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 TechnologiesDrawing from UNAVCO PBO website

6. Long-term Trends

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

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

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

16. A Time-independent View

17. Express as Strains in Slab-oriented Coordinates

18. Effect of Orientation Uncertainty

19. Rotation Invariant: Maximum Shear

20. Model Calculations

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

22. Effect of Up-dip Limit

23. Effect of Rake

24. Best Choice for 2008 Event: Oblique Slip

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

26. 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