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Results from Strainmeter Recordings of Cascadia “ETS” events

Results from Strainmeter Recordings of Cascadia “ETS” events. Work by Evelyn Roeloffs, Wendy McCausland, and Paul Silver 2 March 2009. Areal strain data from many PBO borehole strainmeters in Cascadia are of poor quality

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Results from Strainmeter Recordings of Cascadia “ETS” events

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  1. Results from Strainmeter Recordings of Cascadia “ETS” events Work by Evelyn Roeloffs, Wendy McCausland, and Paul Silver 2 March 2009 Cascadia 2007 and Beyond Workshop, Seattle

  2. Areal strain data from many PBO borehole strainmeters in Cascadia are of poor quality Shear strain (Differential extension and engineering shear) coupling is generally good For strainmeters that could be tidally calibrated, the tidal calibrations yield shear strains that are 3 to 1.5 times larger than the PBO calibration coefficients Recent work by Duncan Agnew indicates shear strain tides are not well modeled More work needed on removing surface-loading effects and detrending Horizontal Strain Components derived from Strainmeter Data Cascadia 2007 and Beyond Workshop, Seattle

  3. Cascadia Aseismic Slip Events Cascadia 2007 and Beyond Workshop, Seattle

  4. PBO Borehole Strainmeters in Cascadia Cascadia 2007 and Beyond Workshop, Seattle

  5. What do we expect? • Simple Model: 1cm slip, 10km-wide fault centered on 40 km depth contour. • Example: B004 for 2007 event. Northward Rupture Southward Rupture Cascadia 2007 and Beyond Workshop, Seattle

  6. 2007 Tremor and Slow Slip Event Tremor locations within 50-60 km of strainmeter coincide with onset of strain signals North N N Tremor locations from A Wech of Univ. of Washington and H. Kao of Geol. Survey of Canada South Cascadia 2007 and Beyond Workshop, Seattle N

  7. 2008 Tremor and Slow Slip Event Tremor locations within 50-60 km of strainmeter coincide with onset of strain signals North Bilateral migration Tremor locations from A Wech of Univ. of Washington and H. Kao of Geol. Survey of Canada South Cascadia 2007 and Beyond Workshop, Seattle

  8. Which comes first, the tremor or the slow slip? For all the data, the onsets are simultaneous to within a few hours. At B018, based on 30-minute strainmeter data and 30-minute tremor counts the onsets of the tremor and slow slip are simultaneous. Cascadia 2007 and Beyond Workshop, Seattle

  9. Close-up of B012 Tremor locations from H. Kao of Geol. Survey of Canada Cascadia 2007 and Beyond Workshop, Seattle

  10. Simultaneous Onset within 1/2 hour Tremor locations from A. Wech of Univ. of Washington Cascadia 2007 and Beyond Workshop, Seattle

  11. Slip does accompany inter-ETS tremor bursts • - For 2 of the largest 3 inter-ETS events, there is discernible strain consistent with known tremor and slow slip events. • Possible exception for June 2007 tremor • Results indicate that even when there is tremor comparable to that observed during recurrent ETS events, the strain is much (1/6th as much) smaller than during the recurrent ETS events. • Exception at B012 for October 2007 Cascadia 2007 and Beyond Workshop, Seattle

  12. Slow slip event recorded by strainmeter, little or no GPS signal 2006 2006 K. Wang et al., GRL, 2008 Cascadia 2007 and Beyond Workshop, Seattle

  13. Question 2: Inter-Event Tremor Activity- 4 Candidate Events June 2007 No discernible strain Washington Located Tremor Counts March 2008 Discernible strain Vancouver Island Located Tremor Counts October 2007 Discernible strain Tremor locations from A Wech of Univ. of Washington and H. Kao of Geol. Survey of Canada Cascadia 2007 and Beyond Workshop, Seattle Days after 1/1/2007

  14. October 2007Inter-Event TremorBursts Southward propagation past B012 Tremor locations from H. Kao of Geol. Survey of Canada Northward propagation stopping near B007 Cascadia 2007 and Beyond Workshop, Seattle Tremor locations from A. Wech of Univ. of Washington

  15. March 2008 B018 B018 March 2008 blowup - Tremor and strain have indistinguishable onsets (< 30 min) - Southward propagating rupture - Located at least 10 km updip from B018 Cascadia 2007 and Beyond Workshop, Seattle

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