Right-lateral-reverse surface rupture along the San Andreas and Sargent faults associated with the October 17,1989, Loma Prieta, California, earthquake

The Loma Prieta earthquake produced bewildering arrays of surface fractures of all possible modes in the epicentral area. Preliminary reports of the fracturing, though, suggested that there was no evidence of right-lateral surface slip above the fault as it is defined seismically. Through detailed mapping of selected localities along the San Andreas and Sargent faults, however, we have been able to show that strands of the San Andreas and Sargent faults ruptured to the surface with right-lateral-northward thrust components of slip in amounts of 10 to 20 cm.