Estimation of Subsurface Structure in the Western Fukuoka City from Gravity Data

Our purpose is to predict earthquake disaster in Fukuoka city. A predictive estimation of ground motion requires an accurate subsurface structure. But the detailed subsurface structure in the western Fukuoka city was still unknowndue to lack of gravity data and uneven distribution of deep boring data. So we conducted densed gravity measurements to obtain a detailed Bouguer anomaly map and estimated subsurface structure based on the measured gravity data. We used Scintrex CG-3+ and CG-5 relative gravimeters for the gravity measurement and we measured the elevation of the measurement point by using network RTK-GPS.As a result of drawing a map, we found a steep slope of Bouguer anomaly which runs from west to east across the western Fukuoka city. And we also found out the slope is a boundary which divides the area into the granite layer in the south and the Paleogene layer in the north. By the prism modeling of the subsurface structure in this area, granite was found in about 30m depth ground level in the south and in about 250∼300m depth in the north. Therefore the depth of the granite is approximately 200∼250m deeper on the north side of the boundary than that in south.