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Episodic Tremor and Slip in the Pacific Northwest

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Episodic Tremor and Slip in the Pacific Northwest winter/spring 2007 featured science: Episodic Tremor and Slip in the Pacific Northwest Every 14 months the Pacific Northwest experiences slow slip on a aultf that is the equivalent of about a magnitude 6.5 earthquake. While a typical earthquake of this magnitude happens in less than 10 seconds, the duration of these slip events is two to several weeks. The most recent event occurred from January 14 through February 1, 2007. In the Pacific Northwest, the Juan de onSitenewsletter Fuca plate is subducting (or dipping) beneath the North American plate from northern California From The Principal Investigators to Vancouver Island. EarthScope’s multi-disciplinary approach These plates slide past to improving our understanding of the structure each other along the and formation of continents and the physical solid green, dashed Figure 1. Courtesy of H. Dragert, Geological Survey of Canada processes that control earthquakes and yellow and dashed red lines in Figure 1. The Cascadia subduction zone, as it is volcanoes has spurred the development of new called, experiences large earthquakes, perhaps as large as the 2004 Sumatra techniques for analyzing and interpreting many earthquake, once every 500 years on average. The last one was about 300 years types of geophysical data. Throughout this issue, ago in January 1700. The slip during these earthquakes, occurring on the “locked” we highlight some of EarthScope’s integrated zone in the figure, is thought to accommodate most, if not all, of the relative activities. motion between the North American plate and the Juan de Fuca plate. Down dip of this locked zone (red dashed lines), the plates must still slide past each In our science feature, two EarthScope researchers, Ken Creager of the University of other. However, instead of rupturing in devastating earthquakes, much of that slip Washington and Tim Melbourne of Central appears to be occurring during the Episodic Tremor and Slip (ETS) events (labeled Washington University, collaborated to present “slip” in the figure). some of their ongoing work on Episodic Tremor EarthScope and other research projects have installed Global Positioning and Slip in the Pacific Northwest. These events, System (GPS) stations near the Northwest Pacific coast to record ETS events. also referred to as ‘slow earthquakes,’ occur These instruments are continuously moving to the northeast relative to the stable with an amazing degree of predictability. The interior of North America because the Juan de Fuca plate is pushing on the most recent event took place in January 2007 North American plate (green arrow in Figure 1). During an ETS event, the stations and was recorded by EarthScope seismometers above the ETS zone slip backwards (the red arrows in the figure). Seismometers and strainmeters. Installation of instruments also record the slip. These signals initially were considered to be noise from wind in harsh and sensitive environments poses or other sources, however, by filtering and analyzing the seismograms, it was unique challenges and we describe on page discovered that tremor produces signals that are similar on many seismometers 4 the special GPS monument used in Alaska. Another article provides an overview of the and that the signals move across a network of seismometers at the speed of EarthScope National Meeting held earlier this waves generated by earthquakes. Tremors typically originate where the two year in Monterey, California. In conjunction plates meet at depths of 30-45 km. These tremors may be related to high fluid with this meeting, a special workshop was pore pressure in the rock resulting from dehydration reactions as the subducting conducted for middle and high school teachers plate heats up and undergoes increasing pressure. to introduce them to EarthScope and to One of the most exciting areas of improve their foundation in geophysics. We also research to emerge from joint GPS and report on the completion of the first EarthScope seismic monitoring is the discovery of component, the Permanent Array, and ETS. Japanese researchers were the first announce a new web-based tool that displays to identify periodic slip events, however, when and where future Transportable Array with the installation of more GPS stations, stations will be installed and allows the user to this phenomena has been detected in suggest a specific location for an Earthquake Monitoring Station. Updates on the progress of Cascadia and many other subduction the Transportable Array and the Plate Boundary zones around the world. In Cascadia, Observatory are presented in this and every initial recognition of slow faulting led issue of onSite. to the identification of eight additional events with a regular 14-month periodicity We value your continued interest in and the forecasting of future events. EarthScope. If there is a topic of special