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

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 M. Gladwin,
GTSM Technologies
Figure courtesy of Alan Linde,
Carnegie Institution
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
B004 2007343_0700_0900
CH0
CH1
CH2
CH3
nanostrain
10
0
-10
-20
-30
2000
2500
3000
3500
4000
4500
seconds after 073430700
5000
5500
6000
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
2008
N 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 Caldera
1997 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