The basement rocks of this volcanic area consist of the so-called green tuff formation of Neogene age. These are overlain by thick welded rhyolitic tuffs of the latest Pliocene or earliest Pleistocene age. Covering these tuffs there are many kinds of basalts and andesites (pyroclastics and flows), which can be grouped into six volcanic groups such as Hachimantai, Yakeyama, Moriyoshi-yama, Iwate-san, Komagatake and Kayodake. From the lithologic characters both mineralogic and chemical, the six groups are divided into the northern type of the hypersthenic rock series (calc-alkali rock series) and the southern type of the pigeonitic rock series (tholeiitic rock series). The forty seven chemical analyses from the mentioned six volcanic groups show that the rocks are rich in CaO and poor in Na₂O and K₂O; this is the characteristics of the Japanese volcanic rocks. In the differentiation trend of the pigeonitic rock series in MgO-FeO+Fe₂O₃-Na₂O+K₂O triangular diagram, the enrichment of iron in the middle stage is noticed; whereas in the trend of the hypersthenic rock series the enrichment of Na₂O + K₂O is marked from the early to later stage without concentration of iron. These differentiation trends coincide well to that of the Izu and Hakone volcanic rocks and to the other districts of the Nasu volcanic zone.

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1) Nach einem Uberblick uber die fruheren Abteilungen der vulkanischen Zonen in Japan (Abb. 1 ist reprasentativ.) schlagt der Verfasser eine neue Abteilung (Abb. 2) vor. Er teilt Vulkane in Japan in Ostjapan Vulkanzone und Westjapan Vulkanzone ab. Es ist besser als die fruheren Abteilungen aus verschiedenen Grunden. 2) Im Laufe des Stadiums der magmatischen Differentierung (hier von Molekularproportion (Na₂O + K₂O)/Al₂O₃ ausdruckt ist), eine Steigerung des Kieselsauregehalts in Gewichtprozent erkannt wird (Abb. 4-6, 9). Der Verfasser stellt statistische Betrachtungen uber die obigen zwei Veranderlichen in Bezug auf die Vulkane in Japan an. a und b in SiO₂ = a + b (Na₂O + K₂O)/Al₂O₃ werden nach der Methode der kleinsten Quadrate gefunden. Darauf druckt der Vrefasser, wie viel tholeiitisch Basalt ist, von θ = SiO₂ - 47(Na₂O + K₂O)/Al₂O₃ aus. 3) Mann kann nach den θ-Werten zwischen den Vulkanen der einigen Magmenstamme (d.h. des stark tholeiitischen Stamms, des schwach tholeiitischen Stamms, und des alkalischen Stamms) unterscheiden. 4) Die geographische Verbreitung des Mittelwerts θ^-zeigt sich in Abb. 10・11. 5) Es stimmt mit der Ansicht 1) uberein. In §7 wird eine Anwendung der Formel auf den Fall der Vulkane in Indonesia besprochen. 6) Der Vergleich einer geographischen Verteilung des Werts θ^- mit derjenigen der Tiefenlage des Bebenherds (Abb. 11 gegen Abb. 14 in Japan; Abb. 12 gegen Abb. 13 in Indonesia) erscheint aufschlussreich. 7) Die Ahnlichkeit der beiden Verteilungen weisen auf eine kausale Verknupfung der Bildung des Magmas mit der Anhaufung der elastischen Verziehung hin.

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On the basis of seismometrical investigations, the writer studied the relations between the eruptions and the earthquakes from various volcanoes in Japan. Judging from reports with respect to volcanic eruptions and earthquakes originating from volcanoes, it seems that seismic activities preceding the eruption depend not only on the magnitude of eruption and the structure of volcano but also on the viscosity of lava at the time of eruption. This is rather natural because higher viscous lava will receive larger resistance in the course of its elevation from the magma chamber to the earth’s surface, and as a result, larger stress will be produced inside and under the volcano. In this paper, the writer reports in a condenced form a statistical relation concerning the frequency of earthquakes and explosive eruptions of Volcano Asama. The Asama earthquakes dealt with in the report are mainly those observed by comparatively low magnificatian (T = 1.0sec, V = 350) at the Asama Volcano Observatory which is situated at 4.2km E from the center of the summit crater. From the present investigation, it was made clear that most of the explosive eruptions were preceded by an increase in unfelt earthquakes. In addition, an experimental formula useful for predicting volcanic eruptions was given by the statistical relations based on the frequency of earthquakes originating from the volcano and the occurrence of explosive eruptions of Asama. In the forthcomming report (Part II), the same problem will be discussed on the basis of observations made by means of seismographs of higher magnification which were set nearer the summit crater.

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Firstly, the writers will give an outline of the seismographs and the method of seismometrical observations used at Volcano Asama over the past several years. For the recent seismometrical investigations of volcanoes in Japan, the writers used a series of telerecording seismographs. At Asama, transducers were set near the summit crater, on the mountainside and at the foot of the volcano and as these transducers were connected with the respective recorders in the Observatory by means of sealed wires, their records could be obtained at the Volcano Observatory. The earthquakes originating from Asama discussed in this paper are mainly those observed by the transducers which were set at 0.9km (No. I) and 2.5km (No. II) from the crater. These seismographs were adjusted to 4000 in magnification of displacement amplitude ranging from 0.3sec to 0.7sec of vibration period. It may be remarked that earthquakes originating from Asama are markedly similar to those (1956～1957) of Sakura-zima in the nature of earthquake motion and in the mode of seismic activity. Therefore, it may be said that the two volcanoes Asama and Sakura-zima are similar not only in the seismological features but also in chemical and physical properties of erupted lava, the position of eruption (i.e. at the summit crater), and in the nature of eruption.

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