中部地方北西部の地震活動

Abstract

Seismic activity in the northwestern Chubu region has been investigated mainly frommicroearthquake observations near the Atotsugawa fault, one of first class active geologicalfaults in Japan, and its environment, together with epicentral distribution of major historicalearthquakes and moderate-size earthquakes determined by the JMA. Six majar past earthquakesin this region might be associated with fault movement of the Atotsugawa fault.Seismicity of microearthquakes near the fault has been routinely monitored since 1971. Theresults obtained to date show: 1) a clear lineation of microearthquakes along the Atotsugawafault, with an intermittent zone of low seismicity, 2) less clear lineation but considerablyhigh activity near a latent smaller fault along the Yamada river, 3) high activity withclustered epicenters just south of Mt. Norikura, and 4) extremely low seismicity over aregion southeast of the Atotsugawa fault.The above pattern of seismicity does not greatly vary with time except in the region 2).Focal depths of these microearthquakes along the Atotsugawa fault are mainly confined toshallower than 15-20 kin, which appears to suggest the lower boundary of the fault plane, orpossible change of crustal properties there.Composite distribution of first motion of P wave from a number of earthquakes appears toindicate that maximum compressional, tectonic stresses lie in a derection slightly deviatingfrom the E-W direction.