Investigation of Low-Level Cascadia Tremor Between Major Episodes

Major episodes of episodic tremor and slip (ETS) located under the Olympic peninsula, Washington, and further north occur every 14 months and have been well-studied using both seismic and geodetic data. The next major episode is expected in November 2006. We look at the interval between major events using seismic data and find that there are still tremor episodes, although of smaller duration and magnitude. We examined vertical component data from 22 Pacific Northwest Seismograph Network seismometers broadly distributed across western Washington and Oregon. The data were band-pass filtered at 1 to 6 Hz. An envelope function calculated from these waveforms was then low-pass filtered at 0.03 Hz. We classify tremor as an emergent increase in seismic activity across multiple stations lasting anywhere from 10 minutes to many hours and exhibiting similarly shaped waveforms for nearby stations. Crude estimates of tremor location are based on the geographic variability of tremor amplitude. Typical smaller tremor bursts occur over 2 to 3 days, containing periods of up to 8 continuous hours exhibiting similar waveforms, and appear in 2 to 8 stations at a time. The longest period without any tremor was 10 days. Preliminary analysis shows that more tremor occurs under Washington than Oregon and occurs with a possible two-week periodicity, suggesting a potential effect of tidal forcing.