Abstract
Many thermal springs are known in Okiura district along the Aseishigawa River, a tributary of the Iwaki-gawa River which flows down the Tsugaru Plain, Aomori Prefecture. In this district, three faults F1-F3 of N-S direction and one fault F4 of NE-SW direction are recognized geologically. The western part of the F2 fault consist of the Okiura basalt member and the Itadome Formation both of Miocene age, and the eastern part of the fault consist of the Aoni Formation of Pliocene age; these rocks and terrace deposits are covered by volcanic ash. In a block between the F2 fault and the F3 fault, many naturally issuing thermal springs of high temper-ature are scattered, and high ground temperature is obtained by shallow drilling. Ground temperature is comparatively low at eastern part of the F2 fault and western part of the F3 fault, especially at the latter. The F3 fault is a distinct boundary of chemical composition of thermal waters; the waters in eastern part of the fault belong to Cl>SO4 type and dissolve 0.95-1.82 g/kg of substances, but the waters in western part of the fault belong to SO4>HCO3 type and dissolve 0.25-0.35 g/kg of substances. The F3 fault is considered to form a barrier of flowing of thermal water. It is assumed that the block between the F2 fault and the F3 fault is a pass zone of thermal water and especially the F2 fault is an excellent pass, then relatively high temperature and slightly high thermal water level are obtained by drilling near the F2 fault.
