Gravity measurements in and around the Seto Inland Sea

根据作者所编的日本列岛重力资料汇编（KONO and FURUSE，1989），濑户内海是最大的重力资料空白区之一。本文利用Lacoste & Romberg重力仪和TSSG重力仪分别进行了陆侧和海侧重力测量。我们甚至可以通过RV上的小船进入一些小岛，比如无人居住的岛屿。科比大学理学部海洋研究实验室的Onokoro-maru（6.5吨）。利用R.利用东京大学海洋研究所的V. Tansei-maru（650吨）重力仪在整个濑户内海进行了重力观测，在该海域陆地（岛屿）上增加了600个重力测点，海上增加了6000个测点，经过数据整理，得到了详细的濑户内海重力异常图。这幅图显示了该地区重力分布的几个新发现。最相关的发现是至少从别府湾到大坂湾通过四国岛北方部分的低重力异常带的延续。该低重力异常带的南楔与中央构造线大致重合，但不延续到纪伊半岛，而是延续到大坂平原，进而延伸到琵琶湖和敦贺半岛。作为研究的副产品，我们在香川县高津市南部发现了火山口状的低重力异常分布。这可以用撞击或火山口来解释。

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.