The damaging earthquake (M=6.8) on September 14, 1984 was a strike-slip faulting trending ENE-WSW across an area of earthquake swarm near Ontake volcano, central Japan. The rupture was 12 km long, about 10 km wide and very shallow, though there was no break on the surface. The earthquake occurred near the boundary of crustal blocks between two adjacent earthquake swarms that have been markedly active since 1976. The expansion of aftershock activity to the west and to the east was obvious, though the rupture pattern was complicated. The largest aftershock of M=6.2 to the west of the main shock was conjugated with the main fault. The largest to the east was a reverse faulting. However the pressure axes of these faultings were the same as that of the main shock. The difference between these fault types might be controlled by block boundaries with different directions around the volcano.
The detailed three-dimensional P and S wave velocity structure in the focal region of the 1984 Western Nagano Prefecture Earthquake occurring close to an active volcano, Mt. Ontake, central Japan, is derived from a tomographic inversion of travel time data obtained by the 1986 Joint Seismological Research in Western Nagano Prefecture. The data set includes 7,693 P-wave and 6,070 S-wave arrival times observed at 49 stations from 212 local earthquakes and 2 explosions. The velocities in the shallow portion of upper crust are determined at each grid point with its spacing of 1-2 km and good resolutions are obtained from the Earth's surface to a depth of 4 km. There exist strong lateral heterogeneities especially from the surface to a depth of 1 km. The seismic velocity map obtained in the height of 1 km above sea level well corresponds to the maps of surface geology and Bouguer gravity anomaly. This correspondence, however, is not so clear in the deeper layers. And, the greater the depth, the less heterogeneous the velocity structure. The velocity distribution on the fault seems to correspond to the distribution of dislocation and rupture front estimated from an analysis of strong motion and geodetic data: the low velocity region has the large amount of dislocation and the retarded rupture front.
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