...... PNSN ETS HISTORICAL RECORD

Summer 2011 ETS

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• Sep 6, 2011 - This ETS is over. The last burst of tremor seen at the north end of this ETS in BC occurred on Sep 3. It was followed on Sep 4 by a burst back in the south end of Puget Sound but nothing has been seen since then. Thus we declare the ETS over. It lasted from Jul. 23 until Sep. 4, longer than any previous ETS well recorded and processed by us. Besides starting much earlier than expected it started farther south than most previous ones and didn't proceed quite as far north.
  - A team of 8 are currently in the field recovering all of the AofA instruments and the remaining CAFE ones. Between Sep 6 and 8 over 90 seismic stations will be recovered from the field and preparations made to ship them back to the PASSCAL instrument pool. The data collected for this ETS will not be available for analysis for a week or two but should be an interesting set.
• Sep 2 - This ETS seems to be the longest lived of any that we have well monitored. It will have lasted 6 weeks tomorrow. But maybe it is about to quit. In the past 24 hours there seem to be decreasing tremor. On the summary spectrograms near the end of August little can be seen on the traditional tremor-sensitive station, HDW (though it is far enough south that moderate level tremor up in BC probably can not be seen. However this ETS has not yet progressed as far north as the past several ones have. To extend to its more usual stopping place there should be another week or so left in the puppy.
• Aug 30 - The tremor continues to slog northwestward, but slowly. On Aug 27 it seemed to jump 20 km to the northwest but then has hung in the backwoods just north of the Leech River fault since then. Plans are being made for an assault on the AofA next week to rip them out of the field and send all equipment back to their PASSCAL home. After three years of periodicaly slogging through clear cuts, ranch and forest land to service, repair, replace, repower these stations it will be time for one last big "vacation" trip to the northern Olympic tourist detinations of CL, BS, DR, GC, PA, TB, BH and of course our favorite, LC (particularly LC13).
• Aug 26 - Well this ETS seems to again be acting a bit differently. Tremor continues at a good clip but the tremor front has slowed down considerably and seems stalled under the southern end of Vancouver Island. In the past week rather than moving at the normal rate of about 10 km per day the motion is no more than about 4 km per day. Maybe its hanging around so the AofA can get a good look at it.
• Aug 23 - Today the tremor is centered under the southern Vancouver Island coast west of Victoria. It has now left the area directly under the AofA and probably will be out of range for the best studies within a week. Reports from the field crew who were checking on stations over the past two days are that out of over 80 stations only two were down because of battery problems. Good job AofA crews!
  - There is a very fine PBO Strainmeter summary page for the current ETS with background, comparisons and an animation provided by Kathleen Hodgkinson of UNAVCO.
• 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. Tomorrow and Monday an AofA field team is checking on and possibly replacing weak batteries at the more vulnerable array stations.
• Aug 18 - Its clear this ETS is moving north as usual. The leading edge should reach Victoria tomorrow evening and by Sunday the trailing edge will have left Washington. One can see that the two week's RMS plot shows greatly decreased levels on HDW and increasing levels to the north on MCW (San Juan Islands). On the summary spectrograms one can easily see that the tremor was passing directly under HDW on Aug 13-15.
  - To illustrate the geodetic changes going on I have made a summary page that is a static GPS data summary page derived from the plots at the CWU Pacific Northwest Geodetic Array. I have also made a summary page of strain data provided by Kathleen Hodgkinson of UNAVCO from processing PBO strain meter data that allows for comparisons of strain records for the past few ETSs. Please don't expect this blogger to understand what any of these plots mean or how they are derived. Seismology is simple compared to strain and geodesy.
• Aug 17 - Your pathetic blogger has been out of sector for the past five days goofing off (riding my bike 350 miles over the Cascades and back) and thus have gotten behind on what is going on with this ETS. So to catch up, here are some of the highlights.
  - Tremor locations have progressed more or less as normal in a northwesterly direction but with what might be a bit more jumping around than in previous ETS. For example between Aug 13 and 14 the tremor changed from a relatively compact bunch centered over Hood Canal to a much more dispersed grouping extending from southern Puget Sound to the north coast of the Olympics. Currently the leading edge of the tremor has crossed into the Straights and thus is pretty much centered under the AofA. We expect that the PGC folks will see the tremor under their house in just a couple of days.
