Formation of left‐lateral fractures within the Summit Ridge Shear Zone, 1989 Loma Prieta, California, Earthquake

Publisher Website
Google Scholar
Abstract
The 1989 Loma Prieta, California, earthquake is characterized by the lack of major, throughgoing, coseismic, right‐lateral faulting along strands of the San Andreas fault zone in the epicentral area. Instead, throughout the Summit Ridge area there are zones of tension cracks and left‐lateral fracture zones oriented about N45°W, that is, roughly parallel to the San Andreas fault in this area. The left‐lateral fractures zones are enigmatic because their left‐lateral slip is opposite to the right‐lateral sense of the relative motion between the Pacific and North American plates. We suggest that the enigmatic fractures can be understood if we assume that coseismic deformation was by right‐lateral shear across a broad zone, about 0.5 km wide and 4 km long, beneath Summit Ridge. The shear zone has an orientation of about N70° W and is oblique to the San Andreas fault zone, which has a trend of N45° to 50°W, so that right‐lateral shearing, along with some dilation, can account for the orientation of the fracture zones. Using an analog, kinematic model of the Summit Ridge shear zone and the observable geometric parameters (a shear zone about 0.5 km wide, an acute angle of 25° between the tension cracks and the shear‐zone walls, left‐lateral slip of 5 cm, and spacing of about 100 m in the tension cracks) we estimate that the amount of right‐lateral shift across the Summit Ridge shear zone was on the order of 1.4 m. This is comparable to the amount of slip for coseismic faulting at depth, 1.9 to 2.4 m, as computed by several investigators. Thus contrary to most previous reports on the Loma Prieta earthquake, which assert that coseismic, right‐lateral ground rupture was restricted to considerable (greater than 4 km) depths in the epicentral area, we find that nearly all the right‐lateral offset is represented at the ground surface by the Summit Ridge shear zone.