Deformation of Compliant Fault Zones Induced by Nearby Earthquakes

Book excerpt: Using dynamic modeling of earthquake rupture on a strike-slip fault and seismic wave propagation in a three dimensional inhomogeneous elastoplastic medium, I investigate the inelastic response of compliant fault zones to nearby earthquakes. I primarily examine the plastic strain distribution within the fault zone and the displacement field that characterizes the effects of the presence of the fault zone. I find that when the fault zone rocks are close to failure in the prestress field, plastic strain occurs along the entire fault zone near the Earth's surface and some portions of the fault zone in the extensional quadrant at depth, while the remaining portion deforms elastically. Plastic strain enhances the surface displacement of the fault zone, and the enhancement in the extensional quadrant is stronger than that in the compressive quadrant. These findings suggest that taking into account both elastic and inelastic deformation of fault zones to nearby earthquakes may improve our estimations of fault zone structure and properties from small-scale surface deformation signals. Furthermore, identifying the inelastic response of nearby fault zones to large earthquakes may allow us to place some constraints on the absolute stress level in the crust. I also investigate how to distinguish inelastic and elastic responses of compliant fault zones to the nearby rupture. I explore in detail the range of plastic parameters that allow plastic strain to occur and examine its effect on the displacement field around compliant fault zone. I find that the sympathetic motion (i.e., consistent to long-term geologic slip) or the reduced retrograde motion (i.e., opposite to long-term geologic slip) observed in residual displacement on fault parallel horizontal direction can be directly used to distinguish the inelastic deformation from the elastic deformation. This may help better interpret the geodetic observations in the further. In addition, I conduct models with various fault zone geometries (i.e., depth, width and shape) and rigidity reduction properties to test their effects on the displacement field. The results from elastic models suggest that to the same dynamic rupture source, the deeper and wider pre-existing nearby fault zone will result in larger residual displacement. But this only applies to fault zones with large depth extent. For shallow fault zones, residual displacement tends to keep the same magnitude or even decreases with fault zone width. While in plastic models, where plastic strain is allowed, displacement field is more complex. The magnitude of the residual displacement will be enhanced by the occurrence of plastic strain. Then I extend the theoretical simulations of an idealized planar rupture fault system into one in a geometrically complex real fault system in the East California Shear Zone (ECSZ). I compare our simulation results of the 1992 Landers Earthquake with the geodetic observations. Responses of the Calico and Rodman compliant fault zone are better understood by taking into account of both inelastic and elastic responses of compliant fault zones to the nearby Landers rupture. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/152529