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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
MT
– SAFZ on west
– Walker Lane FZ on east
YS
• Local seismicity bands:
–
–
–
–
–
ID
NV
west-central NV
Southern NV
SW UT - YS
YS - W central ID
YS - NW MT
UT
1975-2000 Epicenters from CNSS
Thatcher 2008 IGR
Walker Lane /
Eastern
California
Shear Zone
Wesnousky (2005 Tectonics)
Differences between SAF and
Walker Lane/ECSZ due to
• Differences in cumulative
geologic slip
San Andreas
Fault system
• Additional component of
extension / merging with
Basin & Range
• Role of underlying mantle
asthenosphere in dragging
the crust
Fault zone locations / strain localization are the product of:
1) Coupling of mantle flow with the crust
2) Pre-existing weaknesses
3) Gravitational potential energy
4) Laterally variable crust and mantle viscosity structure
5) 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
How do the lower crust and upper mantle deform after an earthquake?
Upper crust
Upper crust
Lower crust
Lower crust
Upper mantle
Upper mantle
Afterslip
Relaxation
Figures courtesy of Liz Hearn
1999 M7.1
Hector 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 and
Thatcher
(2010)
Post-earthquake (M7.5 1959 Hebgen Lake, Idaho, earthquake)
Nishimura and Thatcher
(2003)
Geologic inference of rheology
Figures courtesy of Bruce Bills
Paleolake Lahontan, Nevada
High-viscosity crust
Low-viscosity mantle
Bills et al (2007)
Thatcher and Pollitz
(2008)
Western US rheology based on laterally homogeneous models
Modified from Dixon et al. (2004)
Strike-slip faults often localize along sharp structural boundaries
 Influence of laterally heterogeneous rheology structure
Molnar and Dayem (2010)
Postseismic relaxation following 2001 M7.8 Kokoxili earthquake
Kunlun fault
Ryder et al. (2011)
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)
Seismic structure around 2010 M7.2 El Mayor-Cucapah earthquake
50 km depth
Pollitz et al. (2012)
Pollitz et al. (2012)
Interseismic
velocity field
# velocity vectors
PBO:
1595
Various:
2414
Payne et al.:
672
Total:
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
Lateral Crustal Rigidity Variations
Mantle Seismic Shear-Wave
Velocity (40 km depth)
Pollitz and Snoke (2010)
Elastic plate thickness
Mantle Seismic Shear-Wave
Velocity (40 km depth)
Lowry et al., 2000
Pollitz and Snoke (2010)
Correlation of low-vp/vs
with actively deforming
areas

Long-term weakening
role of high-silica crust
Lowry and PerezGussinye (2011)
CONCLUSIONS
• Both sharp lateral discontinuities and
localized `weak’ zones can concentrate strain
in the crust.
• Crustal strain accumulation is largest within the
major strike-slip fault zones (SAF, ECSZ) and
eastern 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.