丹那断層の過去3,000年間の活動履歴と単位変位量

Abstract

The amount of slip per event is one of the most important parameters establishing a recurrence model of large earthquakes. In Japan, previous works have usually exposed only two trench walls, and recognition of faulting events and estimation of holizontal slip have some uncertainty. This study tried to estimate accurate slip per event and identify faulting events precisely using threedimensional trenching and Geoslicer techniques. The Tashiro basin was selected as the study site along the Tanna fault which ruptured during the 1930 Northern Izu earthquake (Ms 7.3); the trench site is covered with continuous alluvial deposits and is expected to contain numerous piercing-points of offsets across the strike-slip fault.At the Tashiro site, the four earthquake events are recognized in the last 3, 000 years. The ages of the four events are: event 1: after AD 1442, event 2: AD 1610-1296, event 3: AD 1398-685, event 4: 1671-ca. 2800 y. B. P. This result is consistent to the one obtained at the 1982 Tanna-Myoga site. Event 1 can be correlated to the 1930 Northern Izu earthquake, and a lot of geological sections revealed right-stepping en echelon cracks associated with the 1930 earthquake at Tashiro site. Event 2 is correlated to a possible event estimated by previous works and event 2 may have ruptured the surface from Tashiro site to Osawa-ike site to the south. Event 3 is probably correlated with the 841 Izu-koku large earthquake according to historic records and previous paleoseismological works. The recurrence intervals of the last four events are estimated as follows: 500-630 years between events 1 and 2, 460-590 years between events 2 and 3, assuming the correlation of event 3 with the AD841 event, c. a. 500-1600 years between event 3 and 4. Average recurrence interval during the past 3000 years ranges from 650 to 750 years. This average recurrence interval is significantly shorter than that of previous work at Myoga site, 700 to 1000 years in the last 7, 000 years.Reconstruction of offset channel-fill gravel led us to idendify accurate 50cm left-lateral and 15 to 20cm vertical slip along the 1930 rupture. Another offset channel allowed us to estimate cumulative 60-80cm slip by the 1930 event and the penultimate event. Slips of the last two faulting events are estimated at 50cm and 10-30cm, respectively. Cumulative vertical slip associated with 4 faulting events is estimated at 1.7 to 1.8m. These data suggest that in the last 3, 000 years the Tanna fault has possibly not moved with characteristic slip during individual seismic events. Taking short reccurence intervals of the Tanna fault into consideration, recent activity of the Tanna fault may not support slip-predictable reccurence model, because intervals between events 1-2 and 2-3 are not proportional to the amount of slip of the following faulting events, events 1 and 2.