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Apr 18 / Dave Schumaker

102 Years Ago…

One hundred and two years ago today, at 5:12am, the earth around the San Francisco Bay Area started to violently shake thanks to a ~M7.8 earthquake. A minute and a half later, buildings lay in ruins, while north of the city in Marin and Tomales Bay, fence posts laying across the “newly discovered” San Andreas fault (which didn’t have a name at that time) were offset by nearly 30 feet. The mayhem was just beginning however, as fires from knocked over furnaces and stoves began to rage out of control and completely destroy much of the city.

Despite the tragic loss of life in San Francisco (up to 3,000 people by some estimates), some good did result from the� catastrophe. It marked the dawn of earthquake science and seismology. The Lawson Report pegged the cause of the 1906 earthquake on the San Andreas fault. You can actually read the Lawson Report (published in 1910) online here: Volume I, Part I; Volume I, Part II; and Volume II.

The preliminary report (published May 31, 1906) details the initial recollection of the San Andreas Fault.

One of the remarkable features of the Coast Ranges of California is a line of peculiar geomorphic expression which extends obliquely across the entire width of the mountainous belt from Mendocino County to Riverside County. The peculiarity of the surface features along this line lies in the fact that they are not due, as nearly all the other features of the mountains are, to atmospheric and stream erosion of the uplifted mass which constitutes the mountains, but have been formed by a dislocation of the earth’s crust, or rather a series of such dislocations, in time past, with a differential movement of the parts on either side of the plane of rupture. In general this line follows a system of long narrow valleys, or where it passes through wide valleys it lies close to the base of the confining hills, and these have a very straight trend; in some places, however, it passes over mountain ridges, usually, at the divide separating the ends of two valleys; it even in some cases goes over a spur or shoulder of a mountain. Along this line are very commonly found abrupt changes in the normal slope of the valley sides giving rise to what are technically known as scarps. These scarps have the appearance of low precipitous walls which have been usually softened and rounded somewhat by the action of the weather. Small basins or ponds, many having no outlet, and some containing saline water, are of fairly frequent occurrence and they usually lie at the base of the small scarps. Trough-like depressions also occur bounded on both sides by scarps. […]

I love living in San Francisco, but it baffles my mind that one day this will happen again. And somehow, my choice of living here means I accept that fact and am okay with it. Crazy.

[Via USGS]

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