Earthquake of 3.4 magnitude shakes San Francisco on 105th anniversary of 1906 quake
On April 19, 2011 At 12:00 am
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Residents of San Francisco got a bit of a shake up today when a 3.4 magnitude earthquake disrupted power for 10,000 residents in the Bay area. The US Geological Survey reported that theearthquake hit about 2:57 Pacific Time, with the epicenter located near San Mateo.
Though the outages occurred right around the quake, PG&E is still investigating what cause a circuit breaker at one of their substations to open, Tell said. Around 4:30 p.m., the circuit breaker was closed as power has been restored, Tell said.
The initial shaker was followed by two quakes – a 1.4-magnitude at 3:25 p.m. and a 1.2-magnitude at 3:28 p.m., said USGS spokesperson Paul Laustsen.
“It’s a very weak earthquake,” said Laustsen. “And a very common one for California.”
USGS initially reported the quake as a 3.8-magnitude, but has since been scaled at 3.41, said Laustsen.
Rupturing the northernmost 296 miles (477 kilometers) of the San Andreas fault from northwest of San Juan Bautista to the triple junction at Cape Mendocino, the earthquake confounded contemporary geologists with its large, horizontal displacements and great rupture length. Indeed, the significance of the fault and recognition of its large cumulative offset would not be fully appreciated until the advent of plate tectonics more than half a century later. Analysis of the 1906 displacements and strain in the surrounding crust led Reid (1910) to formulate his elastic-rebound theory of the earthquake source, which remains today the principal model of the earthquake cycle.At almost precisely 5:12 a.m., local time, a foreshock occurred with sufficient force to be felt widely throughout the San Francisco Bay area. The great earthquake broke loose some 20 to 25 seconds later, with an epicenter near San Francisco. Violent shocks punctuated the strong shaking which lasted some 45 to 60 seconds. The earthquake was felt from southern Oregon to south of Los Angeles and inland as far as central Nevada. The highest Modified Mercalli Intensities (MMI's) of VII to IX paralleled the length of the rupture, extending as far as 80 kilometers inland from the fault trace. One important characteristic of the shaking intensity noted in Lawson's (1908) report was the clear correlation of intensity with underlying geologic conditions. Areas situated in sediment-filled valleys sustained stronger shaking than nearby bedrock sites, and the strongest shaking occurred in areas where ground reclaimed from San Francisco Bay failed in the earthquake. Modern seismic-zonation practice accounts for the differences in seismic hazard posed by varying geologic conditions.
In comparison, the earthquake today was very minor. There have been no reports of damage or injuries.