伊賀・伊勢地方の地震活動

Abstract

In order to study seismic activity of microearthquakes in the Iga and isa regions, atemporary ohservation was carried out at four high sensitive stations in the region fromAugust 22 to December 6 in 1974. About 40 shocks could be located within this area, although several hundreds microearthquakes were recorded at each station during thisperiod. Some following results were obtained;1) The seismic activity is very low in this area except northwest of Iga-Ueno city.Particularly, no earthquakes were detected in an area from the middle of the IsePlain to the Iga-Basin, in spite of very high detection capability.2) In the northwestern part of Jga-Ueno city, where the Iga-Ueno Earthquake of 1854took place, a relatively high level of seismic activity has been detected around thesouthwestern half of a presumed fault. Their focal depths are confined to 5-15km.Its lower limit appears to suggest the deeper boundary of the presumed fault plane.3) The focal mechanism of the largest earthquake (M=3. 3) that occurred near Iga-Uenocity during this observation period, shows the radiation pattern consistent with rightlateral faulting, suggesting that the Iga-Ueno Earthquake also might have been causedby the same type of fault movements.4) The area concerned has a number of active geological faults, but many of them werenot traced by the present observation of microearthquakes.