四国の中央構造線断層帯の最新活動に伴う横ずれ変位量分布

抄録

Recent paleoseismological studies revealed that the entire length of the right-lateral Median Tectonic Line active fault zone in Shikoku ruptured after the 16th century A.D. Based on aerial photograph interpretation and field observations, we identified geomorphic and artificial features offset along the fault zone that record the amount of right-lateral slip associated with the most recent and previous earthquakes. The amount of offset during the most recent earthquake varies considerably among the active faults that comprise the fault zone: about 7m along the Chichio and Ikeda faults in eastern Shikoku and 2-3m along the Shigenobu and Iyo faults in western Shikoku. In general, surface slip associated with the latest event is greater than 5m between the Zunden and Okamura faults, and decreases gradually to the east and west. There is a correlation between horizontal slip rates of the individual faults and surface slip during the most recent event. This is consistent with the paleoseismic observation that the recurrence intervals of the individual faults do not vary considerably along the fault zone and are in a range of 1000-2000 years. Based on the surface offsets and segmentation model by Goto and Nakata (2000b), we are able to calculate moment magnitudes for scenario earthquakes from the fault zone. Separate ruptures of the western Shikoku, Okamura, and eastern and east-central Shikoku segments would produce earthquakes with magnitudes of M_w7.3-7.4, M_w7.2-7.4, and M_w7.7-7.8, respectively. These ruptures would be as large as or greater than the largest recorded historical earthquake, the 1891 M_w7.4 Nobi earthquake.