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