Gravity measurements in and around the Seto Inland Sea

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According to one of the author's compilatation of gravity data all over the Japanese Islands (KONO and FURUSE, 1989), the Seto Inland Sea is one of the biggest vacant region of gravity data. This region is geologically important since the Median Tectonic Line and zonal distribution of many geological units are exist.We carried out gravity measurements from both land and sea side by using Lacoste & Romberg gravimeter on land and TSSG gravimeter on sea. We accessed even small islands such uninhabited islers by employing a small boat on RV. Onokoro-maru (6.5 ton) of Marine Research Laboratory, Faculty of Science, Kobe univesity. Marine gravity measurements were carried out by using R. V. Tansei-maru (650 ton) of Ocean Research Institute, university of Tokyo, all over the Seto Inland Sea.600 gravity stations were increased in this area on land (islands) and 6, 000 points on sea.After data reduction, detailed gravity anomaly map over the Seto Inland Sea were obtained. This map indicates several new findings of gravity distribution in this region. The most relevant finding is a continuation of low gravity anomaly zone at least from the Beppu-wan to Osaka-wan through the northern part of Shikoku island. Southern wedge of this low gravity anomaly zone is roughly coincide with the Median Tectonic Line but do not continue to the Kii peninsula but to the Osaka plain and furthermore to the Lake Biwa and the Tsuruga peninsula.As a by-product of this study, we found a crater-like low gravity anomaly distribution in the south of Takazatsu, Kagawa prefecture. This may be explained by either an impact or volcanic crater.