‘Array of arrays’ offers better understanding of unfelt seismic tremor events

WASHINGTON - University of Washington researchers have devised a new technology that gives a better picture of how episodic tremor events relate to potentially catastrophic earthquakes, perhaps as powerful as magnitude 9, that occur every 300 to 500 years in the Cascadia subduction zone in western Washington, Oregon and British Columbia.

Scientists discovered episodic tremor about a decade ago and have been trying to understand how it figures in the seismic hierarchy of the earthquake-prone Pacific Northwest.

In 2008 on Washington’s Olympic Peninsula, UW scientists deployed an array of 80 seismic sensors that act something like a radio antenna, except that instead of bringing in distant radio waves it collects signals from tremor events.

It was known that tremor events generally start near Olympia, Wash., and march slowly northward on the Olympic Peninsula, eventually reaching Canada’s Vancouver Island and running their course in several weeks.

Abhijit Ghosh, a UW doctoral student in Earth and space sciences, has found the tremor movement to be far more complex. The source of the tremor generates streaks that travel 60 miles per hour back and forth along a southwest-northeast track.

The arrays are producing enough data for scientists to locate the precise latitude and longitude where a signal originates, Ghosh said, but more work must be done to determine precise depths.

“Because the signal is very different from our garden variety earthquakes, we need new techniques to determine the source of the signal, and this is one step toward that.

“With the array of arrays we should be able to see a greater quantity of clear signal, and we do. We see more tremor - way more tremor - than with conventional methods,” he said.

The findings were presented at the annual meeting of the American Geophysical Union in San Francisco. (ANI)