S. Barbot, Y. Fialko, Y. Bock, "Postseismic Deformation due to the Mw6.0 2004 Parkfield Earthquake: Stress-Driven Creep on a Fault with Spatially Variable Rate-and-State Friction Parameters", Journal of Geophysical Research, 2009, submitted

Abstract - We investigate the co- and postseismic deformation due to the Mw6.0 2004 Parkfield, CA earthquake. We produce co- and postseismic slip models by inverting data from an array of 14 continuous GPS stations from the SCIGN network. Kinematic inversions of postseismic GPS data over a time period of three years show that afterslip occurred in areas of low seismicity and low coseismic slip, predominantly at a depth of 5 km. Inversions suggest that coseismic stress increases were relaxed by predominantly aseismic afterslip on a fault plane. The kinetics of afterslip is consistent with a velocity-strengthening friction generalized to include the case of infinitesimal velocities. We performed simulations of stress-driven creep using a numerical model that evaluates the time-dependent deformation due to coseismic stress changes in an visco-elastoplastic half-space. Starting with a coseismic slip distribution, we compute the time-dependent evolution of afterslip on a fault plane and the associated displacements at the GPS stations. Data are best explained by a rate-strengthening model with frictional parameter (a-b) = 7 ×10-3, at a high end of values observed in laboratory experiments. We also find that the geodetic moment due to creep is a factor of 100 greater than the cumulative seismic moment of aftershocks. The rate of aftershocks in the top 10 km of the seismogenic zone mirrors the kinetics of afterslip, suggesting that post-earthquake seismicity is governed by loading from the nearby aseismic creep. The San Andreas fault around Parkfield is deduced to have large along-strike variations in rate-and-state frictional properties. Velocity strengthening areas may be responsible for the separation of the coseismic slip in two distinct asperities and for the ongoing aseismic creep occurring between the velocity-weakening patches after the 2004 rupture.

S. Barbot, Y. Fialko, D. Sandwell, "Three-Dimensional Models of Elasto-Static Deformation in Heterogeneous Media, with Applications to the East California Shear Zone", Geophysical Journal International, submitted

Abstract - We present a semi-analytic iterative procedure for evaluating the three-dimensional (3-D) deformation due to faults in an arbitrary heterogeneous half space. Spatially variable elastic properties are modeled with equivalent body forces and equivalent surface traction in a "homogenized" elastic medium. The displacement field is obtained in the Fourier domain using a semi-analytic Green function. We apply this model to investigate the response of 3-D compliant zones (CZ) around major crustal faults to coseismic stressing by nearby earthquakes. We constrain the two elastic moduli, as well as the geometry of the fault zones by comparing the model predictions to Synthetic Aperture Radar interferometric (InSAR) data. Our results condirm that the compliant zones models for the Rodman, Calico and Pinto Mountain faults in the Eastern California Shear Zone can explain the coseismic InSAR data from both the Landers and the Hector Mine earthquakes. For the Pinto Mountain fault zone, InSAR data suggest a 50% reduction in effective shear modulus and no significant change in Poisson's ratio compared to the ambient crust. The large wavelength of coseismic line-of-sight displacements around the Pinto Mountain fault requires a fairly wide (~1.9 km) CZ extending to a depth of at least 9 km. Best fit for the Calico CZ, north of Galway Dry Lake, is obtained for a 4 km deep structure, with a 60% reduction in shear modulus, with no change in Poisson's ratio. We find that the require effective rigidity of the Calico fault zone south of Galway Dry Lake is not as low as that of the northern segment, suggesting along-strike variations of effective elastic moduli within the same fault zone. The ECSZ InSAR data is best explained by CZ models with reduction in both shear and bulk moduli. The observations suggest pervasive and widespread damage around active crustal faults.

S. Barbot, Y. Fialko, D. Sandwell, "Effect of a compliant fault zone on the inferred earthquake slip distribution", Journal of Geophysical Research, Volume 113, Issue B6, CiteID B06404

Abstract - We present a new semi-analytic method to evaluate the deformation due to a screw dislocation in arbitrarily heterogeneous and/or anisotropic elastic half plane. The method employs integral transformations to reduce the governing partial differential equations to the integral Fredholm equation of the second kind. Dislocation sources, as well as spatial perturbations in the elastic properties are modeled using equivalent body forces. The solution to the Fredholm equation is obtained in the Fourier domain using a method of successive over-relaxation, and is mapped into the spatial domain using the inverse Fast Fourier Transform. We apply this method to investigate the effect of a soft damage zone around an earthquake fault on the co-seismic displacement field, and on the earthquake slip distribution inferred from inversions of geodetic data. In the presence of a kilometer-wide damage zone with a reduction of the effective shear modulus of a factor of 2, inversions that assume a laterally homogeneous model tend to underestimate the amount of slip in the middle of the seismogenic layer by as much as 20%. This bias may accentuate the inferred maxima in the seismic moment release at depth between 3–6 km suggested by previous studies of large strike-slip earthquakes.

S. Barbot, Y. Hamiel. Y. Fialko, "Space geodetic investigation of the coseismic and postseismic deformation due to the 2003 Mw 7.2 Altai earthquake: Implications for the local lithospheric rheology", J. Geophys. Res., 113, B03403, doi:10.1029/2007JB005063

Abstract - We use Envisat Advanced Synthetic Aperture Radar data and SPOT optical imagery to investigate the coseismic and postseismic deformation due to the 27 September 2003, Mw 7.2 Altai earthquake, which occurred in the Chuya Basin near the Russia-China-Mongolia border. On the basis of the synthetic aperture radar (SAR) and SPOT data, we determined the rupture location and developed a coseismic slip model for the Altai earthquake. The inferred rupture location is in a good agreement with field observations, and the geodetic moment from our slip model is consistent with the seismic moment determined from the teleseismic data. While the epicentral area of the Altai earthquake is not optimal for radar interferometry (in particular, due to temporal decorrelation), we were able to detect a transient signal over a time period of 3 years following the earthquake. The signal is robust in that it allows us to discriminate among several commonly assumed mechanisms of postseismic relaxation. We find that the postearthquake interferometric SAR data do not warrant poroelastic rebound in the upper crust. The observed deformation also disagrees with linear viscoelastic relaxation in the upper mantle or lower crust, giving rise to a lower bound on the dynamic viscosity of the lower crust of the order of 1e19 Pa s. The data can be explained in terms of fault slip within the seismogenic zone, on the periphery of areas with high coseismic slip. Most of the postseismic deformation can be explained in terms of seismic moment release in aftershocks; some shallow slip may have also occurred aseismically. Therefore the observed postseismic deformation due to the Altai earthquake is qualitatively different from deformation due to other similarly sized earthquakes, in particular, the Landers and Hector Mine earthquakes in the Mojave desert, southern California. The observed variations in the deformation pattern may be indicative of different rheologic structure of the continental lithosphere in different tectonically active areas.