Annual Meeting - SCEC.org

Poster Abstracts | Group 1 – CDM
and either low or high asthenosphere viscosities to examine the effect of model assumptions on slip
rate estimates. In particular, we are developing a new block models for southern California in
which faults are modeled in an elastic lithosphere overlying a viscoelastic asthenosphere. The 3D
viscoelastic block model is an extension of 3D elastic block models previously developed. Our 3D
viscoelastic cycle model also uses the concept of a “steady-state” deformation field in the absence
of fault locking, similar to elastic block models. The interseismic velocity field is obtained by
modifying the steady-state velocity field with contributions from interseismic fault locking and
periodic earthquakes on faults.
Preliminary results suggest that models with low asthenosphere viscosity infer low slip rates on
the above-mentioned faults, consistent with the results of elastic block models. Decreasing the
asthenosphere viscosity raises the estimated fault slip rates. We show that the model slip rates
approach the high geologic slip rates on the Mojave segment and San Bernadino segment with
asthenosphere viscosities less than 10^19 Pa s.
1-120
METHODS FOR ESTIMATING FAULT SLIP RATES USING GEODETIC AND
GEOLOGIC DATA Johnson KM, and Fukuda J
Significant discrepancies between slip rates estimated using geodetic data and slip rates estimated
from geologic data are surfacing in the literature. Perhaps the most recognized is the discrepancy
between geologic and geodetic estimates of fault slip rate on the Altyn Tagh fault in northwestern
Tibet where inversions of geodetic data infer a slip rate of 4-10 mm/yr and geologic estimates
range from 8-34 mm/yr. Inversions of GPS data in southern California illustrate not only
discrepancies between geodetic and geologic estimates of fault slip rates, but also discrepancies
between different models of geodetic data. Inversions using elastic half-space block models by
Becker et al. [2004] and Meade and Hager [2005] infer fault slip rates in the Mojave desert region of
southern California that are a factor of two or more lower than geologic estimates on the Garlock
fault and the San Andreas and San Bernadino segments of the San Andreas fault. In contrast, a joint
inversion of GPS, geologic, and focal mechanism data by McCaffrey [2005] results in slip rate
estimates on these fault segments that are consistent with the geologic estimates.
It is not yet clear the extent to which the noted discrepancies between geodetic and geologic
estimates of fault slip rates are real differences due to sampling of time-varying slip rates at
different times, or errors in the calculation of geodetic or geologic rates. One possibility for the
discrepancies is that the model used to estimate slip rate from geodetic data is wrong or that the
inversion method is insufficient.
We outline a method for modeling long-term and interseismic deformation of elastic lithospheric
blocks moving over a viscoelastic asthenosphere. We present a fully probabilistic scheme for
inverting geologic and geodetic data for model parameters including fault slip rates, earthquake
recurrence times, lithosphere thickness, and asthenosphere viscosity. The forward model and
inverse method are designed to address the limitations of previous methods and to allow for the
integration of multiple data sets, such as geodetic and geologic data, into a unified probabilistic
estimation of the uncertainties in our knowledge of long-term block motions and fault slip rates.
132 | Southern California Earthquake Center