A microgravity survey was carried out in Izumi Cityto investigate subsurface geological structure around the hugelandslide, which killed 21 people and destroyed 19 houses atHarihara district in July, 1997.Locations of 128 gravity stations were dicided by the fast-staticmode of GPS survey with a simultaneous measurement at a basestation of the Geographical Survey Institute (GSI) in Akune City.The terrain effect was precisely corrected using the 50 meter meshtopographic data provided by GSI.The F-H relationship method is improved for the automatic andsimultaneous estimation of a Bouguer density and a regional trend.In this method, unknown parameters such as density andcoefficients of 2-D surface function are estimatedby the least square algorithm.Analysis of gravity data sets gives an appropriate density valueof 2.18g/cm3, around the Hariharagawa debris flow. On the otherhand, a classical F-H method determines an unreasonable densityvalue because of high correlation between Bouguer anomalies and terrain.Residual anomalies without a regional trend show that theHariharagawa landslide located in the boundary of high gravityanomalies. The gravity anomalies and the surface geology suggestthat there was an intrusive dyke and the lanslide occurred near theboundary wall.
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