2-3 March 2009 Cascadia 2007 and Beyond Workshop, Seattle Results from Strainmeter Recordings of Cascadia “ETS” events Work by Evelyn Roeloffs, Wendy McCausland, and Paul Silver 2 March 2009

2-3 March 2009 Cascadia 2007 and Beyond Workshop, Seattle Horizontal Strain Components derived from Strainmeter Data 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

2-3 March 2009 Cascadia 2007 and Beyond Workshop, Seattle Cascadia Aseismic Slip Events

2-3 March 2009 Cascadia 2007 and Beyond Workshop, Seattle PBO Borehole Strainmeters in Cascadia

2-3 March 2009 Cascadia 2007 and Beyond Workshop, Seattle What do we expect? Southward Rupture Northward Rupture Simple Model: 1cm slip, 10km-wide fault centered on 40 km depth contour. Example: B004 for 2007 event.

2-3 March 2009 Cascadia 2007 and Beyond Workshop, Seattle 2007 Tremor and Slow Slip Event North South Tremor locations within km of strainmeter coincide with onset of strain signals Tremor locations from A Wech of Univ. of Washington and H. Kao of Geol. Survey of Canada N N N

2-3 March 2009 Cascadia 2007 and Beyond Workshop, Seattle 2008 Tremor and Slow Slip Event Bilateral migration South North Tremor locations within km of strainmeter coincide with onset of strain signals Tremor locations from A Wech of Univ. of Washington and H. Kao of Geol. Survey of Canada

2-3 March 2009 Cascadia 2007 and Beyond Workshop, Seattle 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.

2-3 March 2009 Cascadia 2007 and Beyond Workshop, Seattle Close-up of B012 Tremor locations from H. Kao of Geol. Survey of Canada

2-3 March 2009 Cascadia 2007 and Beyond Workshop, Seattle Simultaneous Onset within 1/2 hour Tremor locations from A. Wech of Univ. of Washington

2-3 March 2009 Cascadia 2007 and Beyond Workshop, Seattle - 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 Slip does accompany inter-ETS tremor bursts

2-3 March 2009 Cascadia 2007 and Beyond Workshop, Seattle Slow slip event recorded by strainmeter, little or no GPS signal K. Wang et al., GRL,

2-3 March 2009 Cascadia 2007 and Beyond Workshop, Seattle Question 2: Inter-Event Tremor Activity- 4 Candidate Events Washington Located Tremor Counts Vancouver Island Located Tremor Counts Days after 1/1/2007 October 2007 Discernible strain June 2007 No discernible strain March 2008 Discernible strain Tremor locations from A Wech of Univ. of Washington and H. Kao of Geol. Survey of Canada

2-3 March 2009 Cascadia 2007 and Beyond Workshop, Seattle October 2007 Inter-Event Tremor Bursts Southward propagation past B012 Northward propagation stopping near B007 Tremor locations from H. Kao of Geol. Survey of Canada Tremor locations from A. Wech of Univ. of Washington

2-3 March 2009 Cascadia 2007 and Beyond Workshop, Seattle 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