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Outline of Talk

• Relation of surface and subsurface velocity fields

• Western US velocity field

• Where Earthscope can help

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Western North America

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How to characterize the deformation: Possibilities

• Plate-like Faults penetrate lithosphere as high strain areasShear in mantle lithosphere ‘localized’

• Floating blocks in continuumWeak faults extend only through thin brittle upper layerDistributed shear in mantle/lower crust

• Plates with wide boundariesCombination of above

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Thatcher, W., International Geology Review, 45, p. 191, 2002.

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Thatcher, W., J. Geophysical Res., March 1995.

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Marlborough region NZ

Bourne et al., 1998

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Flesch, L., et al. Science 287, 2000.

Deviatoric stress from gravitational potential energy variations

Implied lithospheric viscosity from stress and strain rate estimates

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Thatcher, W., International Geology Review, 45, p. 191, 2002.

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Pollitz, F., Geophys. J. Int. 153, 2003.

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Let’s examine the plate possibility for western US

Analysis• Use geodetic, geologic, seismologic data to estimate simultaneously

crustal block rotation poles, coupling on block-bounding faults, internal strain rates, and GPS reference frame

• Each GPS velocity solution rotated into reference frame by least-squares fit

• No velocity data excluded due to proximity to faults• 3D coupling distribution on faults parameterized by nodes along fault

contours• Minimize reduced

2 by simulated annealing & downhill simplex

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M. K. Savage, K. M. Fischer, and C. E. Hall, Strain modeling, seismic anisotropy and coupling at strike-slip boundaries: Applications in New Zealand and the San Andreas Fault, Geol. Soc. London Special Publications, 227, 9-40, in press.

Wallace, Laura, et al., in prep.

Surface velocity field

First a stop in NZ:

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Rotational and Elastic parts of velocity field

Wallace, Laura, et al., in prep.

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In North Island rotation accommodates 2/3 and faulting 1/3 of transverse motion (gray lines) -- rotation is our friend

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Region is divided into ‘blocks’, contiguous areas that are thought to rotate.

Each block rotates about a pole.

The rotating blocks are separated by dipping faults.

Velocities due to fault locking are added to rotations to get full

velocity field.

The relative long-term slip vectors on the faults

are determined from rotation poles.

Back-slip is applied at each fault to get surface

velocities due to locking.

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The strain rate tensor near a locked fault represents a spatial transition from the velocity of one block to the velocity of the other. In other words, a locked fault allows one block to communicate information about its motion into an adjacent block.

Program described at www.rpi.edu/~mccafr/defnode/defnode.html

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DataGPS velocities

• PNW1, our PNW solution• SCEC CDM3, Southern California • WUSC version 2, Western US (Bennett

et al.)• Northern California (Freymueller et al.,

1999) • BARD (Murray and Segall, 2001)• Sierra Nevada (Dixon et al., 2000)• ECSZ (McCluskey et al. 2001, Gan et

al. 2001)• Basin and Range (Thatcher et al. 1999)• Baja (Dixon et al. 2002)• Pacific –North America (Beavan et al.

2002)

Slip vectors • Harvard CMT, NUVEL-1, C. DeMets, Jackson & Molnar (1990)

Transform azimuths• C. DeMets

Slip rates• NUVEL-1, C. DeMets, several compilations

Fault outline data• Jennings

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Block model

GPS Fault slip rateEQ slip vector

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North America reference frame is estimated by minimizing 248 GPS velocities (Nrms=1.1, Wrms=1.0mm/yr).

Pacific angular velocity from 5 spreading rates, 73 eq slip vectors (Nrms = 1.2), and 56 GPS velocities (Nrms = 1.2, Wrms=1.0mm/yr).

Juan de Fuca Euler vector from 28 PAC-JdF spreading rates, 1 transform azimuth (Blanco FZ; res = 1º).

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Slip vectorsObservedCalculated

Block boundaries placed alongmajor fault systems.

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Block motion

Total

NW component NE component

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Rotational component

N component E component

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GPS residuals with 70% confidence ellipses

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Predicted fault slip rates

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Lamb, S., Earth Planet. Sci. Lett. 84, p. 75, 1987.

Block rotations

IRIS June 2004 26Jackson & Molnar, J. Geophys. Res., 1990.

Luyendyk, B. GSA Bull., 1991.

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Vertical axis rotations in North America frame

(Negative is clockwise)

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GPS residuals in Transverse Ranges don’t indicate rapid rotations (presently)

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Strain rates – residual inverted Inverted strain rates – sigma < 10 ns/yr10 < sigma < 20 ns/yr

10 ns/yr = 1 mm/yr over 100 km distance

Strain rates within blocks needed in 5% to 15% of area

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Velocity field for Pacific Northwest derived from campaign and continuous sites.

Reference frame is North America and ellipses are 70% confidence

In collaboration with Tony Qamar, Bob King, Herb Dragert, Charles Williams

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42.5N

44.5N

46N47.2N

Distance from deformation front, km

East profiles of East component

W E

mm

/a

East profiles of North component

Distance from deformation front, km

42.5N

44.5N

46N

47.2N

mm

/a

42.5N

44.5N

46N

47.2N

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Block rotations relative to North America.Block rotations relative to

NE Oregon.Cape Blanco (0.72)

SE Oregon (0.20 o/Ma)

NE Oregon (0.72)

W Washington (0.69)

NE Washington (0.19)

Allowing Oregon and Washington to behave as 5 independent, rotating blocks shows which regions take up the slip.

• The poles of 4 of the Oregon blocks fall close the the ‘whole Oregon’ pole. • All rotations are clockwise.• Could be ~ 1 mm/a extension along arc• No indications of strike-slip along arc

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Rotation rates

GPS – derived

P’mag – Ray Wells12 Ma Pomona15 Ma Ginko

Rotating Oregon block

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North America frame Hotspot frame

Western US ‘discontinuous’ velocity field

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Summary of shear-wave splitting measurements in California and Nevada. From M. K. Savage, K. M. Fischer, and C. E. Hall, Geol. Soc. London Special Publications, 227, 9-40, in press.

Splitting observations on map of shear wave velocities at 150 km depth. (Silver, P., and W. Holt, Science 295, 2002)

IRIS June 2004 37Silver, P., and W. Holt, Science 295, 2002

Surface velocity Mantle velocity (hot spot frame)

Observations

Mantle = hotspot

Mantle moves east

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Straining block inversion

Rigid block inversion

Steady-state, discontinuous, hot-spot frame velocity field

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Thoughts:

• ‘Block’ representation appears to work for about 85 to 95% of western US at the mm/yr (2%) level

• PBO will provide improved surface velocity distributions

• USArray can provide length-scales of strain variations in mantle

• USArray can provide length-scales of mantle wavespeed variations (viscosity) in mantle

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THE END