Long-term characteristics of geological conditions in Japan. Pt. 3. Characterization of time-space distribution of volcanoes in the northeast Japan arc

It is very important to evaluate the long-term stability of geological conditions such as volcanism, uplift-subsidence, earthquakes, faulting and sea level change when the long-term safety performance of HLW geological disposal is investigated. In particular volcanism is one of the most important items among them because of serious damage to the disposal facilities caused by magmatic intrusion or effusion. For the purpose of obtaining basic information to construct the logic for prediction of volcanic activities in the future, we investigated the time-space distribution of volcanic centers by using the literature on the igneous rocks distributed in the Northeast Japan arc since 14 Ma when the back-arc opening had almost ceased. Following results were obtained. (1) Volcanic front (VF) (defined as eastern border of the volcanic field in each period) has shifted 20-30 km westward to the present VF since 14 Ma. (2) The distribution of volcanoes in each period (in every three million years) is similarly characterized by the zone composed of 'trunk' on the VF side (which has N-S trend) and 'branch' on the back-arc side (which extends westward from 'trunk' on the VF side). (3) Volcanic centers had been distributed scatteringly within the volcanic field during the period between 14 to 8 Ma. Since 8 Ma those had been focusing to several clustering areas, and since 2 Ma these phenomena have been more conspicuous. (author)