Borehole Strainmeters: Instruments for Measuring Aseismic Deformation in Subduction Zones

1. Borehole Strainmeters: Instruments for Measuring Aseismic Deformation in Subduction Zones Evelyn Roeloffs U.S. Geological Survey, Vancouver, WA

2. Acknowledgments • PBO borehole strainmeters are part of the NSF Earthscope initiative • PBO being constructed by UNAVCO, Inc. • Many UNAVCO staff working on strainmeter installation and data… • Dave Mencin, Kathleen Hodgkinson, …

4. Figure courtesy of Alan Linde, Carnegie Institution Figure courtesy of M. Gladwin, GTSM Technologies

5. Borehole Diagram

6. They do not measure displacement Low-frequency stability limit is unclear Strainmeters Complement Seismology and GPS

8. B004 Fiji Islands M7.8 9 Dec 07

9. Surface waves

10. Main use of strainmeters is for signals lasting hours to days Output from borehole strainmeter gauges Derived Strains

12. 2007 ETS Event, model by T. Melbourne based on GPS

13. 2007 N Cascadia Slow Slip Event

14. 2008N CascadiaSlow SlipEvent

15. Cascadia Aseismic Slip Events

16. Tremor and Strain Onset at B018

17. Slow slip event recorded by strainmeter, little or no GPS signal K. Wang et al., GRL, 2008

18. Transient Aseismic Slip Throughout Cascadia Brudzinski & Allen, Geology 2007

19. Grants Pass PBO Borehole Strain

20. Fluid Pressure and Borehole Strain • Fluid pressure is in some sense a proxy for strain • Subsurface fluid pressures fluctuate in response to strain induced by earth tides and atmospheric pressure • Can use fluid pressure tidal response to convert fluid pressure data to “units” of strain (typically order of 1 m H2O/microstrain) • But: • not all fluid pressure changes can be attributed to strain • some strain changes look a lot like fluid pressure changes

21. Iceland Dilatometer Array Figure courtesy of Alan Linde, Carnegie Institution

22. Eruptions of Hekla Volcano Figure courtesy of Alan Linde, Carnegie Institution

23. Possible Decoupling of Strainmeters? • Very high vertical diffusivity • Regional strain deforms fractures but produces no local strain near strainmeter

24. Long Valley Caldera1997 Seismic Swarm Roeloffs et al., J. Volc. Geotherm. Res., 2003

25. Transient Strain Following 1992 M7.3 Landers Earthquake • No other deformation detected, although seismicity was triggered

26. Earthquakes Affecting Water Levels in Long Valley

27. Normalized Time Histories of Water-Level and Strain Changes

28. Time histories of strain transients are consistent with diffusive decay of a fluid pressure increase near, but not at, the strainmeter 1-D Diffusion Models for Strain Transients

29. Beyond Pore Pressure Monitoring as a Proxy for Strain: Fluid Pressure and Strain are Independent Variables • Fluid pressure has unique ways of interacting with static or dynamic tectonic deformation • Fluid pressure changes cannot necessarily be computed from strain observations • Strain cannot necessarily be inferred from fluid pressure changes

32. Quantify requirements for rock modulus and quality Custom build strainmeters to match in-situ properties? Best practices for grouting Depends on temperature, pressure Very consistent procedure required Refine understanding of strainmeter coupling to formation strain Partitioning between horizontal and vertical sensitivity How are crescent-shaped strainmeters coupled? With what instruments can strainmeters share boreholes? Open interval for pore pressure recording Active instruments whose current may cause heat transients Aspects needing engineering development