Abstract
The cooperative microearthquake observation with seven portable seismometers and one fixed seismometer was carried out in the northeastern part (Tohoku) of Honshu, Japan, for twenty days in 1967. Twenty recording sites were used to locate the foci of about fifty microearthquakes during this period. One-half of the events were located in the zone (Morioka-Shirakawa tectonic line) between the western edge of Kitakami mountain and the volcanic front, the others were located on the westward dipping seismic plane. The crust bounded by these two seismic zones is aseismic.The Morioka-Shirakawa tectonic line, which was inferred from the gravity anomaly, is a shallow seismic belt with a NS trend, however the level of the activity is not high; seven small shocks along this line in the surveyed area had been located by JMA network during the last 15 years. This tectonic line was characterized by the occurrence of small earthquake swarm.
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