Loading ...While the annual Sound Shake exercise on Wednesday produced a simulated magnitude 6.7 earthquake on the Seattle fault, a real, though unfelt seismic event is taking place beneath western Washington.
Earlier in the week, seismographs in the southern Hood Canal area began recording bursts of low-level shaking associated with what is called an episodic tremor-and-slip event. If this episode behaves true to form, the tremor will move north beneath the Olympic Mountains and across to Vancouver Island during the next two to three weeks.
This the fifth so-called slow-slip event to be recorded since the phenomenon was discovered in 2002, and it will be the most closely studied such event so far. University of Washington scientists and students are hurrying to deploy a special set of instruments, 100 temporary seismographs set in a close formation in the Olympic mountains, to record the current episode. The temporary stations will augment readings from the permanent seismograph network that covers all of Washington and Oregon.
“We hope to record unprecedented detail as the tremor moves beneath the seismometer array,” said John Vidale, a UW professor of Earth and space sciences and director of the Pacific Northwest Seismic Network.
Slow-slip events, or silent earthquakes, occur at a depth of about 25 miles and can last for several weeks. Though they are unfelt by humans, they can release as much energy as a large earthquake.
Since they were first discovered in the Puget Sound region, such events have occurred regularly about every 14 months. The current slow-slip event was expected to start between mid-February and mid-April, and the first evidence that it had begun turned up on Sunday.
It is expected that as the tremor runs its course, GPS stations in western Washington will move ever so slightly — about one-tenth of an inch — to the southwest. Then they will resume their normal slow march to the northeast at a rate of about a half-inch per year as the North American plate that lies under much of the Pacific Northwest is compressed by the Juan de Fuca plate where the two meet just off the Pacific coast.
Scientists have found that the fairly continuous tremor associated with slow-slip episodes is very difficult to locate precisely using standard techniques, so they hope special processing and the use of the temporary seismic arrays will help to pinpoint the exact location and source of the tremor, as well as its relationship to earthquake faults.
Future such studies could help to determine when the region might experience major earthquakes, and provide an understanding of just how large such quakes will be, Vidale said.
Details of the current work, along with scientific commentary, can be found at http://www.pnsn.org/WEBICORDER/DEEPTREM/winter2008.html .
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NEWS (latest at the top)
* Mar 7, 2008 - Tremor continues at moderate to strong levels during periods over the past two days but does not seem to have moved from its position in southern Puget Sound. Tim Melbourne from Central Washington University reports that no anamolous tilt is being observed on a long baseline tilt meter located at Shelton. Herb Dragert of Pacific Geoscience Centre reports that the GPS network of stations in the region are NOT showing any lateral motions consistent with an ETS. However, Wendy McCausland and Evelyn Roeloffs of the Cascade Volcano Observatory report that their analysis of the borehole strain meter B018 (south of Olympia) does show a subtle signal that may be the beginning of an anamolous strain event.
* Mar 5, 2008 - Tremor is continuing at moderate levels but variable at different times. 90 high-frequency verticals were installed today in a tight array in the central Olympics with the hope that the tremor will soon move north west right under the array. TA site, C04A has had an autonomous broad-band instrument installed. The experimental automatic tremor location routine by Aaron Wech is plotting contours of number of locations per area and shows where tremor is currently concentrated.
* Mar 4, 2008 - Tremor continues today at similar levels. A large crew is headed to the field tomorrow to deploy a heard of Texans. A CAFE station is being moved to the abandoned TA site of C04A to run autonomously for at least a month. The automatic tremor locator process is being updated with some improvements but is not yet plotting correctly. With everyone in the field tomorrow it will not be fixed until Thursday. A simple press release announcing the ETS start is being prepared.
* Mar 3, 2008 - Tremor has been detected in the South Puget Sound area starting yesterday (Mar 2). It is moderately strong on station HDW from time to time but quite weak on most other stations in the area. Unfortunately a key TA station C04A was removed a couple of weeks ago and thus is no longer available for detecting tremor. The CAFE autonomous stations were all serviced over the past two weeks and so should be working fine now. Plans are being made to do the installation of a many component array of high-frequency, vertical only sensors (Texans) at the Big Skidder site on Wednesday, Mar 5.
An experimental auto-locator for tremor is being developed by Aaron Wech. While only in the early testing mode we are putting up its results on this web page as of today.
* Feb 25, 2008 - No ETS starting yet. Unfortunately, the USArray has been pulling stations out of western Washington to move to their next deployment much farther east. This and some heavy snow conditions causing other station failures has reduced the number of good stations for studying the next ETS. However, the Texan array deployment project is just about ready to go. Most instrument sites have been found and marked so when tremor starts we should be able to deploy most instruments in a very short time.
* Feb 7, 2008 - No obvious tremor for the past week or so. The second half of Feb 5 looks like it might be tremor on the RMS-RSAM plots but while the over all envelope rises slowly and falls together on several stations the short term (minutes to tens of minutes) details don’t match. This pattern is consistent with general wind noise due to a storm. For much higher details we are now providing large PDF files of hourly envelopes for February, 2008 and for March, 2008.
* Feb 1, 2008 - The semi-realtime seismic amplitude (envelope) plots have been started again and should automatically update every hour or so. You will find these at: RMS-RSAM Envelope Plots.
* Jan 31, 2008 - A whole herd of Texans (RT125) have arrived at the PNSN labs. The PASSCAL expert, Steve Alzevedo is training a group of us to configure and operate the Texans. A siting trip will be made within the next week or so.
* Jan 29, 2008 - Again yesterday there were several hours with moderate tremor in the central Puget Sound. It seems that there will be several quiet days and then a day with some activity, but not enough to indicate a major ETS is underway. A simple plot of hours per day with tremor since the beginning of 2008 shows a few such peaks. An explanation of our current analysis process and a summary of 2007 activity is available under a Tremor Tracking Page.
* Jan 14, 2008 - During yesterday and today several hours of weak to moderate tremor was observed, mostly in the north Puget Sound region.
* Jan 10, 2008 - Discussions within the PNSN scientists are centering around doing a prototype experiment this winter using a bunch (~100) Reftek-Texan recorders in a very dense array to look for P-wave coherence from deep tremor. This experiment will be used to see if it is worth using these short-period, vertical-component only in a much larger experiment during the following ETS. Disccusions with PASSCAL indicate that we could get a set of Texans by the end of January, in time to deploy just as we first detect the arrival of the next ETS. These units will be run in continuous mode for one, two or three four day periods during the strongest tremor. They will be installed at or around the Big Skidder array site where there are 6 three-component instruments currently running.
* Jan 8, 2008 - New Tremor page started for the expected ETS of winter, 2008. According to Herb Dragert of PGC and based on his average GPS start-time interval for the 14 month ETS events of 450 days the estimated start time on southern Vancouver Island would be Apr 24, 2008 +/- 76 days. Since there may be a tendency for a long interval (previous interval was 506 days) to be followed by a shorter interval and the tremor often starts in Puget Sound area before Vancouver Island he estimates it could start as early as mid February.