IRIS June 20041

2 Outline of Talk Relation of surface and subsurface velocity fields Western US velocity field Where Earthscope can help

IRIS June Western North America

IRIS June How to characterize the deformation: Possibilities Plate-like Faults penetrate lithosphere as high strain areas Shear in mantle lithosphere ‘localized’ Floating blocks in continuum Weak faults extend only through thin brittle upper layer Distributed shear in mantle/lower crust Plates with wide boundaries Combination of above

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

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

IRIS June Marlborough region NZ Bourne et al., 1998

IRIS June Flesch, L., et al. Science 287, Deviatoric stress from gravitational potential energy variations Implied lithospheric viscosity from stress and strain rate estimates

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

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

IRIS June 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

IRIS June 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:

IRIS June Rotational and Elastic parts of velocity field Wallace, Laura, et al., in prep.

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

IRIS June 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.

IRIS June 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

IRIS June Data GPS 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

IRIS June Block model GPS Fault slip rate EQ slip vector

IRIS June 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).

IRIS June Slip vectors Observed Calculated Block boundaries placed along major fault systems.

IRIS June Block motion Total NW component NE component

IRIS June Rotational component N component E component

IRIS June GPS residuals with 70% confidence ellipses

IRIS June Predicted fault slip rates

IRIS June Lamb, S., Earth Planet. Sci. Lett. 84, p. 75, Block rotations

IRIS June Jackson & Molnar, J. Geophys. Res., Luyendyk, B. GSA Bull., 1991.

IRIS June Vertical axis rotations in North America frame (Negative is clockwise)

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

IRIS June Strain rates – residual inverted Inverted strain rates – sigma < 10 ns/yr 10 < 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

IRIS June 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

IRIS June N 44.5N 46N 47.2N 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

IRIS June

IRIS June 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

IRIS June Rotation rates GPS – derived P’mag – Ray Wells 12 Ma Pomona 15 Ma Ginko Rotating Oregon block

IRIS June North America frameHotspot frame Western US ‘discontinuous’ velocity field

IRIS June 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 Silver, P., and W. Holt, Science 295, 2002 Surface velocity Mantle velocity (hot spot frame) Observations Mantle = hotspot Mantle moves east

IRIS June Straining block inversion Rigid block inversion Steady-state, discontinuous, hot-spot frame velocity field

IRIS June 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

IRIS June THE END