interest Since then, five predicted events have to you, please let us know by contacting one of occurred with the same periodicity. the EarthScope onSite editors (USArray: dorr@iris. Further study has shown that ETS events edu or PBO: [email protected]). likely occur all along the Cascadia subduction zone but with different Figure 2. Plot of East-West position vs time for periodicity (see Figure 2.). For example, 23 GPS sites (eastward motions are positive). The blue vertical lines mark ETS events where ETS events occur beneath Northern multiple stations temporarily move westward. California every 11 months and off the David W. Simpson Michael E. Jackson (continued on page 3) USArray Principal Investigator Director, Plate Boundary Observatory 2 project status: USArray Where is the Transportable Array? As of April 1, more than 335 earthquake monitoring stations are operating in eight western states. Installation of the Transportable Array network is complete in California, Oregon, Washington and Nevada with stations located approximately 70 km apart in a grid-like fashion. Throughout the fall and winter months, activities migrated from the northern mountain states to the southwest region where USArray Status both installation and construction as of crews have been working. Significant April 1, 2007 progress is being made in Arizona with all installations expected to be completed before the desert heat settles in for the summer. Now that the snow is beginning to melt in the higher elevations, plans are underway to resume station construction in Utah and Idaho. Site reconnaissance activities are taking place in New Mexico and will begin later this spring in Operating Station: 337 Colorado, Wyoming, eastern Montana, Construction Phase: 37 and the Big Bend area of Texas. Permitting Phase: 71 Future Site In an effort to utilize the safest as well as the most efficient station construction and installation procedures, a two-day “best practices” workshop was recently REAL-TIME STATION STATUS: To view a map of current Transportable Array stations, visit http://www.earthscope.org and click on ‘Current Status’. held in Phoenix, Arizona, for Transportable Array personnel. Construction and To view seismograms recorded at a USArray station, go to http://usarray.seis.sc.edu/ and enter installation crews participated in activities the station code. You can also enter a zip code to view the recordings from the USArray station which included both the construction closest to that area. of a seismic station and installation of TransportaBLE ARRAY COORDinatinG OFFICE: [email protected] 1-800-504-0357 instrumentation and ancillary equipment. During the debriefing sessions, techniques and suggested improvements were discussed. ■ On the Move . Permanent Array Facility Completed With the installation of a seismic operated by the U.S. Geological Survey. station near Conover, Wisconsin, in Data from the Permanent Array provide September 2006, EarthScope completed high-quality real-time information that the construction of the EarthScope is used to help scientists explore the Permanent Array facility. This is the first structure of the deep Earth, better EarthScope component to be completed understand earthquake processes, and to and was constructed on time and within identify and mitigate seismic hazards. ■ budget. The Permanent Array activities involved l NEW NLWA l PKME HAWA EGMT DGMT the upgrade or installation l l l AGMN l l EYMN l of stations at 39 sites COWI l l LONY WVOR HLID l throughout the U.S. These ECSD l l GLMI stations become part of l JFWS SCIA l AAM l lPDAR ACSO l HDIL the Advanced National l DUG NVAR l l Seismic System (ANSS), l ISCO l KSU1 l SRU BLA MVCO l a nationwide l l CNNC l network of lWUAZ WMOK MIAR earthquake l l GOGAl EGAK monitors l BRAL Transportable Array personnel review station JCT l installation procedures during the “best practices” TXAR l workshop. WRAK l l KVTX l ATTU l 3 featured science: Suggest A Site . Episodic Tremor and Slip When will the Transportable Array be in my friend’s community? in the Pacific Northwest (continued from front) As the Transportable Array continues the bottom left-hand side of the page) coast of central Oregon its journey from the west to east, you might to identify the suggested location on the approximately every 18 months. wonder when it will arrive in a friend’s map. Placing a marker will automatically or family member’s neighborhood. You open the site suggestion form, which will The 2007 ETS event began under can find out by going to our “Suggest slide open on the right-hand side of the the southern Puget Sound area to the A Site” web page at www.iagt.org/ map. Or you can click on the ‘Show southwest of Seattle and propagated earthscope/suggestasite/. Zoom to a Panel’ tab, located in the upper right north northwest into Vancouver Island, place by entering an address, a zip corner of the map to display the site Canada at a rate of 10 km per day code, a city and state (City, State), or suggestion form.
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