Symmetry properties, pulverized rocks and damage architecture in fault zones as signatures of earthquake ruptures.

摘要

Structural symmetry properties were mapped across faults of the San Andreas Fault (SAF) and North Anatolian Fault (NAF) systems, at various scales and several sites on each fault. Fractures on a fault-core scale, subsidiary faults and fault rocks on a fault-zone scale and pulverized rocks on a damage-zone scale show systematically asymmetry. On the SAF, San Jacinto and Punchbowl faults the northeast side is more damaged. On the NAF 1943 and 1944 rupture sections the south and north sides, respectively, are more damaged. Asymmetric erosion patterns along the NAF including locations of river valleys with respect to the fault and contrast in drainage density and other morphometric parameters across the fault, are consistent with the geologically mapped structural asymmetry. These asymmetric patterns are compatible with preferred rupture directions northwestward on faults of the SAF system, and eastward and westward on the 1943-1944 rupture sections of the NAF, respectively (as occurred in these two earthquakes). Tomographic studies show that the northeast side of the SAF and the San Jacinto fault have faster seismic velocities at depth. Significant damage content in sedimentary rocks of the Juniper Hills formation near the SAF in the central Mojave section indicates that dynamic generation of damage can occur close to the Earth surface, in agreement with other indications for minimal exhumation of damaged fault zone rocks. An asymmetric shallow damage structure correlated with the velocity structure at depth is a predicted outcome for rupture along a bimaterial interface (Ben-Zion and Shi, 2005). Microfractures in the Juniper Hills rocks near the fault, orientated preferably normal to its strike, are compatible with the transient stress field associated with seismic slip events on frictional rough surfaces (Chester and Chester, 2000). The damage fabric is anisotropic, rich with compressional features, and therefore not compatible with an absolute tension.; Structural analysis of orientation and slip data of 115 slip surfaces in the hanging-wall of the Sierra Madre fault near JPL shows that their geometry and kinematics are compatible with Mohr-Coulomb failure associated with the stress field of propagating mode II ruptures with slip weakening (Rice et al. 2005).