The accumulation and release of interseismic strain on the Cascadia Subduction Zone

David Schmidt (das@uoregon.edu)Ray WeldonReed Burgette Randy KrogstadHaiying Gao

Department of Geological SciencesUniversity of Oregon

Funding provided by USGS NEHRP

Graphic: Hyndman & Wang (1995)

Talk Overview

• Sources of deformation on the plate interface

• Making observations of deformation

• Strain accumulation

• Transient release of strain

(Safer and Tobin, 2011)

Deformation in Cascadia

Coast moves east and up

between earthquakes.

Coast moves west and down

during earthquakes.

Graphic: Leonard et al. (2009)

Observations of Interseismic Deformation

• Uplift rates from spirit leveling– Observations span 1930-present.– Uplift referenced to eustatic sea level.– 1- uncertainties ~0.5 mm/yr.

(Burgette et al., JGR, 2009)-3 -2 -1 0 1 2 3 4

Uplift (mm/yr)

Differenced Tidal Records, CC-AS

1940 1960 20001980

year

Sea

leve

l (m

)

0

-0.25

0.25

Observations of Interseismic Deformation

• Uplift rates from spirit leveling– Observations span 1930-present.– Uplift referenced to eustatic sea level.– 1- uncertainties ~0.5 mm/yr.

Upl

ift ra

te (m

m/y

r)

West East

East from 20-km-depth slab contour (km)(Burgette et al., JGR, 2009)

-3 -2 -1 0 1 2 3 4Uplift (mm/yr)

Horizontal Observations of Interseismic Deformation

• Horizontal GPS Velocities from PBO data products and McCaffrey et al. (2007).

• Limited to stations with data for > 2 years

Horizontal Observations of Interseismic Deformation

Original Velocity Field Rotation of Oregon Removed(pole of McCaffrey et al., 2007)

Horizontal Observations of Interseismic Deformation

0 50 150100

Compressional Strain Rate (nanostrain/yr)

Original Velocity Field Rotation of Oregon Removed(pole of McCaffrey et al., 2007)

Modeling Methodology• Fixed parameters

– Defined Convergence rate– JDF-OCR Euler pole for the southern end (Wells & Simpson, 2001).– JDF-NA Euler pole for the northern end (Mazzotti et al., 2003)

– Plate geometry of McCrory et al. (2006).– Upper edge of locked zone begins at trench.

5 15 25

0

1

2

3

4

Depth of Slab (km)

Slip

Rat

e De

ficit

(cm

/yr)

Down-Dip Locking

Graphic: Geological Survey of Canada

Modeling Methodology• Fixed parameters

– Defined Convergence rate– JDF-OCR Euler pole for the southern end (Wells & Simpson, 2001).– JDF-NA Euler pole for the northern end (Mazzotti et al., 2003)

– Plate geometry of McCrory et al. (2006).– Upper edge of locked zone begins at trench.

• Backslip calculation (Savage, 1983).• Free parameters

– Depth of locking; Depth of transition zone.

• Optimization by grid search of parameter space– Step 1: Depths constrained along west-east leveling profiles (fit both uplift and strain).– Step 2: Optimize along-strike locking with

entire dataset.

0

1

2

3

4

Slip RateDeficit(cm/yr)

1 3 5 7 9 11

Misfit (χ2)

Depth of locked zone (km)

Dept

h of

tran

sition

zone

(km

)

5 15 35255

15

25

35

Narrow Locked Zone

NarrowTransition

Zone

• Combined inversion (uplift and strain)• Data misfit weighted by data uncertainties

0

1

2

3

4

Slip RateDeficit(cm/yr)

1 3 5 7 9 11

Misfit (χ2)

Depth of locked zone (km)Dept

h of

tran

sition

zone

(km

)

5 15 35255

15

25

35

Misfit for combined Leveling & Horizontal Strain

Observed Predicted Residual

-3 -2 -1 0 1 2 3 4Uplift (mm/yr)

0 1 2 3 4| Misfit | (mm/yr)

Observed Strain Rate Predicted Strain Rate

0 50 150100Compressional Strain Rate (nanostrain/yr)

Lohman (2010)

Episodic Tremor and Slip (ETS)

• Transient slip on the plate interface• Accompanied by emergent tremor• Located 30-45 km depth• Recurrence 10-20 months• Magnitude ~6

time (days)

east

(mm

)ea

st (m

m)

east

(mm

)

estimated slip (cm)

2007 Slow Slip Event

P420

P423

KTBW

P420

P423

KTBW

time (days)

east

(mm

)ea

st (m

m)

east

(mm

)

predicted

estimated slip (cm)

P420

P423

KTBW

2007 Slow Slip Event

3

2

1

0

Slip rate (mm/day)

Consecutive Slow Slip Events

Slow Slip and Tremor in Oregon• Slip inferred from GPS time series

• Mosaic of events (June 2007, May 2008, August 2009)

• Tremor from 2005-2007 Boyarko and Brudzinski (2010)

54321Slow Slip (cm)

3210Slip Rate Deficit (cm/yr)

54321Slow Slip (cm)

3210Slip Rate Deficit (cm/yr)

54321Slow Slip (cm)

Locked

TransitionZone

ETS Zone

West East

Newport Leveling Profile

coastline

Figures by Randy Krogstad (UO)

Summary

• A Cascadia locking model constrained by leveling and horizontal strain rates from GPS.

• Wide locked zone in WA and CA; Narrow in OR.

• Mo Accumulation Rate: 0.9-1.4 x1022 Nm/century• Mw Accumulation Rate: 8.5-8.7 per century

• Spatial separation of locked and ETS zones in OR.

• <10% long term locking in ETS zone

3210Slip Rate Deficit (cm/yr)

54321Slow Slip (cm)

OR

WA

CA

BC

Holtkamp and Brudzinski (2010)

1000

800

600

400

200

Nor

th (k

m)

0 2 4 6

0 1 2 3 4Slip Rate Deficit (cm/yr)

Seismic Moment Rate (1018 Nm/yr)

0 10 20 30 40Position of 1 cm/yr slip rate deficit contour (% of models)

0

1

2

3

4

Slip RateDeficit(cm/yr)

Horizontal GPS Observations and Predictions

Leveling Observations and Predictions

0 50 150100

Compressional Strain Rate (nanostrain/yr)

Strain Rate from Original GPS Strain Rate from GPS w/ rotation removed

Observations of Interseismic DeformationU

plift

rate

(mm

/yr)

West East

East from 20-km-depth slab contour (km)

Elev

ation

(m)

(Burgette et al., JGR, 2009)-3 -2 -1 0 1 2 3 4

Uplift (mm/yr)