  - The AofA field crew was very busy over last weekend. As of Sunday night all arrays have been restarted with only a few missing stations. Unfortunately, because many of the aircells are on their last legs there were a few station failures just over the previous week. If one cell goes bad out of five it takes down the whole site. Rechargable batteries were used for many stations but we have access to only a limited number of these and they are very HEAVY to get to some sites. Plans are being made to do a quick field check of stations this coming weekend.
• Aug 11 - The tremor front is just crossing Hood Canal onto the Olympic Peninsula and thus is now on schedule to be under the Array of Arrays in four to five days. Speaking of AofA, today plans were made for a return trip to the Olympics to finish getting all (most) of the arrays running again. Another shipment of dataloggers from PASSCAL arrived yesterday and some older rechargeable batteries were all charged up. With 18 such batteries and a dozen partially used air-cells we think we have enough power to get all arrays other than LC going again. The field team led by the FOC (Justin Sweet) is off first thing tomorrow.
• Aug 10 - Over the past four days the tremor front has been moving at about 10 km per day to the NNW as is normal for this part of the ETS. However in comparing this ETS with previous (as found on the Comparison Maps) it would seem that we are now in a typical week #1 of an ETS rather than half way through week #3. This ETS seems to have started further south than previous ones, moved much slower during the early parts, but is now progressing more or less normally.
  - In response to my sending out a note on the start of this ETS I received comments from a number of people. Andrew Miner of CWU and Shelley Olds of UNAVCO both called my attention to a tremor period in July in northern Oregon. Indeed this appears to be a northern Oregon ETS starting about Jun. 3 near Eugene and Salem, coalescing and then moving north to Longview and ending on Jul. 3. This ETS reportedly has a strong GPS signal similar to one in 2009. Andy suggests that the early Washington ETS may have been triggered by what may be an unusual amount of ETSing over much of Cascadia in the previous couple of months including: Southern Oregon - May 15 - 23; Vancouver Island - May 22 - Jun 6; Northern California - May 29 - June 10. Because the UW research efforts have primarily been concentrated in the Olympics we do not keep a running blog on other ETS even though the automated Interactive Tremor Map shows tremor locations for most of Cascadia.
  - Evelyn Roeloffs of the USGS and Kathleen Hodgkinson of UNAVCO report that there doesn't seem to be any evidence of the current Washington ETS on PBO strainmeters; however a couple of critical ones for detecting the start of this (B018) are having telemetry problems and thus are currently missing some recent data.
  - Anne Trehu from Oregon State University reports that a bunch of OBSs were deployed just off the southern Washington coast at the end of July as part of the Cascadia Initiative Expedition Team. While the data from these instruments will not be available until the OBSs are recovered in a year they could provide an interesting view of what goes on up-dip of the start of an ETS event.
• Aug 8-9 - With a mighty effort, requiring shuffling various air-cell batteries around from different sites because of battery failures, and two long days we got two and two-half arrays restarted. BS and CL were already running and BH and TB were totally restarted while half of the stations in GC and DR were started before all the air-cells with any life left were used up. Array LC is a lost cause and not worth restarting and so some of its few good air-cells were removed to help at other arrays. Of course we were convinced that such heroic efforts would turn the ETS off; but it continues.
• Aug 7, 2011 - Based on the tremor locations provided by the Wech-o-meter (Interactive Tremor Map) and the strength of the tremor as seen on the spectrograms and after consultations with UW tremor honchos (not to mention throwing some dice, consulting soothsayers and reading tea leaves) Justin Sweet, the FOC (Field Organizing Czar) rallied troops to head to the field. He reserved three vehicles (on a Sunday) and cajoled 5 others to load them up with equipment and to leave early the next morning for the Olympic Peninsula.
  Tremor seemed to be clearly moving north of Olympia. A check of the Central Washington University regional GPS plots for Willapa Hills (WWH) summary plots show that several of the key stations have significant changes starting in July. Those in the Olympics (WOP) don't yet seem to be showing anything unusual.

