Asymmetric Structural Properties Across the San Jacinto Fault, Anza and the San Andreas Fault, Gorman: A Possible Indication of Preferred Rupture Direction

We studied the structure and asymmetry properties of gouge and damage zone along the San Jacinto fault near Anza, CA. The NE side of the Principal Shear Zone (PSZ) expresses higher fracture density (FD) and more fault-parallel shear fractures with respect to the SW. In a road-cut exposure 2 m below the original ground surface, where the fault juxtaposes middle and late Pleistocene alluvial deposits, the gouge is 1.6 m wide and can be divided into distinct fracture domains. On the SW side of the recently active PSZ, the gouge is massive and dark, with a FD of 0.08 (cumulative fracture length, cm/cm2). Immediately to the NE of the PSZ, the gouge is highly sheared and foliated, with a FD of 0.82. The next gouge layer to the NE is shattered but shows no evidence of shearing and has a FD of 0.58. The SW wall-rock shows near-zero FD while the NE wall-rock has a FD of 0.17, higher than that of the massive SW gouge layer. We exposed the same gouge zone in a trench, 3.5 m below the road-cut and 5 m away from the original ground surface where it is only 15 cm wide; its face was moistened, than carefully scraped to preserve the natural fabric. This narrow gouge expresses similar division and asymmetry as the exposure above it, except that it is more compressed. Artificial exposures on two additional slopes with opposing exposure directions, located 100 and 140 m SE of the road-cut, respectively, were excavated to a depth of several meters. The sheared gouge layer adjacent to the PSZ from its NE side and within it has, respectively, 1.8 and 2.2 higher FD than that of the massive gouge layer on the SW side. The general fracture pattern is similar to the one of the road-cut exposures. We are now in the process of studying additional exposures along the Anza section of the San Jacinto fault and the Gorman area of the San Andreas fault. Our observations show several manifestations of asymmetry in gouge and fracture properties that are compatible overall with theoretical predictions for a wrinkle-like rupture pulse along an interface between different elastic materials (Ben-Zion, JMPS, 2001, and references therein). The wrinkle-like pulse has a preferred propagation direction that is the same as the loading direction on the slower side of the fault, and dynamic dilation at the rupture front with large fault-normal motion on the slower side. If such ruptures occur repeatedly on a fault that separate different media, we expect the following signals on the slower side: 1. higher FD in the gouge and in the host rock, 2. fault-parallel tension fractures with increasing density towards the PSZ, 3. higher morphological degradation driven by the increased FD, 4. reasonable consistency of items 1-3 along the fault. Our initial observations agree with these expectations, assuming that the slower side of the faults in our study areas is on the NE and the preferred direction of rupture propagation is to the SE. If these signals indeed represent the mechanical behavior of wrinkle-like rupture pulses, continuing studies can provide improved understanding of earthquake behavior and refined estimates of ground shaking associated with large faults separating different media. Updated results will be presented in the meeting.