Triggered Tremor from Half a World Away

The cutting edge of seismology these days is on slow and subtle things, among them being triggering and tremor. A particularly rich example of both at once will be presented next week in San Francisco. Triggering is when the seismic waves from big, distant earthquakes set off small jiggles elsewhere, the way a boat's deck might creak in heavy seas. Since 1992, when the Landers quake was shown to trigger seismicity all over western North America, the field has been lively. Tremor is a gentle juddering best known in connection with moving magma under volcanoes (I liken it to the familiar intestinal gurgling known officially as borborygmus), but the frontier phenomenon is non-volcanic tremor, detected only in the last decade. A team led by Abhijit Ghosh of the University of Washington will present what happened when the world's biggest quake in a generation, the magnitude-9 Sumatra event of 2004, shook the world's best-instrumented place, the San Andreas fault at Parkfield (itself a site of mysterious ongoing tremor). Triggered tremor was detected under Mustang Ridge north of Parkfield, where the same thing happened in 2002 after the great Denali quake. Ponder Ghosh's detailed records here and read more in a related press release.