Notable Earthquake of the Week
San Francisco Ms 8.2, Mw 7.8 Earthquake
The San Andreas fault zone is the dominant fault system in California. It forms the boundary between the Pacific and North American tectonic plates. At 5:12 a.m., April 18, 1906, a powerful earthquake measuring Ms 8.2, Mw 7.8 occurred along the northern San Andreas fault segment. (See some background photos of the San Andreas fault)

Surface ruptures occurred for over 270 miles, from a fault splay at Shelter Cove, Humboldt County, to Pajaro Gap at the southern extent of the Santa Cruz Mountains. Displacements of 10-15 feet were common with a maximum offset of 21 feet measured at Olema in Marin County. (See fault offset photos)

Shaking was locally violent, lasting up to 45-60 seconds with peak shaking intensities of XI on the Modified Mercalli scale (a relative shaking intensity rating from 1-12 and shown in Roman numerals).

The severe shaking brought down buildings 60-80 miles away from the rupture zone, and caused cracked walls up to 100 miles away. The ensuing fire in San Francisco was devastating and is well documented. The commonly quoted total of 700 deaths is believed to be underestimated by 3-4 times. (See more damage photos).

Interestingly, no significant earthquakes have struck the northern segment of the San Andreas fault since this great quake and scientist believe it is locked and poised for another strong jolt.


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