Microearthquake Activity in the Focal Area of the 1962 Northern Miyagi Earthquake (M 6.5)

The northern part of Miyagi Prefecture is one of the most seismically active areas in the northeastern Japan arc. At present, many shallow earthquakes occur in and around the focal area of the 1962 Northern Miyagi Earthquake (M 6.5). The daily number of these earthquakes occurring now coincides with that expected from the lapse time-aftershock frequency relation, the extended Omori's law, of the 1962 event. A temporary seismic network set up in this area has revealed that the present seismicity is distributed on a plane dipping to the west-northwest at an angle of about 50°. Hypocenters of aftershocks within one month of the main shock occurrence are relocated by using S-P time data of the aftershocks. Relocated aftershocks are also distributed on a plane dipping to the west-northwest at approximately the same angle which corresponds to one of the nodal planes of the focal mechanism solution of the main shock. These observations indicate that the 1962 event ruptured along a plane inclined toward the west-northwest at an angle of -50° and that aftershocks of this event are still actively occurring now, more than 30 years after the main shock occurrence, along the fault plane or its northward extension.