The September 5, 1970 Sea of Okhotsk Earthquake: A multiple event with evidence of triggering

The September 5, 1970 Sea of Okhotsk earthquake consisted of two possibly causally related but dissimilar events: a small (Mag 4.5) event at the hypocenter given by the International Seismologic Centre (52.28 N, 151.49 E, 560 km) followed 5.16±.06 seconds later by a larger (Mag 5.7) event almost at the same epicenter but 23.0±1 km deeper. The fault planes and the principal stress axes of the two events are significantly different; furthermore, the second event does not lie on either of the nodal planes of the first event. We interpret this as a possible triggering of the second event by the smaller first event and not as a change in direction of rupture propagation during a single continuous faulting episode.