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 eŠects 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.
Destructive earthquake (MJ 6.8) occurred on October 23, 2004 in the Niigata-ken Chuetsu area. Many active foldings are found in this area, and it is known that the subsurface structure is complex. In case we simulate the earthquake ground motions during the main shock in such a complex area, we need detailed information with respect to the subsurface structure. This kind of information, however, is not provided enough currently. Therefore, we carried out the gravity survey around Chuetsu area and estimate a 3-D shape of the upper boundary for the gravity basement. Through the comparison of the proposed model of the gravity basement and the results from some previous researches, we can conclude that the proposed model is improved by the newly observed gravity data. Furthermore, to discuss the validity of the proposed model, we perform two simple numerical simulations of the earthquake ground motions: one is based on the proposed basement model and the other, based on a model estimated using existent gravity data without our contribution. As a result, the proposed model can provide better explanation of the earthquake ground motions than the model from the existent gravity data.
Gravity survey is useful tool to estimate subsurface structure for large area. However, we may face sometime difficult problem to fix the trade-off between depth and density. One solution to solve this problem is to introduce different physical quantities, such as magnetic anomaly etc. Chandler et al. (1981) proposed a technique to estimate location of boundary of medium using Poissons' relation, which provides a relation between gravity and magnetic potentials. We propose a method to improve the Chendler's method for obtaining better solutions. Furthermore, we applied the proposed method to some numerical examples and a real data-set, and showed that appropriate results can be obtained.
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