by Mary Caperton Morton Thursday, October 2, 2014
The San Andreas Fault (SAF) in California is one of the most active in the U.S., but the 1,300-kilometer-long strike-slip fault seems to only be susceptible to small-scale dynamic triggering. After the magnitude-9 Tohoku quake in Japan in 2011, the SAF experienced an elevated incidence of tremor up and down its length. The tiny tremors were recorded at depths between 16 and 30 kilometers, below the fault’s seismogenic zone.
“If a regular earthquake sounds like firecrackers, the tremors along the San Andreas Fault can be likened to chattering,” says Zhigang Peng, a geophysicist at Georgia Tech in Atlanta. “Tremor is more of a continuous vibration.”
Tremor is common along the SAF, but the behavior after Tohoku was different. “We weren’t just seeing random spots of tremor, but coherent propagation of tremor events lasting tens of minutes,” says David Shelly, a geophysicist at the U.S. Geological Survey (USGS) in Menlo Park, Calif. That observation led Shelly and other researchers to hypothesize that the seismic waves were triggering individual patches of the fault that were propagating out into slow-slip events.
The observation offers potential clues as to why some triggered events are delayed long after seismic waves have passed. “Slow slip can last for minutes to days to weeks,” Shelly says. “So if seismic waves are capable of triggering drawn-out slow-slip events, which could in turn trigger larger events down the line, that may explain the delay we see with some of these triggered earthquakes.”
In that case, the tremors would be dynamically triggered but the following quakes could be considered statically triggered by local forces. “In effect, the triggering of these tiny tremors has revealed a level of complexity deep in these faults that we couldn’t see before,” Shelly says.
Similar episodes of triggered tremor have been observed at strike-slip faults elsewhere in the world, including Haiti and Greece. “The next step is to look at tremor in the deep parts of fault zones and see how it links to activity in the shallower seismogenic regions of the fault,” says Tom Parsons, a seismologist at USGS in Menlo Park. “The tremor may be able to tell us something about how fluids move up and down fault zones and how seismic waves passing through affect the plumbing and pressures in the fault zone.”
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