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 F
1-F
3 of N-S direction and one fault F
4 of NE-SW direction are recognized geologically. The western part of the F
2 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 F
2 fault and the F
3 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 F
2 fault and western part of the F
3 fault, especially at the latter. The F
3 fault is a distinct boundary of chemical composition of thermal waters; the waters in eastern part of the fault belong to Cl>SO
4 type and dissolve 0.95-1.82 g/kg of substances, but the waters in western part of the fault belong to SO
4>HCO
3 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 F
2 fault and the F
3 fault is a pass zone of thermal water and especially the F
2 fault is an excellent pass, then relatively high temperature and slightly high thermal water level are obtained by drilling near the F
2 fault.
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