Space Geodetic Constraints on the Structure and Properties of Compliant Damage Zones Around Major Crustal Faults

Geologic and seismologic studies of large crustal faults indicate that the fault interface that accommodates most of seismic slip is often surrounded by heavily damaged material characterized by high crack density and reduced seismic velocities. Recently such damage zones were imaged by space geodetic observations. I present results of Interferometric Synthetic Aperture Radar (InSAR) observations of deformation across kilometer-wide compliant fault zones in response to nearby earthquakes. In particular, a number of faults in the Eastern California Shear Zone, including the Calico, Rodman, Pinto Mountain, and Lenwood faults, were strained by both the 1992 Landers and the 1999 Hector Mine earthquakes. Analysis of deformation on these faults indicates that the fault zone displacements depend on the magnitude, but are independent of the sign of the co-seismic stress changes, implying a linearly elastic deformation. Other examples include faults adjacent to the North Anatolian fault (Turkey) that were strained by the 1999 Izmit earthquake. Analytic and numerical (finite element) modeling of the observed deformation suggests that the compliant fault zones have width of 1-2 km, depth extent of several km (or greater), and reductions in the effective shear modulus of about a factor of two. Stacked interferometric data from the Eastern California Shear Zone spannig a time period of more than 10 years reveal time-dependent (post- or inter-seismic) deformation on some of the inferred compliant fault zones. In particular, the fault zone associated with the Pinto Mountain fault was subsiding over several years following the Landers eartquake, with the total amplitude of subsidence comparable to the amplitude of the co-seismically-induced uplift. This behavior may be indicative of the poro-elastic deformation of the fluid-saturated fault zone.