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In 2001, CWU researchers with the continuous GPS network Pacific Northwest Geodetic Array discovered periodic slow-slip across the Cascadia Subduction Zone. Previously undetected by seismic networks, these slip events exhibit regular recurrence intervals thus changing current understanding of earthquake behavior. Since this time, definitions for this newly discovered phenomenon have evolved. At first, the term "silent-earthquake" was employed to illustrate the absence of a seismic signature. Subsequent investigations and recent discoveries have led to a change in characterization. Now these slow-slip events are defined as eposodic tremor and slip (ETS).
In short, an ETS is a discreet time interval (episode) of relative tectonic plate movement (slip) coupled with high frequency seismic energy bursts (tremor). ETS usually last for around a few weeks duration as opposed to regular earthquakes where energy is released within seconds to minutes.
Due to continuous plate motion, the daily solution of any GPS measurement is recorded as a velocity; reflecting the overall strain accumulating in the upper crust. During an ETS, GPS velocities change direction until the event passes. This minute signal would go unnoticed without extended timeseries. With the proper corrections and long-term stablizations, GPS allows accurate measurements for each day. Since ETS usually last for over 10 days, this provides a nice measurement of offset that could not be recorded on the longest period seismometer.
There were nine arrays installed of ~10 continuous, three-component stations each starting in summer of 2009 through fall of 2010. Because of power limitations, the need for dataloggers for other PASSCAL experiments elsewhere and the amount of effort to service this many stations most arrays have been shut down for long periods when ETS was not expected. The 2011 ETS was expected to start in late October or early Novemeber based on the normal inter-ETS of 14-15 months. The arrays were to be restarted in early to mid Octorber.... BUT ------
On July 23 a tremor burst near Olympia (which we all thought was a typical short-lived inter-ETS tremor burst) just kept going. We waited for the tremor to die out, but it didn't. First it moved a bit south and then east and then hovered near Olympia until early August. We kept thinking it couldn't be a real ETS. It was too soon and seemed to start way too slowly. Besides much of the tremor was not as strong as we were used to seeing during previous ETS. But, after more than a week of significant tremor we started to panic and contacted the PASSCAL instrument center to see about getting some dataloggers back in the field sooner than originally requested.
PASSCAL came through and shipped us 48 datalogers on very short notice. THANK YOU PASSCAL STAFF! On Aug 5, when the tremor started clearly moving slowly north we double paniced and made preparations to head to the field ASAP.
Aug 20 - The leading edge of the tremor band reached Victoria this evening with the trailing edge now under the AofA. Indeed Honn Kao's TAMS Report from the PGC shows many coherent tremor locations under and just south of the southern tip of Vancouver Island. Today and tomorrow an AofA field team is checking on and possibly replacing week batteries at the more vulnerable array stations.