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New seismic observations indicate that the Himalayas and Pacific Northwest may be primed for major earthquakes.
The Cascadia subduction zone, which stretches from northern California to Vancouver Island, han’t experienced a major seismic event since 1700, when an 8.7-9.2 magnitude earthquake created a tsunami that reached Japan.
While many geophysicists believe a similar-scale event is now, there’s little earthquake data for the region, making it difficult to predict how ground motion from a future event would propagate in the area around Seattle, Portland and Vancouver.
But Stanford postdoctoral scholar Annemarie Baltay has found that measurements of small seismic tremors in the region can be used to work out how ground motion from larger events might behave. By measuing low amplitude tectonic tremors 30 kilometers below Earth’s surface and analyzing how the tremor signal decays along and away from the subduction zone, she says she can calculate how ground motion activity from a larger earthquake will dissipate.
“We can’t predict when an earthquake will occur, but we can try to be very prepared for them,” says Baltay. “Looking at these episodic tremor events can help us constrain what the ground motion might be like in a certain place during an earthquake.”
Meanwhile, Stanford geophysics Professor Paul Segall has used computational models of the region to determine whether the cumulative effects of many small events can trigger a major earthquake.
“You have these small events every 15 months or so, and a magnitude 9 earthquake every 500 years. We need to known whether you want to raise an alert every time one of these small events happens,” he says. “What our calculations have shown is that ultimately these slow events do evolve into the ultimate fast event, and it does this on a pretty short time scale.”
Perhaps thinking of what happened to a group of Italian seismologists earlier this year, he adds: “We’re not so confident in our model that public policy should be based on the output of our calculations, but we’re working in that direction.”
Meanwhile, it’s been known for some time that India is gradually sliding under Asia, previous observations had indicated a relatively uniform fault plane – the Main Himalayan Thrust (MHT) – that dipped a few degrees to the north.
But new seismic data shows not only this gentle dip, but also a segment of the thrust that dips at a much steeper 15 degrees downward for 20 kilometers. Such a ramp could be a starting point for massive earthquakes in the Himalayas, with the MHT historically responsible for a magnitude 8 to 9 earthquake every few hundred years.
“What we’re observing doesn’t bear on where we are in the earthquake cycle, but it has implications in predicting earthquake magnitude,” says Warren Caldwell, a geophysics doctoral student at Stanford.
“From our imaging, the ramp location is a bit farther north than has been previously observed, which would create a larger rupture width and a larger magnitude earthquake.”