Abstract
A new gravity map of the northeastern part of Kyushu has been published. It is an issue of the 1 : 200, 000 gravity map series published by GSJ/AIST (Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology). New gravity data at 445 stations were obtained around Oita City, Shonai Town, Kuju Town, and Inukai Town. They were compiled together with existing data of 11,748 land stations. The Bouguer gravity anomaly map with an assumed density of 2.3 g/cm3 revealed some structural boundaries. Volcanic areas including active volcanoes, Mt. Tsurumi-dake, Mt. Yubu-dake, Mt. Kuju, and Mt. Aso are included in a negative gravity region, which consist of five major gravity lows. Granite rocks and metamorphic rocks are distributed in the positive gravity region at northern part of Oita district. Bouguer gravity anomalies clearly indicate the boundaries of basement structure in this area. The result of 2-D two-layer modeling shows that maximum thickness of Cenozoic deposits in the Beppu bay is more than 4,000 m.
