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Observing Details of Transient Aseismic Slip with Borehole Strainmeters

Observing Details of Transient Aseismic Slip with Borehole Strainmeters. Evelyn Roeloffs U.S. Geological Survey, Earthquake Science Center. Acknowledgments. All of this work was made possible by the UNAVCO-PBO strainmeter field teams and analysis staff!. Cascadia Subduction and Slow Slip.

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Observing Details of Transient Aseismic Slip with Borehole Strainmeters

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  1. Observing Details of Transient Aseismic Slip with Borehole Strainmeters Evelyn Roeloffs U.S. Geological Survey, Earthquake Science Center

  2. Acknowledgments All of this work was made possible by the UNAVCO-PBO strainmeter field teams and analysis staff!

  3. Cascadia Subduction and Slow Slip Figures from Geological Survey of Canada

  4. PBO Borehole Strainmeters

  5. Strainmeter Design and Installation Drawing (modified) from GTSM Technologies Drawing from UNAVCO PBO website

  6. Long-term Trends

  7. Strainmeters Detect the Slip Events May-June 2008 Tremor locations from A Wech of Univ. of Washington and H. Kao of Geol. Survey of Canada; figure by W. McCausland

  8. Strainmeters “See” Events GPS Doesn’t Deformation during these October 2007 tremor bursts was below the resolution of GPS

  9. Cascadia Slow Slip Events: Questions • What is the up-dip limit of slip? • Where is the tremor relative to the slip? • Is the slip pure thrust or oblique? • What stops the slow slip events?

  10. Data Processing Needs Improvement …if we want to answer those questions Tremor locations from A Wech of Univ. of Washington and H. Kao of Geol. Survey of Canada; figure by W. McCausland

  11. Borehole Strain Challenges • Need to remove long-term trends without distorting signals from slow slip events • Need to remove seasonal signals without distorting signals from slow slip events • Calibration parameters (scale factors) uncertain • Orientation uncertain

  12. Removing Long-term Trends -Edit out initial part of record -Fit linear function + fractional power of time Note that the trends diverge on the different channels – may reflect tectonic shear strain.

  13. Trend Removal Reveals Seasonal Variations Seasonal variations are as large as the slow slip events...

  14. Suppression of Seasonal Signals …but at B004, seasonal signals are common to all gauges, which allows them to be removed

  15. A Time-independent View

  16. Express as Strains in Slab-oriented Coordinates

  17. Effect of Orientation Uncertainty

  18. Rotation Invariant: Maximum Shear

  19. Model Calculations

  20. Simple Forward Models • Elastic half space • Uniform slip • Test: • Depth limits • Rake • Propagation speed

  21. Effect of Up-dip Limit

  22. Effect of Rake

  23. Best Choice for 2008 Event: Oblique Slip

  24. Velocity Varies… …or slip is distributed behind slip front

  25. Take-away Messages • The PBO borehole strainmeters offer lots of information about the Cascadia slow slip events that cannot be obtained using GPS • But, it’s necessary to improve the ways we process the data

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