December 4, 2012
Scientists Warn Of Major Earthquakes In Himalayas And Pacific Northwest
Lee Rannals for redOrbit.com — Your Universe Online
Scientists at the American Geophysical Union meeting in San Francisco this week reported that the Himalayas and Pacific Northwest could experience major earthquakes.Researchers have begun studying the complexity of the Himalayan range in greater detail, particularly the fault that separates the two plates known as the Main Himalayan Thrust (MHT).
Previous observations indicated a fault that dipped a few degrees to the north. In order to produce a clearer image of the fault, Warren Caldwell, a geophysics doctoral student at Stanford University, analyzed seismic data from 20 seismometers deployed for two years across the Himalayas.
The data showed a thrust dipping in two to four degrees northward, and also revealed a segment of thrust that dips 15 degrees downward. A ramp such as the one discovered by the researchers could be a nucleation point for massive earthquakes in the Himalayas.
Caldwell pointed out that the MHT has historically been responsible for magnitude 8 to 9 earthquakes every several 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," Caldwell said in a statement. "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."
Stanford Geophysics Professor Simon Klemperer said recent detections of magma and water around the MHT indicate which segments of the thrust will rupture during an earthquake.
"We think that the big thrust vault will probably rupture southward to the Earth's surface, but we don't expect significant rupture north of there," said Klemperer.
The team said the findings are important for creating risk assessments and disaster plans for the heavily populated cities in the area.
Stanford postdoctoral scholar Annemarie Baltay presented measurements of small seismic tremors in the Cascadia subduction zone, which has not experienced a major seismic event since it ruptured in 1700. Her research involves measuring low amplitude tectonic tremors that occur at the intersections of tectonic plates, roughly over the course of a month each year.
By analyzing how the tremor signal decays along and away from the Cascadia subduction zone, Batlay 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," Baltay said in the statement. "Looking at these episodic tremor events can help us constrain what the ground motion might be like in a certain place during an earthquake."
Paul Segall, a Stanford geophysics professor, who studied the slow slip and tremor events in Cascadia, said his team is doing sophisticated numerical calculations to simulate these events and see whether they relate to big earthquakes over time.
"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," Segall noted.