This shallow crustal earthquake was followed 13 hours later by a deep focus magnitude 6.8 quake roughly to the west, below the Sea of Japan. The two earthquakes were generated by different mechanisms. The first earthquake was caused by deformation within the crust of the Okhotsk Plate and the second quake was likely caused by faulting resulting from internal deformation of the subducted Pacific Plate. Given their different mechanisms and physical separation of at least 10 rupture lengths, the second earthquake is not considered an aftershock of the first.