Elastic stress relaxation: A mechanism for opposite sense of secondary faulting with respect to a major fault

The motion along a major fault perturbs the local stress field and produces numerous secondary fractures. Among these, the secondary fractures that have the opposite sense of motion with respect to the main fault are particularly intriguing. For example, the right-lateral/reverse motion of the M(s) 7.1 Loma Prieta earthquake of 18 October 1989 produced a series of left-lateral/normal fractures, located in a narrow zone about 1 km in width subparallel to the trace of the San Andreas fault. Through a rigorous inversion of geodetic data associated with the earthquake that allowed the slip to vary on the rupture surface, we studied the origin of the left-lateral/normal fractures by comparing the location, orientation and slip sense of the observed fractures with the perturbed elastic stress field calculated from the estimated slip distribution of the earthquake. The perturbed state of stress at the Earth's surface is complex, and the greatest principal stress changes from horizontal to vertical. The area of left-lateral/tensile state of stress coincides with that of the observed left-lateral/normal fractures. The magnitudes of the induced fault-normal tensile stresses of 60-120 bars and left-lateral fault-parallel shear stresses of 0-4 bars in the area are large enough to produce the ground failures. Thus, the observed left-lateral/normal fractures adjacent to the San Andreas fault are interpreted to be related to coseismic elastic stress relaxation. The same mechanism can also be used to explain the normal faulting in the upthrown blocks of the thrust faults which produced the 1980 El Asnam, Algeria and the 1968 Tokachi-Oki, Japan earthquakes.