The accumulation and release of interseismic strain on the Cascadia Subduction Zone
David Schmidt ([email protected])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)