A Shallow Crustal Structure as Derived from an Explosion Seismic Experiment

The key subjects of the 1986 Joint Seismological Research in Western Nagano Prefecture (JSR'86) were to determine hypocenters of aftershocks of the 1984 Western Nagano Prefecture Earthquake and the shallow crustal structure precisely. Therefore, we deployed a temporary seismic network of closely spaced stations with a high quality in the southern foot of Mt. Ontake, the epicentral area of the 1984 earthquake. As one of the key subjects of JSR'86, a three-dimensional velocity modeling was executed using the natural earthquake data observed during the experiment. In addition to this inversion of observed data of earthquakes, we conducted an explosion seismic experiment to obtain information on the shallow crustal structure more precisely to improve hypocenter determination as well as to find characteristics of the structure. The two-dimensional profile was derived from observed data of seismic waves by two shots at 43 temporal stations. The length of the profile was 20 km, crossing the Otaki River which may be concordant with the structural boundary as pointed out geologically and topographically. The derived time-term distribution represents the two-dimensional surface-layer thickness, and has correlations to the distribution of the Bouguer gravity anomaly and the hypocenter distribution in the area. The rim of high consistency of contour lines on the Bouguer gravity anomaly map and the time-term distribution map may coincide with the boundary of the seismically active area.