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Abstract:A new gravity map of the Fukuoka district was published. It is a map of the 1:200,000 gravity map series by the Geological Survey of Japan, AIST. The gravity data were compiled from 11,937 stations including 1,513 new stations. These new gravity surveys were carried out around the Fukuoka area. We show the table with gravity data for 706 new stations around the central part of the Fukuoka City area. This Bouguer gravity anomaly map with an assumed density of 2.3 g/cm 3 clearly revealed the structural boundaries between the central part of the Fukuoka City and its surroundings.

A gravity survey was conducted in and around the Nagamachi-Rifu Fault. The density for both terrain and Bouguer corrections was chosen to be 2,300 kg/m 3 , because volcanic rocks are dominant over the whole measurement area and the surface layer density is estimated to be low from geological considerations. The Bouguer anomalies are characterized by a low anomaly similar to those of a caldera and the basement structure inferred from two and three-dimensional analysis shows that the depth is often more than 1 km in the caldera region and that there is a circular structure. The gradient of basement is steepest at the southern margin, and it resembles to rim of funnel-shaped caldera.