Mechanics and tectonic implications of the 1966 and 1970 Peru earthquakes

Abstract

The source mechanism of two large earthquake which occurred off the coast of Peru on October 17, 1966 and on May 31, 1970, are determined on the basis of long-period surface-wave data at many worldwide stations. The wave-form equalization method is used for the interpretation. The 1966 earthquake represents a low-angle thrust faulting whose slip vector is almost perpendicular to the trench axis. The 1970 earthquake definitely represents, unlike the 1966 earthquake, a normal faulting; the fracture extends to a depth of about 100 km. These large earthquake reflect major tectonic processes in the Peruvian active belt. The large-scale thrust faulting is considered to result from an elastic rebound which took place at the friction-coupled interface between the underthrusting oceanis Nazca plate and the South America continent. The normal faulting is regarded as a large-scale extensional fracture of the underthrusting Nazca plate; the extensional stress is caused by a gravitational pull exerted by the denser sinking plate. Such large-scale normal faulting within the plate and the subsequent detachment of the sinking portion account for the marked gap in seismic activity at depths and also the occurence of deep earthquakes beneath the South American continent. The source parameters of the 1966 earthquake are: seismic moment, 2.0 × 10 28 dyne · cm; average dislocation, 2.6 m; stress drop, 42 b. The source parameters of the 1970 earthquake are: seismic moment, 1.0 × 10 28 dyne · cm; average dislocation, 1.4–1.8 m; stress drop, 23–35 b; rupture length, 130 km; rupture velocity, 2–3 km/s towards N 160° E.