Abstract
The seismic method is the best way to recognize clearly the under ground structures of alluvial plains. This report shows the results of our investigations on the three alluvial plains as follows:
1. Ofunato plain: This plain is the small delta that was construct ed in the drowned valley. We set 4 lines of measurement as shown in Fig. 1. Fig. 2-3 are the results of this investigation along the lines. The layers, in which the velocity of the longitudinal wave are 5.0 & 5.7km/sec, are palaeozoic and 6.0 & 4.2km/sec are granit. The surface of these rocks is the fossil valley before it drowned. The layer of 2.4 km/sec is the diluvial gravel bed, and the others are alluvial. Under the delta, several faults were discovered.
2. Joganji plain: This is the alluvial fan which was made by River Joganji. Fig. 4-11 are the results. The layers of 2.8, 3.0 and a part of the 2.6km/sec are Tertiary, and the others are the gravel beds. At the middle of the fan, it became clear that a buried hill exsists under the gravel beds, and the ancient river course ran more west than now. E line was set at near the coast, and the altitude of the surface of the bed rock on the east end of the line was 450m high. The old river would run more east and deeper than this point.
3. Upper part of Nobi plain: This part of the plain is the fan made by River Kiso. The positions of the lines and results of the in-vestigation are shown in Fig. 12-14. The layers of 4.5km/sec is ther pre-tertiary rock. The others are all gravel beds from Tertiary to, alluvial. We can read the geological history of this area from these-geological sections.
