1984年長野県西部地震の地変および大加速度域の調査

Abstract

Surface ruptures and a high acceleration area were revealed from the survey at the sourceregion of the Western Nagano Prefecture Earthquake of September 14, 1984 (35 49.3'N, 137 33.67E, H=2 km, M=6.8, JMA).Cracks on paved roads and on ridge crests were developed within 10 km in length and 2 kmin width from Yanagase to north of Matsukoshi, trending N70 E which is consistent with thestrike of one nodal plane of the main shock as well as with the trend of the aftershock area. Sinceeach of the cracks is believed to be tectonic in nature, the rupture region can show the im-bedded earthquake fault, though the systematic displacement was not found on the surface.More than 80 of boulders and fallen-trees on horizontal ground overturned off or thrownoff out of their sockets were found near the epicenter of the main shock. The ground velocitycalculated from the displacement of the boulder is about 200 cm/s, therefore, the ground accele-ration is as high as 4-l3 g at the assumed wave frequency of 2-10 Hz. We ascertained sucha high acceleration by a simple experiment, using accelerometers and small stones. Additionalevidence of the high acceleration was found; size segregation was observed for the boulders inthe cracks. The size segregation must be caused by the high acceleration. However, housesin the Otaki Village were not severely damaged by the main shock. This is probably attributedto the high frequency (about 10 Hz) and the short duration time of the high-acceleration seismicwaves.The high acceleration area was clearly defined by detailed field surveys for the dislodgedboulders and fallen trees. Most of them were found on the ridge crests but some in the valley.Boulders were shot out in the area of 1 km by 2 km and they were only shifted off in the areaof 2 km by 4 km. Although the areas are much smaller than the aftershock area, the trend ofthe areas is consistent with that of the one nodal plane as well as the trend of the rupture region.The results of the field surveys must be important for the analysis of the detailed source processesof the main shock.