Implications of interseismic deformaton in the western United States for the mechanics of strain localization

Fred Pollitz, USGS Menlo Park

Data sources: UNAVCO, IRIS DMC

Thatcher 2008 IGR

Western US Seismicity

• Seismicity ~bounds

Sierra Nevada microplate– SAFZ on west

– Walker Lane FZ on east

• Local seismicity bands:– west-central NV

– Southern NV

– SW UT - YS

– YS - W central ID

– YS - NW MT

YS

UT

MT

NV

ID

1975-2000 Epicenters from CNSS

Thatcher 2008 IGR

Wesnousky (2005 Tectonics)

Differences between SAF and Walker Lane/ECSZ due to

•Differences in cumulative geologic slip

•Additional component of extension / merging with Basin & Range

•Role of underlying mantle asthenosphere in dragging the crust

San AndreasFault system

Walker Lane /Eastern CaliforniaShear Zone

Fault zone locations / strain localization are the product of:

1) Coupling of mantle flow with the crust2) Pre-existing weaknesses3) Gravitational potential energy4) Laterally variable crust and mantle viscosity structure5) Other crust and mantle thermal / compositional heterogeneities

Laterally variable crust and mantle viscosity structure

• Crustal rheology -- felsic vs. mafic lower crust

• Mantle rheology -- dry vs. wet olivine -- low vs. high temperature

• Lateral rheological discontinuities -- sharp discontinuities -- broad weak zones surrounded by strong zones -- variations in effective elastic plate thickness

Thatcher and Pollitz (2008)

GPS Crustal velocity field Seismic shear-wave velocity

Geodetic inference of rheology

Upper crust

Lower crust

Upper mantle

Upper crust

Lower crust

Upper mantle

Figures courtesy of Liz Hearn

How do the lower crust and upper mantle deform after an earthquake?

Afterslip Relaxation

1999 M7.1Hector Mine earthquake

Freed et al. (2007):

• Time-dependent GPS displacements after 1999 Hector Mine earthquake best explained with mantle relaxation

• Acceptable rheologies have `strong’ lower crust and weak upper mantle

Pollitz andThatcher(2010)

Post-earthquake (M7.5 1959 Hebgen Lake, Idaho, earthquake)

Nishimura and Thatcher (2003)

Geologic inference of rheology

Figures courtesy of Bruce Bills

Bills et al (2007)

Paleolake Lahontan, Nevada

High-viscosity crust

Low-viscosity mantle

Thatcher and Pollitz(2008)

Western US rheology based on laterally homogeneous models

Modified from Dixon et al. (2004)

Molnar and Dayem (2010)

Strike-slip faults often localize along sharp structural boundaries

Influence of laterally heterogeneous rheology structure

Ryder et al. (2011)

Kunlun fault

Postseismic relaxation following 2001 M7.8 Kokoxili earthquake

Postseismic relaxation following 2010 M7.2 El Mayor-Cucapah earthquake

Pollitz et al. (2012)

Postseismic relaxation following 2010 M7.2 El Mayor-Cucapah earthquake

Pollitz et al. (2012)

Postseismic relaxation following 2010 M7.2 El Mayor-Cucapah earthquake

Pollitz et al. (2012)

50 km depth

Pollitz et al. (2012)

Seismic structure around 2010 M7.2 El Mayor-Cucapah earthquake

Pollitz et al. (2012)

Interseismic velocity field

# velocity vectorsPBO: 1595Various: 2414Payne et al.: 672Total: 4681

Viscoelastic-cycle (`blockless’) model

• Time-dependent viscoelastic relaxation of the lower crust and mantle from earthquakes occurring on a few major faults, dominated by faults close to the major plate boundaries (SAF system; Pacific-Juan de Fuca transform faults and spreading centers; Cascadia megathrust)

•Time-independent relaxation from numerous minor faults

• Viscoelastic relaxation from broadly distributed dislocation sources over a ~106 km2 area within the plate interior

• Lateral variations in effective (vertically-averaged) rigidity

• Steady slip on creeping faults

Data Inverted parameters:

• Fault slip rates

• Lateral variations in effective rigidity

• Slip distribution of past large quakes, e.g., 1700 Cascadia eq

Components of Model Velocity Field Total

Mantle Seismic Shear-WaveVelocity (40 km depth)

Pollitz and Snoke (2010)

Lateral Crustal Rigidity Variations

Lowry et al., 2000

Pollitz and Snoke (2010)

Mantle Seismic Shear-WaveVelocity (40 km depth)Elastic plate thickness

Lowry and Perez-Gussinye (2011)

Correlation of low-vp/vs

with actively deformingareasLong-term weakeningrole of high-silica crust

CONCLUSIONS

• Both sharp lateral discontinuities and localized `weak’ zones can concentrate strain in the crust.

• Crustal strain accumulation is largest within themajor strike-slip fault zones (SAF, ECSZ) andeastern boundary of the Basin & Range (ISB).

• Zones of low depth-averaged rigidity help concentrate crustal strain around the SAF, ECSZ, and ISB. There is likely upper mantle control on these zones of strain accumulation.