AFTERSHOCK ACTIVITY OF THE 1984 WESTERN NAGANO PREFECTURE EARTHQUAKE, CENTRAL JAPAN, AND ITS RELATION TO EARTHQUAKE SWARMS

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.