総合報告: 活断層からの長期地震予測の現状 -糸魚川-静岡構造線活断層系を例にして-

Abstract

A long-term prediction of earthquakes for the ltoigawa?Shizuoka Tectonic Line Active Fault System was made public in 1996 by the Earthquake Research Committee of the Headquarters of the Earthquake Research Promotion. The prediction is that an earthquake of magnitude about 8(M 71/2-81/2) will occur within a few hundreds years with high probability from the Itoigawa?Shizuoka Tectonic Line Active Fault System including the Gofukuji segment, though the extent of the seismogenic segment can not be specified.This statement was based on the following evaluation on the past activity of the fault system: the last faulting event was about 1200 yrs ago and the surface rupture zone at that time extended over 100 km from Hakuba to Kobuchizawa, and the slip was 6?9 m at Gofukuji and about 6 m at Chino. The recurrence interval of faulting at the Gofukuji segment was estimated at about 1000 years from ages of the last three events, which is consistent with the long-term slip rate, 8 mm/yr, of the segment.The magnitude of the next earthquake was estimated from the rupture length (about 100 km) and the amount of slip (6?9 m) in the last event and the time elapsed since the last event.The probability of the earthquake occurrence was estimated based on the step-model in which an active fault at a point or a segment moves with a characteristic displacement D and a definite recurrence interval R, as shown in relation of R=D/S, where S is the long-term slip rate on the fault. When R is close to time (t) elapsed since the last faulting, we think the probability of the occurrence is high.In this case, R is about 1000 years, and t is about 1200 years. So, the probability of the occurrence is expressed in the statement as high. A preliminary estimation, although it was not included in the statement, shows the probability of occurrence of the next earthquake is about 30% in coming 100 years, assuming the lognormal distribution of variability in recurrence time and standard deviation 0.3 in the renewal model.