The Peru earthquake of May 31, 1970: Some preliminary seismological results

The 1970 Peru earthquake was a multiple shock. The first widely-recorded shock had a magnitude of about 5; within 20 sec there were at least four larger earthquakes, at least two of which exceeded magnitude 7. Focal mechanisms appear to be complicated, with at least one major shock yielding compressions at distances of 25° to 30° to the northeast. Most aftershocks occur at depths of 40 to 70 km; their first motions are sometimes reversed with respect to the main shocks. The seismic history of the region includes one comparable earthquake in 1725 but no events of similar magnitude thereafter. The town of Ranrahirca was not destroyed by the 1970 mud flow but by an earlier event in 1962.