Scripps seismologist learns state regions' aftershocks dissimilar
It's lunchtime, and old friends are gathered at a restaurant, chatting away. Suddenly, their table starts shaking. Glasses clink. Silverware shifts. Everything has been set in motion by an earthquake that most of the diners ignore.
Eventually, one friends asks another: "How big is it?" The blithe reply: "4.0."
Actor Steve Martin delivers the verdict in the 1991 movie "L.A. Story," a social satire that suggests that Californians are so savvy about earthquakes that they overlook anything but a catastrophic event.
It's an insightful moment; many of us have stood around guessing the size of a quake rather than taking cover. Such behavior is odd, because seismologists like Debi Kilb will tell you that scientists only have a rudimentary understanding of quakes.
Kilb works at the Scripps Institution of Oceanography, where she studies the basic nature of earthquakes, particularly aftershocks. She also studies how a quake can trigger another shaker at a distant location. Kilb told us what scientists have been learning during a recent series of interviews.
Q: You've been studying the aftershocks of moderate-sized quakes in Southern California. Any surprises in the data?
A: Yes, which is exciting because so often an unexpected finding or observation can be a clue to help answer unanswered scientific questions. In my postdoctoral work at Princeton, I studied aftershock sequences in Northern California along the Hayward, San Andreas and Calaveras faults. I expected the signature of aftershocks in Southern California to be similar, but that's not the case. We are seeing more evidence that seismicity patterns in the south are much more complicated than patterns in the north.
We're also finding deeper earthquakes in the south. They run from the surface to a depth of about 12 miles. Quakes in Northern California typically extend to only about six miles in depth.
The "footprint" of aftershock sequences also is different in Southern California. Generally speaking, aftershocks here tend to occur within a volume, whereas in Northern California the aftershocks tend to be more constrained to a single fault plane. So, it's a 3-D distribution versus a 2-D distribution.
And there's a notable difference in the size of aftershocks. Typically for magnitude 5.0 mainshocks in Northern California, the largest aftershocks will be about magnitude 4.0. In Southern California, the largest aftershocks to 5.0 mainshocks measure about 3.0.
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View entire article here: http://www.signonsandiego.com/news/2010/oct/11/exploring-a-new-take-on-quakes/
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