April 18, 2011

105th Anniversary of SF’s 1906 quake

Posted by Austin Elliott

5:12 a.m. PDT this morning marked the 105-year anniversary of “the San Francisco earthquake,” the >400 km rupture of the northern San Andreas fault that began just offshore and propagated along the fault in both directions for several minutes, all the while churning and twisting the blossoming city above it.

Maximum shaking intensity as a result of the 1906 earthquake: the results of a large concerted effort by the USGS and several California universities to simulate major earthquake ruptures throughout the state

It’s helpful to take a moment and reflect upon the reality of the event. I get the impression that such high-profile events lose their meaning as we talk so commonly about them. “The 1906 quake” is surely a part of the California lexicon, if not the country or world, however it still stuns me to realize that this event did happen. Just a century ago the gleaming city of San Francisco was shaken to its core and burned even further. We have come a long way (basically the whole way) in understanding earthquakes since then, and we now construct buildings with mitigation of earthquake risk in mind, but in America we have yet to be tested by another such earthquake so near a huge population center.

To help transcribe the impact of that turn-of-the-last-century event into modern consciousness, my friend John McDaris among a group at Carleton College’s Science Education Resource Center compiled an awesome array of informative and illustrative resources for the quake’s centennial 5 years ago. These include photos, videos, maps, and–most importantly–animations. I encourage eye-opening perusal of all of them, but especially the animations of modern structures subjected to the shaking of the 1906 earthquake.

An evocative screenshot from a simulation of the Golden Gate Bridge as it would have been shaken by the actual 1906 earthquake. Fortunately the display is exaggerated 100 times for illustration

Have at it! Happy anniversary.

[Update: 4/18/11 3:33pm] In an amusing little fluke of nature (or is it? –it is.) San Francisco had a nice little 3.4 jolt today, centered squarely within the San Andreas fault zone just south of the city. Mother Nature’s tip-o-the-hat to all our commemorations, if you want to think of it that way. To my utter disappointment it didn’t ripple the ground quite hard enough to make it rattle Davis, but plenty o’ San Franciscans got a mid-day jostling.

USGS "shakemap" showing instrumentally recorded shaking amplitude during this afternoon

Don’t make too much of this; M3-4 earthquakes are par for the course in the Bay Area: just as likely to occur any other day. Plenty more have happened not on major anniversaries.