origin time 19:44:22 UTC Depth 15 km (9.3 mi) | Magnitude 5.7 Mw Type Oblique-slip | |
 | ||
Date March 22, 1957 (1957-03-22) Areas affected San Francisco Bay Area
Northern California
United States | ||
The 1957 San Francisco earthquake (also known as the Daly City earthquake of 1957) occurred on March 22 at 11:44:22 local time with a moment magnitude of 5.7 and a maximum Mercalli Intensity of VII (Very strong). The shock was located in Northern California in the vicinity of the San Andreas Fault. There was both a foreshock and aftershock sequence that lasted through the month of June. Damage was considered minimal, but financial losses amounted to around US$1 million.
Contents
Tectonic settingEdit
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.
EarthquakeEdit
Eight foreshocks (with a maximum magnitude of 3.8) 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.
DamageEdit
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 motionEdit
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 (free field) peak ground acceleration of 0.13g was recorded in Golden Gate Park and .18g was recorded on the fourteenth floor of a building in San Francisco.