March 8, 2013
by Steve Malone
Following the major northern Washington-Vancouver Island ETS (Episodic Tremor and Slip) from July 30 until Oct 12, 2012 there was a reasonably large ETS in northern California from Oct. 15 until Nov. 10. The subsequent three months had comparatively little tremor anywhere in Cascadia; an average of less than 60 hours of tremor per month scattered in small bursts all over. Then beginning about Feb 12 with a three day burst in northern Puget Sound followed by scattered bursts in northern California lasting four to five days activity started really picking up on March 1. Strong tremor just south of Portland Oregon spread south-west for a few days and then is moving south almost to Corvalis now. A few days ago a strong burst started near the Oregon-California boarder and then yesterday there was a strong burst in the southern Puget Sound area under Olympia.
This map shows tremor locations over the past month (Feb 8 - Mar 8). Color coding is by time (blue is oldest, red youngest).
The central Oregon batch bares watching. This is the part of Cascadia for which full ETS episodes are fairly rare (from 18 to 23 months between them compared to 12 to 15 months for northern Washington ones and 9 to 12 months for northern Californa ones). If the current central Oregon activity is the beginning of a full ETS then it should last for another few weeks and extend south to the Roseburg area. The last major ETS in this region was in June, 2011, so one is about due.
Of course all deep tremor bursts may be associated with slip. However, until the geodesits can actually measure it we don't usually concider it a full blown ETS. We will need to wait until the geodetic data are in and processed and a card-carrying geodesist has blessed it before it is offical. In the mean time anyone can follow the progression of this event on our Interactive Tremor Map which is updated nightly.
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