SEGJ Technical Conference

Why was seismic intensity 6-upper at Iizuna in the Nagano prefecture during the 2007 Niigataken Chuetsu-oki Earthquake? Part 2. Geophysical exploration for S-wave velocity structure

Abstract With a seismic intensity of 6-upper on JMA scale and peak ground acceleration up to 620cm/s/s, the fact that many wooden houses in Iizuna town in Nagano Prefecture, which is over 90km from the epicenter, were suffered from damages during the 2007 Niigata-ken Chuetsu-oki Earthquake, suggests that the site amplification is probably one of the reasons for the damages. As a part of our research project, we carried out a series of geophysical exploration such as microtremor observation, array microtremor survey and surface wave survey over the area. Though these surveys, we have obtained various kinds of important data such as the 3-D shear wave velocity structure, the distribution of predominant period as well as the H/V spectrum ratio of the ground. The shear wave velocity structure shows that there is a valley on the geo-engineering basement under the damaged area where the depth of geo-engineering basement is over 35m while that in the other area is only ranged from 10 to 15m. The distribution pattern of predominant period and H/V spectrum ratio also agrees well with the distribution of building damage.