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  BACKGROUND INFORMATION

  The University of Washington seismic group began studying deep tremor associated with Episodic Tremor and Slip (ETS) events in 2003 using both the regional PNSN and small aperture arrays. With NSF funding we installed three small aperture arrays in the spring/summer of 2004 and then Cascadia Arrays for Earthscope (CAFE) for both structural and tremor studies in 2007-2008. The CAFE experiment captured the ETS of fall, 2007 and winter, 2008.

  In the past using stations from the PNSN and the USArray Transportable Stations (TA) we used a semi-automated processing system to track the general occurence of where and when deep tremor takes place. Starting in the fall of 2009 a completely automated tremor detection and location systems was put on line by Aaron Wech with an Interactive tremor map web interface.

  As in the past we collaborate closely with the seismologists and geodesists of the Pacific Geoscience Centre (Natural Resources Canada) and Central Washington University to follow the development of the next ETS.

  Previous Studies of ETS by UW scientists

  Other information about ETS

  • Slow slip events and episodic tremor - from the Pacific Geoscience Centre of the Geological Survey of Canada.
    Note that the PGC provides a very nice near realtime plot of key GPS stations for tracking motions during the next ETS.
  • Geodesy and Slow Earthquakes from the Geodesy Labs of Central Washington University.
    You can find sliding three year windows of daily grouped updates of cleaned, detrended GPS components for all of western North America. Probably the most interesting ones for the 14 month ETS are the longitude components of BC.Cordillera Region and the WA, Olympic Peninsula region though there are others here that show changes as well.
  • Borehole strain meter data from the Plate Boundary Observatory (PBO) are available in near real-time.
  • Processed PBO Borehole Strain meter data by John Langbein (USGS) to detect rate changes.

  Acknowledgments

  • Steve Malone maintains this blog
  • Tony Qamar wrote the original tremor envelope plotting software
  • Ken Creager and Aaron Wech of UW developed the cross-correlation location plots
  • Aaron Wech developed and helps maintain the Interactive Tremor Web pages.
  • Garry Rogers of PGC provides routine update information on tremor observed in Canada
  • Herb Dragert of PGC provides analysis plots of GPS data
  • Tim Melbourne of CWU provides analysis and plots of GPS and long baseline tilt-meters data.
  • Honn Kao of PGC provides routine tremor analysis for Canadian stations.
  • Evelyn Roeloffs and Wendy McCausland of USGS provides analsyis of borehole strain meter data
  • John Langbein of USGS provides strain meter analysis plots.
  • Justin Sweet and Ahbijit Ghosh are the Array of Array field specialists
  • Realtime seismic data is primarily from the PNSN but also uses data from the Canadian National Seismograph Network, the Northern California Seismic Network, Pnd the late Boundary Observatory and USArray, both of EarthScope.

  Financial Support

  • Routine operations of the Pacific Northwest Seismic Network is supported by a cooperative agreement from the US Geological Survey, a contract from the Battelle Pacific Northwest Laboratory, and funding from the State of Washington and the School of Arts and Sciences of the University of Washington.
  • Special funding for deep tremor studies has been made available from EAR-Geophysics of the National Science Foundation and from EAR-EarthScope Science of the National Science Foundation.
  • Ken Creager, John Vidale and Heidi Houston are PIs for the Array of Array experiment supported by EarthScope of the National Science Foundation.