Slow earthquakes recorded in the Pacific Northwest
Beginning in mid February, 2003, the Pacific Northwest Seismograph Network started observing strange, long-lasting, weak signals on seismic station in the north Puget Sound area. These tremor-like signals have a frequency content around 2-6Hz; and show up just above the background noise on some stations without filtering, but show up on more if the signals are filtered. These appear to be the same type of tremor observed in Japan by Obara. Scientists at the Pacific Geoscience Center in British Columbia report observing these same signals on their stations and, in fact, have observed these sorts of tremors in previous years. They associate them both spatially and temporally with slow slip events as detected by the geodetic network. The Pacific Northwest Geodetic Array (PANGA) has a processing lab at Central Washington University where time series from some stations can be seen.

On any individual station, the periods of tremor are usually very subtle. Similar types of signals can also look like noise due to such things as wind, vehicles, or other cultural activity near the station. However, by combining information from several stations its possible to eliminate local noise sources as the cause of the signals.

A typical processing technique is to filter the signals for several stations and then compute a smooth envelope of the signal. Plotting the filtered velocity seismograms and the envelope (in red) together makes it obvious that the period of increased tremor is common to many stations at nearly the same time. By correlating the envelopes between stations it is possible to get a relative delay time between them. When combining these for many stations, one can see a progression from the early arriving stations to later ones, which seem to travel at S-wave velocities.

One can then use a standard earthquake location program with these apparent S-wave arrivals to try and locate the source of these signals. Because the time resolution of these relative times is poorly constrained, the accuracy of such locations in not very good. Applying this method to a few strong tremor periods produces locations in southern Vancouver Island and the northern Olympic Peninsula. Depths are very difficult to constrain, but seem to be in the range of 30 to 40km deep. The map shows the locations of the seismic stations as red triangles. The blue dots show selected GPS sites that have detected motions interpreted to be a slow slip event or "silent earthquake" observed in early March, 2003 (Press Release).

Submitted by Wendy McCausland, Department of Earth and Space Sciences, University of Washington