Hypocenter Locations by a Dense Network

A dense temporary seismograph network was set up in the aftershock area of the 1984 Western Nagano Prefecture Earthquake of M 6.8 by the group for the 1986 Joint Seismological Research in Western Nagano Prefecture. Seismograms of about 1, 500 events were obtained during the observation period of 52 days from September 1 to October 22, 1986, and 530 events in and around the aftershock area were located precisely. Extent of the aftershock area is about 20 km in the east-west and about 7 km in the north-south. The hypocenter distribution in the present study shows that the depths of the deepest aftershocks become deep toward the west. The maximum depth in the eastern part of the aftershock area is about 6 km and it becomes 10 km at the western end. Seismic activity is high at a belt near the maximum depths of the aftershock area and no events were detected below this seismic belt. It was also found that the size and location of the aftershock area found by the present observation are almost the same as that obtained by a temporary seismic observation in 1984, which was made just after the main shock. Since no events below the inclined lower oundary were found in either of the observations, it is suggested that the deeper region below the boundary cannot accumulate stresses sufficient to generate earthquakes, suggesting the existence of large locality in the lower boundary of seismogenic zone.