Borehole Strainmeters: Instruments for Measuring Aseismic Deformation in Subduction Zones

Borehole Strainmeters: Instruments for Measuring Aseismic Deformation in

Subduction Zones

Evelyn Roeloffs

U.S. Geological Survey, Vancouver, WA

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, …

Figure courtesy of Alan Linde, Carnegie Institution

Figure courtesy of M. Gladwin, GTSM Technologies

Borehole Diagram

Strainmeters Complement Seismology and GPS

• They do not measure displacement• Low-frequency stability limit is unclear

B004 Fiji Islands M7.8 9 Dec 07

-30

-20

-10

0

10

20

30

2000 2500 3000 3500 4000 4500 5000 5500 6000

B004 2007343_0700_0900CH0CH1CH2CH3

seconds after 073430700

Surface waves

Main use of strainmeters is for signals lasting hours to days

Output from borehole strainmeter gauges

Derived Strains

2007 ETS Event, model

by T. Melbourne

based on GPS

2007 N Cascadia Slow Slip

Event

2008N Cascadia

Slow Slip

Event

Cascadia Aseismic Slip Events

Tremor and Strain Onset at B018

Slow slip event recorded by strainmeter, little or no GPS signal

K. Wang et al., GRL, 2008

Transient Aseismic Slip Throughout Cascadia

Brudzinski & Allen, Geology 2007

Grants Pass PBO Borehole Strain

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

Iceland Dilatometer Array

Figure courtesy of Alan Linde, Carnegie Institution

Eruptions of Hekla Volcano

Figure courtesy of Alan Linde, Carnegie Institution

Possible Decoupling of Strainmeters?

• Very high vertical diffusivity

• Regional strain deforms fractures but produces no local strain near strainmeter

Long Valley Caldera1997 Seismic Swarm

Roeloffs et al.,

J. Volc. Geotherm. Res.,

2003

Transient Strain Following 1992 M7.3 Landers Earthquake

• No other deformation detected, although seismicity was triggered

Earthquakes Affecting Water Levels in Long Valley

Normalized Time

Histories of Water-Level and Strain Changes

1-D Diffusion Models for Strain Transients

• Time histories of strain transients are consistent with diffusive decay of a fluid pressure increase near, but not at, the strainmeter

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

Aspects needing engineering development• 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