Observing Details of Transient Aseismic Slip with Borehole Strainmeters
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Observing Details of Transient Aseismic Slip with Borehole Strainmeters
Evelyn RoeloffsU.S. Geological Survey,
Earthquake Science Center
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AcknowledgmentsAll of this work was made possible by the UNAVCO-PBO strainmeter field teams and analysis staff!
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Cascadia Subduction and Slow Slip
Figures from Geological Survey of Canada
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PBO Borehole Strainmeters
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Strainmeter Design
and Installation
Drawing (modified) from GTSM Technologies Drawing from UNAVCO PBO website
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Long-term Trends
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Strainmeters Detect the Slip Events
Tremor locations from A Wech of Univ. of Washington andH. Kao of Geol. Survey of Canada; figure by W. McCausland
May-June 2008
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Strainmeters “See” Events GPS Doesn’tDeformation during these October 2007 tremor bursts was below the resolution of GPS
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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?
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Data Processing Needs Improvement
Tremor locations from A Wech of Univ. of Washington andH. Kao of Geol. Survey of Canada; figure by W. McCausland
…if we want to answer those questions
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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
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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.
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Trend Removal Reveals Seasonal Variations
Seasonal variations are as large as the slow slip events...
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Suppression of Seasonal Signals
…but at B004, seasonal signals are common to all gauges, which allows them to be removed
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A Time-independent View
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Express as Strains in Slab-oriented Coordinates
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Effect of Orientation Uncertainty
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Rotation Invariant: Maximum Shear
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Model Calculations
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Simple Forward Models
• Elastic half space• Uniform slip• Test:– Depth limits– Rake– Propagation speed
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Effect of Up-dip Limit
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Effect of Rake
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Best Choice for 2008 Event: Oblique Slip
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Velocity Varies…
…or slip is distributed behind slip front
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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