SUWA BASIN, NEAR KYOTO, JAPAN 63 
In the vicinity of observation station No. 14 the presence of an 
active fault is being considered because of the subsidence of the rail- 
road track and because of the geological features. If the active fault 
is slipping westward, an explanation of the discrepancy between the 
calculated and the corrected observation values can be made. 
If the depth and the dip of the bed rock are assumed, as shown in 
Figure 3, the calculated results satisfactorily coincide with the ob- 
served values. The depth of the bed rock increases as the center of 
the lake is approached. 
CONCLUSION 
The maximum computed depth to bed rock is 450 meters; there- 
fore, the basin can not be considered as formed by weathering only, 
when located in Japan. Its origin should be considered as from sinking 
or from volcanic eruption. 
The isogams of Figure II lead us to believe that Lake Suwa is a 
crater lake. This theory is rejected, however, as unreasonable from 
the geologic standpoint. Therefore, the origin of the lake should be 
regarded as due to a sinking or subsidence following the eruption of 
the Yatsugadake Mountain, near by. If the origin is sinking, the 
occurrence of a fault at the margin of the basin is a matter of course. 
The volcanic lava and debris from the Yatsugadake Mountain were 
carried down into the sunken zone and deposited in the basin, form- 
ing an alluvial fan. The thickness of the deposition is much greater 
on the southern side than on the northern side of the lake. 
The gravity field of the basin is negative, while that of the moun- 
tains which surround the basin is positive. Hence, from the standpoint 
of isostasy, the deficit of the subterranean mass is clear, and shows 
that the basin is a result of faulting or subsidence. 
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