Monday, October 3, 2011

All California Earthquakes Happen on the San Andreas Fault: Quiz Question #2

An offset curb on the Calaveras fault in Hollister, CA. It isn't the San Andreas...
Question #2 on our short quiz on what everyone "knows" about earthquakes was:

All California earthquakes happen on the San Andreas fault - True or False?

The crowd at my recent lecture, California residents all, were up to date on this one. Not a single audience member said "true", and they were right. On the other hand, lots of people don't live in California, and they might not be quite as knowledgeable about all of California's faults (double entendre intended). So a few illustrations may be helpful. First of all, where do the earthquakes in California happen? See the map below...
Map courtesy of NDEDC, UC Berkeley.  Follow the link for an interactive version.

A quick look at the seismicity between 1970 and 2003 shows that earthquakes occur over much of California, and that the San Andreas fault is not even one of the most active. Long sections show little or no activity, meaning that stress is building up, leading to powerful quakes in the future.

But what about the biggest quakes? A map that concentrates only on larger quakes, magnitude 5 and above, shows that large damaging quakes also happen on faults other than the San Andreas (see the map below). Of California's three biggest historical quakes, only two were on the San Andreas. The 1872 Lone Pine quake resulted from movement on the Owens Valley fault east of the Sierra Nevada.
Map courtesy of NCNDE, UC Berkeley. Follow the link for an interactive map.
The good news for those of us who live in the flat, dusty, boring Central Valley is to note how few earthquakes, big or little, ever happen near us. I haven't actually felt an earthquake here since the early 1990s, although most of my students felt one during a class break in 2007. Read the story here, and realize my explanation of magnitude and intensity was misunderstood (magnitude and intensity are two entirely different ways of measuring earthquakes).

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