DEEP SUBSURFACE STRUCTURE ESTIMATED BY MICROTREMORS ARRAY OBSERVATIONS AND GRAVITY SURVEYS IN KASHIWAZAKI AREA, JAPAN

Kashiwazaki City and Kariwa Village, located in the north central part of Japan, were the major areas damaged during the 2007 Niigataken Chuetsu-oki Earthquake (Mw 6.6). Ground motion records for the area indicate the effects of a deep subsurface structure. Microtremor array observations were performed at three sites and gravity surveys at 120 sites in the Kashiwazaki area. The gravity basement estimated from the gravity surveys exists at a depth of about 1000 m around Kariwa Village Hall, and becomes shallow in the southwestern direction around Kashiwazaki City Hall. The gravity basement is compared with the results estimated by the microtremor array observations and validated at two sites, namely, KST and KVH (Kariwa Village Hall). The peak period of the calculated response spectra at Kashiwazaki Village Hall corresponds to the peak period of the observed H/V spectra during the main shock.