Earth Planets Space, Vol. 57 (No. 6), pp. 539-544, 2005LETTER
Hiroaki Yamanaka1, Kentaro Motoki1, Shun'ichi Fukumoto2, Toshiyuki Takahashi2, Nobuyuki Yamada3, and Kimiyuki Asano3
1Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8503, Japan
2Tokyo Soil Research, 2-11-16 Higashigaoka, Meguro-ku, Tokyo 152-0021, Japan
3Disaster Prevention Research Institute, Kyoto University, Gokasho Uji, Kyoto 611-0011, Japan
(Received February 15, 2005; Revised April 25, 2005; Accepted April 25, 2005)
Aftershock observations of the 2004 Mid Niigata Prefecture earthquake were conducted in the central part of Ojiya city, the Niigata prefecture, central Japan, to investigate local site effects. We installed eight accelerographs in the vicinity of the K-NET and JMA stations in the area. The stations of the aftershock observations are situated under different geological conditions including one installed in a mountainous area on Tertiary layers to serve as a reference site. We examined the ground-motion characteristics of the records for a Mj 6.1 aftershock focusing on local site effects. The amplification, at a period of less than 1 sec, is the largest in the vicinity of the K-NET station. The amplification at periods longer than 2 sec is larger in the western part of the city than those in the east. We estimated the S-wave velocity structure in the sediments above the basement with an S-wave velocity of 3.4 km/sec from the inversion of phase velocities measured by the array observations of vertical microtremors. We discuss the amplification factors using the S-wave velocity profile and show that shallow soils over the layer with an S-wave velocity of 0.49 km/sec are responsible for the amplification at periods shorter than 0.4 sec. Deeper sedimentary layers are needed to explain amplification at periods of 1 sec.
Key words: 2004 Mid Niigata Prefecture earthquake, local site effect, aftershock observation, microtremor array exploration.