Results of the measurements of magnetic properties of dacitic rocks collected from a lava dome, Mt. Minakami (Matsushiro, Nagano prefecture) are presented. The results suggest that the geomagnetic anomalies covering Mt. Minakami and its southwest area are caused mainly by stable remanent magnetization of ferromagnetic minerals with very fine grain sizes in the rocks. In an attempt to elucidate the geomagnetic field variations which might be accompanied by the earthquakes in the anomalous area, the pressure effects on the remanent magnetization of rock samples are examined. Irreversible decrease in the remanent magnetization of rocks is within 3 %/kb up to 5 kb of compressional stress.

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On the occasion of Matsushiro earthquake swarm, anomalous variations of the intensity of the geomagnetic field, ca. 10 gammas were observed in relation to occurrence of earthquakes with moderate magnitude. The authors conclude that such geomagnetic field variations may be explained on the basis of the changes of the magnetization of rock owing to changes in stress in the earth’s crust. On the basis of the experimental results on piezomagnetism and rock mechanics of volcanic rocks including the dacite lava of Mt. Minakami in Matsushiro, it is shown that release of 0.1 kb compressional stress in rock within the earthquake volume can cause the observed anomalous changes in the geomagnetic field, as it can be concluded from aeromagnetic survey that the magnetization of the earthquake volume is of the order of 10^-3 emu/cc.

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A sudden decrease in Rn and Tn contents of the hot spring gas from the Bozu-zigoku hot spring pond, Oowakudani, in 1959 was reported by Kamada (1961). He further discussed both the influence of the volcanic earthquakes which happened in the Hakone region at that time and the mechanism of supply of these radioactive components. The Rn and Tn contents of the gas from Bozu-zigoku have been continually measured by the present authors since 1961. A gradual increase in Rn content from 1961 to 1965 and a sudden decrease was observed in 1966, when the earthquakes occurred again. The variations of the main components have not shown any systematic relations to those of Rn and Tn contents. Tn/Rn ratio was almost constant during these years. Rn content of the gases from drilling holes in Oowakuzawa, few hundreds meters apart from Bozu-zigoku, was found to be rather low. These new data enable us to reexamine and to improve the Kamada’s theory on the mechanism of Rn feeding to the volcanic gases. Rn and Tn in the gas seem to be brought out from the source near the ground surface. The gradual increase in these radioactive components from 1961 to 1965 can be explained by any one of the following two mechanisms : (1) The enrichment of Ra and Th by the adsorption on clay minerals from the hot spring waters ; or (2) the increase in the contact area, which is effective to feed Rn and Tn to the gas, due to the alteration. The decreases observed in 1959 and 1966 were concerned with repeated shocks of the earthquakes at those times and can be explained by either one of the two mechanism : (1) Closing of the minute cracks where Ra and Th are enriched, the extraction of Rn and Tn to the hot spring gases and waters is hindered ; or (2) the decrease in the contact area of the source with gas and water. An approximate size of the source volume was also estimated.

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Hakusan Volcano, situated about 50 km south of Kanazawa, central Honshu, is composed of two stratified volcanic cones of different ages, Ko-Hakusan (Old Hakusan) and Shin-Hakusan (New Hakusan), and its volcanic history is divided into the following periods : Ko-Hakusan Period Ko-Hakusan was formed probably in early Pleistocene through repeated eruptions of lava and pyroclastic materials mainly of pyroxene hornblende andesite. The total volume of materials erupted is estimated to be 15 km³. Pyroclastic flow eruptions took place in early stages of the period. The volcanic body has been deeply dissected, and the central portion has been largely removed by erosion. Shin-Hakusan Period After a period of dormancy, volcanism recurred in late Pleistocene, and Shin-Hakusan was formed on the south slope of Ko-Hakusan. Two stages are distinguished in this period. The bulk of the cone was formed during Gozen stage. It was followed by Midoriga-ike stage, during which the center of activity moved a little to the north, and a group of small craters were formed by explosive eruptions including those of historic ages. The materials erupted in this period, being about 1 km³ by volume, are composed mainly of pyroxene hornblende andesite. The basement, on which Hakusan was formed, had been arched possibly by a deep-seated intrusion of magma, and the center of volcanism was located at the top of the swell thus formed. The surface of the basement near the center is more than 1000m higher than in the neighbouring areas of Hakusan. A group of strike-slip faults pass through the Hakusan area from northwest to southeast, and the basement rocks were sinistrally dislocated and densely fractured along them. It is conceivable that the magma conduit of Hakusan was formed in this zone of fracturing.

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