Defining the mechanical fault

Simply defined, a fault is a fracture on which slip is localized during an earthquake. However, as the number of studies on fault zones grows, so too does our understanding of the complexity of fault structure and evolution. We examine the structure and evolution of faults related to the Landers and Hector Mine ruptures in the Mojave Desert, within the eastern California shear zone. Following both the Landers and Hector Mine earthquakes, we used trapped waves to delimit a 100-200 m wide zone of highly damaged rock and found significant velocity and shear modulus reduction within this zone to at least 5 km depth. In addition, we tracked the healing of these fault zones. Our study of both fault zones showed an increase in velocity in the years following the mainshocks. The Hector Mine earthquake shook and re-damaged the Landers fault zone resulting in a temporary reversal of healing. InSAR and shear-wave anisotropy studies complement the refraction wave studies by providing a regionally extensive view of the deformation field. Anisotropy studies show rotation in microcrack orientation along strike of the Hector Mine earthquake and also variable distribution of crack density. Post-seismic InSAR images indicate poroelastic rebound is a major player in the deformation fields following both the Landers and Hector Mine earthquakes. Localized zones of post-seismic deformation detected by InSAR correspond to regions of high crack density and velocity reduction observed by anisotropy and trapped wave studies. While we see healing of the fault zones within the first few years after rupture, long-term reduction in the fault-zone rigidity is evident from coseismic InSAR images spanning both the Landers and Hector Mine earthquakes. We see strain localized on compliant zones of nearby unbroken faults, i.e., the Pinto Mountain, Calico and Rodman faults, indicating that active fault zones are probably permanently softer than the surrounding more intact rock.