Stress fields and rotation of the basement of the Beppu Bay Formation are estimated numerically, and the relationships between them and the fault distribution observed in and around Beppu Bay are discussed. The stress field of the Beppu Bay Formation, representing a pull-apart basin formed by right-lateral motion of the Median Tectonic Line and the Kurume-Hiji Line in the Quaternary, is estimated by reproducing the fault motions on the relevant tectonic lines. The estimated stresses are then reexamined in terms of the tensile strength and Coulomb failure criterion to derive a tectonic interpretation of the fault distribution in and around Beppu Bay. It is shown that the Asamigawa Fault and the Tsuzura-Shonai Fault are secondary faults associated with the motions on the Median Tectonic Line, whereas the Beppu Bay cross fault is a secondary fault formed as a result of movement on the Kurume-Hiji Line. The rotation of the basement, formed by the right-lateral motion of the Median Tectonic Line and the Kurume-Hiji Line, is estimated and compared to the distribution of faults and folding in the sedimentary strata of Beppu Bay. It is shown, based on this comparison, that the rotational motion of the basement, or the right-lateral motion of the Median Tectonic Line and Kurume-Hiji Line as a source of rotational motion, appear to control the formation of tectonic structures in Beppu Bay. However, listric faulting and anticlinal folding around the eastern termination of the Kurume-Hiji Line cannot be explained in terms of the rotation of the basement alone, suggesting the involvement of another unrevealed process of faulting and folding in this area.
Many basaltic monogenetic volcanoes forming groups are distributed on Fukue Island, the largest island of the Goto Islands. The Onidake volcano group, 8×10 km across, is one of the monogenetic volcano groups in the island. It contains eleven monogenetic volcanoes, namely, Daienji, Koba, Nagate, Masuda, Shimosakiyama, Hinodake, Kamiozu, Shirodake, Sakiyamabana, Midake-Usudake, and Onidake, in chronological order. K-Ar and 14C dating results show that these volcanoes were active during 0.5-0.018 Ma. The average interval of monogenetic volcano-forming eruption is fifty thousand to thirty thousand years, while a long interval of about one hundred thousand years exists between 0.25 and 0.15 Ma. By long interval, the group is divided into older volcanoes, Daienji, Koba, Nagate, and Masuda, and younger volcanoes, Shimosakiyama, Hinodake, Kamiozu, Shirodake, Sakiyamabana, Midake-Usudake, and Onidake. At about 0.42 Ma, the activities of the older volcanoes started with Daienji volcano, which produced a lava flow in the north area. It was covered by thick lava flows of Koba volcano at about 0.38 Ma. During or shortly after the activity of Koba volcano, Nagate shield volcano with two small scoria cones became active in the northeastern area. At about 0.30 Ma, in the western area, Masuda volcano erupted to form a lava plateau and two scoria cones. After 0.15 Ma, in the central area, the intermittent activities of younger volcanoes began with lava flows from Shimosakiyama volcano, which was covered soon by Hinodake volcano composed of three lava flows and one scoria cone. Two small cinder cones with a lava flow, Shirodake and Kamiozu volcanoes, erupted on the northern part of Hinodake volcano. At about 0.03 Ma, at the southeastern end, Midake-Usudake volcano, erupted with three cindercone-forming scoria falls and five small plateau-forming lava flows covering an undated scoria cone, Sakiyamabana. At 0.018 Ma, the latest eruption of the group, Onidake volcano, commenced in the central area, the first phase of which was the Abunze lava flow originating from welded scoria falls or a mixture of lava and scoria fall into the lava ponds in craters. Simultaneously, the scoria falls generating Abunze lava flow contributed the formation of Osako scoria cone, which partially collapsed shortly afterwards, probably because Abunze lava carried the parts away. In the second phase, Onidake scoria falls covered a large area of 5×5 km, while Onidake scoria cone was formed in the proximal area. The third phase consisted of five lava flows, Ohma lava 1, 2, and 3 and Onidake lava 1 and 2, all of which erupted from two summit craters of Onidake cone and covered the northern and western foothills of the volcano.
Scientists have been trying to understand how Earth processes occur and influence each other, and are now looking at advanced, next-generation observation technologies. The third International Workshop on the Scientific Use of Submarine Cables and Related Technologies (SSC'03) was held last June at the University of Tokyo to review what has been achieved and to explore future observations technologies. Scientists and engineers started looking at opportunities to conduct observations that would cover the dynamics and structure of the Earth's interior, plate dynamics, natural resources, geohazardous events such as earthquakes and tsunamis, hydrogeological processes in sediment, heat and material exchanges or circulation through the ocean, dynamics of microbiological or ecological processes, deep water currents, physical oceanography, marine meteorology, and air-ocean interactions, etc., from the deep earth to its surface. Cross-disciplinary areas might develop depending on the scales of observations of spatial coverage. Long-term and real-time observatories would surely deepen our knowledge of these processes, about which much is still unknown. We believe that the workshop will lead to the initiation of new steps toward future sea floor observation technologies and scientific findings.