1891年濃尾大地震後の余効変動

抄録

The 1891 great Nobi earthquake is considered to have been the greatest (M-8) intraplate earthquake in Japan in the past 100 years. It provides much information on the long-term prognosis of earthquakes in this district which is affected by postseismic movements. The postseismic movements on and near the ruptured fault zone (the Neodani-Umehara fault) in the period of almost 100 years since the 1891 quake have been inves-tigated by analyzing the data from repetitive leveling and triangulation surveys ever since 1889 in the south Chubu district of central Japan. The postseismic vertical movement due to aseismic slip on and near the edge of the south-eastern section of the ruptured fault zone continued to subside for about 80 years since the occurrence of the quake, but in this last 20 years, the movement has changed to an uplifting movement. That is, following the final stage of o-phase due to aseismic slip in the phase cycle of seismic crustal movement in this district, the first stage of a-phase started around 1970. Two zones showing relatively large maximum shear strains, were detected along the main fault and along the direction of a curved line from WSW to E nearly perpen-dicular to the main fault. The latter zone seems to agree with the leading edge of the Philippine sea plate. These facts show that the strain energies were accumulated elastically by postseismic slow deformation in the period of about 80 years after the 1891 quake and mostly released by generating earthquakes in the periphery of the fault. A non-local anomalous upheaval around Ookuwa-mura near the Atera fault was detected from the leveling data during the period from 1966-7 to 1984 just before the Western Nagano prefecture earthquake (1984 M 6.8). It is necessary to conduct further investigation especially around the Atera fault or Mt. Ontake to comfirm whether the upheaval was released by the earthquake or not.