岐阜県宮川村野首における跡津川断層のトレンチ発掘調査

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The Atotsugawa fault extends 60km or more in the northern Hida Mountains of central Japan, linearly running from ENE to WSW, and forms a master active fault system with predominantly right-lateral component of displacement. A historical large earthquake (1858, M≅7.0) occurred in this region, and many small to micro-earthquakes are taking place along this fault system up the present. A deep trench was excavated across the central part of the Atotsugawa fault where the 5m-high scarplet disturbed the lowest river terrace along the Miya (-gawa) River. The size approximately N-S trending trench was 13m deep in maximum and 22m long across the scarplet (Fig. 1B & 5), and an additional small excavation was made down to clarify the last event at the west side of the main trench, exactly at the southern foot of the scarplet. Several fault planes were exposed on the four trench walls, showing recent movements of the fault which separates the granitic rocks on the hanging-wall side (north) and the younger sediments on the foot wall side. The strike of the main fault is about N70° E and the angle dips 65° N at the surface and 75° N in the trench bottom, increasing the fault angle toward the deeper part. The main results from this survey are summarized as follows : 1) The accumulated vertical offset forming the scarplet is so large at this site that we were not able to correlate the each layers, and sedimentary environment of each side has been quite different due to the displacement, except the basal terrace gravel across the fault. The gravel in the down-thrown side is 2m thicker than that in the up-thrown side. Intrapolating the average vertical slip, at least two events were presumed in the basal gravel. 2) We recognized the “structure D” within the deposits along the main fault disturbed zone, and interpreted that this structure was created by slumping of slope materials from the fault scarplet (see Fig. 13). This phenomenon indicates the sudden movement (earthquake event) of the fault. Dated four events are <820y. B. P., 5200± 200y B. P., 7500±800y. B. P., and 8600±00y. B. P. The latest event (No. 1) was considered to be the 1858 Hietsu Earthquake, as the uppermost humic soils (<820y. B. P.) is thrust up by granitic rocks of the hanging-wall. And, this is also confirmed by existence of a low scarplet at Hayashi (Locality a in Fig. 2), which suggests probably historical fault topography formed within the settlement. 3) Through detailed observation of the trench walls, we found more than 10 events after the formation of the basal gravel which approximate age was estimated to be ca. 12000-13000. However, events of sand deposits at the lower part are not so reliable because it is not easy to detect the key structure in the soft sediments. 4) The average recurrence interval of the main Atotsugawa fault for last 4 events is about 2800years. However, the recurrence interval scatters from 1100 to 5100 years. It is assumed that some events might be missed due to artificial modifications of the fault scarplet and the superficial deposits. In this case, the recurrence interval becomes shorter and the sporadicalness smoother.

岐阜県宮川村野首における跡津川断層のトレンチ発掘調査

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