Seismic Array Studies of Cascadia Deep Tremor

University Of Washington, Seattle WA

Investigators

Abstract

What is being called "Deep non-volcanic seismic tremor" or "Episodic Tremor and Slip" (ETS) events are being studied using a combination of regional network stations of the Pacific Northwest Seismograph Network (PNSN) and three specialized small aperture seismic arrays. These bursts of low-level tremor accompanying geodetically observed slip have occurred at regular 14 month (plus or minus two months) intervals for the past eight years. The next ETS event is expected in May-June, 2004. The seismic arrays will operate simultaneously over a long enough time to catch all of the tremor signals during the next ETS period. Deep tremor has now been observed by the Pacific Geoscience Center and the PNSN in the Cascadia subduction region of northern Washington and southern British Columbia. Periods of relatively large amplitude tremor seem to occur at the same time and in roughly the same areas as silent slip events just down-dip of the locked zone. Regionally spaced seismic stations can detect the tremor but their data can be used for only crude locations. Three small arrays (two in northern Washington and one in southern British Columbia) are used as seismic antenna that can resolve the wave fronts of tremor signals for better time and spatial resolution of the tremor events than is possible with isolated single stations. Better locations and their temporal evolution will help to understand the tremor's relationship to the silent slip events and perhaps future great megathrust earthquakes.