2-3 March 2009Cascadia 2007 and Beyond Workshop, Seattle

Results from Strainmeter Recordings of Cascadia “ETS” events

Work by Evelyn Roeloffs, Wendy McCausland, and Paul Silver

2 March 2009

2-3 March 2009Cascadia 2007 and Beyond Workshop, Seattle

Horizontal Strain Components derived from Strainmeter Data

• Areal strain data from many PBO borehole strainmeters in Cascadia are of poor quality

• Shear strain (Differential extension and engineering shear) coupling is generally good

• For strainmeters that could be tidally calibrated, the tidal calibrations yield shear strains that are 3 to 1.5 times larger than the PBO calibration coefficients

• Recent work by Duncan Agnew indicates shear strain tides are not well modeled

• More work needed on removing surface-loading effects and detrending

2-3 March 2009Cascadia 2007 and Beyond Workshop, Seattle

Cascadia Aseismic Slip Events

2-3 March 2009Cascadia 2007 and Beyond Workshop, Seattle

PBO Borehole Strainmeters in Cascadia

2-3 March 2009Cascadia 2007 and Beyond Workshop, Seattle

What do we expect?

Sou

thw

ard

Rup

ture

Nor

thw

ard

Rup

ture

• Simple Model: 1cm slip, 10km-wide fault centered on 40 km depth contour.

• Example: B004 for 2007 event.

2-3 March 2009Cascadia 2007 and Beyond Workshop, Seattle

2007 Tremor and Slow Slip Event

North

South

Tremor locationswithin 50-60 km of strainmetercoincide with onset of strainsignals

Tremor locations from A Wech of Univ. of Washington andH. Kao of Geol. Survey of Canada

N

N

N

2-3 March 2009Cascadia 2007 and Beyond Workshop, Seattle

2008 Tremor and Slow Slip Event

Bilateralmigration

South

North

Tremor locationswithin 50-60 km of strainmetercoincide with onsetof strainsignals

Tremor locations from A Wech of Univ. of Washington andH. Kao of Geol. Survey of Canada

2-3 March 2009Cascadia 2007 and Beyond Workshop, Seattle

Which comes first, the tremor or the slow slip?For all the data, the onsets are simultaneous to within a few hours.

At B018, based on 30-minute strainmeter data and 30-minute tremor counts the onsets of the tremor and slow slip are simultaneous.

2-3 March 2009Cascadia 2007 and Beyond Workshop, Seattle

Close-up of B012

Tremor locations fromH. Kao of Geol. Survey of Canada

Cascadia 2007 and Beyond Workshop, Seattle 2-3 March 2009

Simultaneous Onset within 1/2 hour

Tremor locations fromA. Wech of Univ. of Washington

2-3 March 2009Cascadia 2007 and Beyond Workshop, Seattle

- For 2 of the largest 3 inter-ETS events, there is discernible strain consistent with known tremor and slow slip events.

- Possible exception for June 2007 tremor

- Results indicate that even when there is tremor comparable to that observed during recurrent ETS events, the strain is much (1/6th as much) smaller than during the recurrent ETS events.

Exception at B012 for October 2007

Slip does accompany inter-ETS tremor bursts

2-3 March 2009Cascadia 2007 and Beyond Workshop, Seattle

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

K. Wang et al., GRL, 2008

2006

2006

2-3 March 2009Cascadia 2007 and Beyond Workshop, Seattle

Question 2: Inter-Event Tremor Activity- 4 Candidate Events

Washington Located Tremor Counts

Vancouver Island Located Tremor Counts

Days after 1/1/2007

October 2007Discerniblestrain

June 2007No discernible strain

March 2008Discerniblestrain

Tremor locations from A Wech of Univ. of Washington andH. Kao of Geol. Survey of Canada

2-3 March 2009Cascadia 2007 and Beyond Workshop, Seattle

-0.06

-0.04

-0.02

0

0.02

0.04

0

10

20

30

40

50

9/20/07 9/25/07 9/30/07 10/5/07 10/10/07 10/15/07 10/20/07 10/25/07 10/30/07

B012 eXYeXX-eYY

B

-0.04

-0.03

-0.02

-0.01

0

0.01

0.02

0

20

40

60

80

100

120

9/20/07 9/25/07 9/30/07 10/5/07 10/10/07 10/15/07 10/20/07 10/25/07 10/30/07

B007

2eXYeXX-eYY

B

October 2007Inter-Event Tremor

Bursts

Southward propagationpast B012

Northward propagationstopping nearB007

Tremor locations fromH. Kao of Geol. Survey of Canada

Tremor locations fromA. Wech of Univ. of Washington

2-3 March 2009Cascadia 2007 and Beyond Workshop, Seattle

March 2008 B018

B018 March 2008 blowup

- Tremor and strain have indistinguishable onsets (< 30 min)- Southward propagating rupture - Located at least 10 km updip from B018