In the Kanto earthquake in 1923 (M=7.9), the land was upheaved nearly 2 meters high at some places along two coasts, the northern coast of the Bay of Sagami and the southern coast of Boso Peninsula, where there are coastal terraces. These Holocene terraces are 20-25 meters above sea level.
Concerning these terraces, A. Sugimura and Y. Naruse (1954, 55) have already pointed out two important notions. (1) On the raised beaches of Holocene terraces along the southern coasts in the Kanto district, the vertical displacement of land is distinctively correlative with vertical changes of the triangulation point of the Kanto earthquake. (2) From the maximum rise of the sea level in the early Holocene (about 4000 B. C.) to the present, the sea level have continued a eustatic fall and the total fall is estimated at about 6 meters.
In the present paper, the authors attempt to determine the time of the formation of these terraces and we can estimate the reasonable age by on-the-spot-investigations and radiocarbon dating methods. According to the results of our field survey at the northern coast of the Bay of Sagami, these terraces are composed of marine sediments, which are generally more than 20 meters thick. Later we found shells among the sediment and at the laboratories of the University of Tokyo and Gakushuin University, radiocarbon dating of these shells was taken place.
The results are as follows, adding the other data from the Numa terraces in the southern part of Boso Peninsula.
From these data and data from the field survey, we conclude our analysis about the formation of these terraces with these summary statements;
(1) These terraces were formed in the early Holocene, that is 7000-5000 years B. P. (2) If the amount of a eustatic fall in sea level is estimated at about 6 meters, the total land upheaval reached about 20 meters in the past 6000 years. (3) The coasts along the Bay of Sagami are characterized by high tectonic activities in the recent geological time.