| Abstract |
We have been conducting a gravity survey with a dense array of measurement points since 1996 to elucidate the detailed subsurface structure of Fukuoka City in Fukuoka Prefecture, Japan. The primary focus of the survey is to evaluate the location, strike and length of the Kego fault, which is one of the active faults in Fukuoka Prefecture. Fukuoka City was hit by an earthquake measuring 7.0 on the Richter scale on 20 March, 2005. The hypocenters of the main shock and the aftershocks exist in the offshore area of Fukuoka City and align with an extension of Kego fault. We measured gravity at 1259 points as of the end of April, 2006 using Scintrex CG-3 and CG-3M gravimeters. As a result, we detected some high gradient zones of Bouguer anomaly, one of which is caused by Kego fault. On the other hand, there are some low-to-moderate temperature hot springs in Fukuoka City. The temperatures of some hot springs in Yokote-Ijiri area, southern part of Fukuoka City, are relatively high (40 - 50?C) as compared with other hot springs in the city. As a result of the gravity survey, we conclude that one of the basement structures where the hot springs are concentrated is part of the Kego fault. We suspect there is a permeable fracture zone created by fault slips that acts as a path for hot water rising from the granite body to the surface in the centre of the hot springs area. |