Post on 23-Feb-2016
description
Observing Details of Transient Aseismic Slip with Borehole Strainmeters
Evelyn RoeloffsU.S. Geological Survey,
Earthquake Science Center
AcknowledgmentsAll of this work was made possible by the UNAVCO-PBO strainmeter field teams and analysis staff!
Cascadia Subduction and Slow Slip
Figures from Geological Survey of Canada
PBO Borehole Strainmeters
Strainmeter Design
and Installation
Drawing (modified) from GTSM Technologies Drawing from UNAVCO PBO website
Long-term Trends
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
Strainmeters “See” Events GPS Doesn’tDeformation during these October 2007 tremor bursts was below the resolution of GPS
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?
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
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
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.
Trend Removal Reveals Seasonal Variations
Seasonal variations are as large as the slow slip events...
Suppression of Seasonal Signals
…but at B004, seasonal signals are common to all gauges, which allows them to be removed
A Time-independent View
Express as Strains in Slab-oriented Coordinates
Effect of Orientation Uncertainty
Rotation Invariant: Maximum Shear
Model Calculations
Simple Forward Models
• Elastic half space• Uniform slip• Test:– Depth limits– Rake– Propagation speed
Effect of Up-dip Limit
Effect of Rake
Best Choice for 2008 Event: Oblique Slip
Velocity Varies…
…or slip is distributed behind slip front
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