A model is presented which explains the temporal relation between an eruption and a succeeding earthquake, taking a basaltic stratovolcano, Izu-Oshima volcano, as an example. In the model, volcano is assumed to consist of an underground reservoir and a long pipe connecting the reservoir to the surface. As the compressional crustal strain is gradually stored toward the earthquakes to occur, the volcano, located near the potential fault, is also deformed and contracts to some degree. Then the magma in the reservoir is squeezed up through the pipe. The rise of the magmatic head above a certain level in the pipe causes an eruption, which, once started, may proceed as a self-moving machine. Later, when the earthquake occurs, the strain that squeezed up the magma is released. And the head of the magma falls off resulting in the end of the eruption, in case it has still continued. The bottom of the summit crater of Oshima volcano showed remarkable rise and fall in this century amounting to some 400 meters. The bottom can be regarded as the head of the magma column, since red hot glow was frequently observed during the period. There were two maxima of the height of the bottom, January 1923 and June 1951. Shortly after each of the maxima, occurred great earthquakes with magnitude larger than 8, September 1923 and November 1953 along the Sagami trough which runs some 20km northeast of the volcano toward northwest, branching off from the Japan trench. The area including the volcano has been under compressional tectonic stress with the maximum pressure axis in a horizontal N30°W direction, during at least these hundreds of thousand years. On the other hand, recent fault-model studies of the 1923 earthquake indicate that the fault trace of the earthquake almost coincides with the Sagami trough and that the slip vector of the southwestern block, in which the volcano is located, is toward northwest almost horizontal with slight down going component. This tectonic situation implies that the strain which had been accumulated prior to the occurrence of the great earthquakes along the Sagami trough was caused by the same origin, probably the motion of the Philippine sea plate against the Japanese plate, with what has produced the compressive stress field of the volcanic area. The model appears to be successfully applied for the interpretation of the relation between the eruption of Akita-Komagatake volcano which started on September 17, 1970 and the October 16 earthquake with the magnitude of 6.2 at the epicentral distance of 55km. The frequency of explosion discontinuously dropped down to one half or lower level, three days after the earthquake together with the cessation of Strombolian type of eruption. The preliminary mechanism study of the earthquake showed that there is some component of thrust motion indicating the accumulation of contractional strain prior to the earthquake. The volcano to which the proposed model is applied is thus able to be regarded as a sensitive natural indicator of tectonic crustal strain, and also at the same time as being in a near critical condition ready to erupt.

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Three different views have been currently proposed for the role of an underthrusting oceanic slab played in the generation of parent magmas. 1. An interaction of the slab with the mantle under the arc gives enough heat to generate the magmas. 2. The slab provides also water to the mantle, so that the melting point may be lowered. 3. A part of the slab itself is molten and becomes the initial liquid for parent magmas. Any combinations of these three are possible. The author prefers to have the first view and substanciates his model here. Two important differences from the other opinions are pointed out. One point is that basaltic magmas may be parent. In this connection, the multiple correlation coefficients with the focal depths are compared between basaltic rocks and other types of rocks. The other point is that the partially molten zone may be certainly above the middle surface of the slab, based on a seismological evidence. It would mean that any considerable part of the slab should not be molten.

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Akita-Komagatake is a dissected strato-volcano, resting on the Tertiary basement. It has well-developed southeast and northwest slopes and a horseshoe-shaped caldera of Hawaiian type, 3×1.5km across, in which lie the central cones of Medake, Kodake and Otsuboike. Of these Medake is the largest and the recent activity occurred near its summit. The summa connecting Odake, Byobudake, Mitsuneyama and Yokodake, consists of lavas and pyroclastics, traversed by many dykes. To the north Katakurayama forms a parastic cone, which has a large explosion crater. Onamedake, the highest peak of Komagatake, lies within this explosion crater where lacustrine sediments are found associated with sulfur deposits. The volcano was built by alternation of lavas and pyroclastic materials of olivine pyroxene basalts and pyroxene andesites, with or without olivine phenocrysts. They belong to the tholeiitic series of types IVc, and Vc, and rarely IIc, representing a fine example of tholeiitic series among the volcanoes of Northeast Japan.

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Soon after the eruption of Akita-Komagatake Volcano had taken place in 1970, the geochemical survey was carried out with the aim at elucidating the present status of the hot springs, rivers and lakes in the vicinity of the volcano. The results of the investigation are summarized as follows : 1. Judging from the chemical constituents of the waters, hot springs in this region are classified into the following two groups ; Chloride ion type : Kunimi (1, 2) and Tsuru-no-yu (5, 6) Sulfate ion type : Mizusawa (3, 4), Ganiba (7), Magoroku (8), Kuro-yu (9), Okama (10) and Ipponmatsu (11) 2. The hot springs in the mid-slope of the volcano is characterized by the high content of dissolved minerals as compared with hot springs in Nyuto spa. 3. From the geochemical point of view, Okama hot sping is the active center of hot springs and fumaroles in Nyuto spa. 4. The quality of waters including hot springs, lakes and rivers in the volcanic region is not influenced by the present eruption.

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Aeromagnetic survey and gravimetric survey were performed over and around Akita-Komaga-take. Positive magnetic anomalies caused by each volcanic cone of Akita-Komaga-take, Eboshi-dake and Takakura-san were observed. Zarumori-san did not create an appreciable magnetic anomaly. Near Me-dake a small magnetic anomaly of negative sense was observed, and it surely concerned to the eruption of 1970. The existence of thermal material concerning this negative anomaly was estimated. From gravimetric data Akita-Komagatake is located in the central area of a positive anomaly which is related to irruptive rocks. In the western side of Akita-Komagatake weak negative magnetic anomaly was observed. It corresponds to negative gravity anomaly in northeastern side of Tazawa lake. These anomalies do not correspond to surface geology, so these may show influence of subterranean structure.

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A mixed eruption took place at Medake, biggest central cone of the composite volcano Akita-Komagatake, in 1970-71. This is the first magmatic eruption of the volcano in the historical age. All the former eruptions on record were phreatic explosions. The new lava is andesite, though older lavas of the central cones are basalts.

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The geothermal survey and the seismic observation were carried out immediately before the outburst of Akita-Komagatake. The only one forerunning phenomenon of the eruptions was the appearance of high ground temperature area with many fumaroles. Neither volcanic-tremor nor B-type earthquake was observed before the beginning of the eruptions.

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The writers intend here to sketch in broad outline the 1970-71 mixed eruption of Akita-Komagatake with special reference to explosion earthquakes. This is the first magmatic eruption of the volcano in the historical time. The present eruption took place near the summit of Medake, one of the central cones of the composite volcano, on September 18, 1970, after a long repose of 38 years, and continued without interruption until January 26, 1971. The total of the new volcanic products was ca. 4×10⁶ tons (lava : ca. 3.34×10⁶ tons, ejecta : ca. 0.67×10⁶ tons). The total kinetic and thermal energies estimated by solid products were ca. 1.6×10²⁰ ergs and ca. 5.0×10²² ergs, respectively.

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A number of lava flow and of volcanic bombs were thrown out from Akita Komagatake when it had been erupting from 1970 to 1971. The volcanic activity was appeared to be proceeded in two different ways and the variations in the dimensions of the lava flows, together with the heights of the cinder cones, was observed. The dimension of lava flow was 1300m² in September 19, 4300m² in October 14, 7000m² in November 18, 80000m² in December 12, 82000m² in January 20. The heights of cinder cone was 5m in September 23, 47m in November 3 and 58m in November 19. The top of the vent was 1035℃ yet in September 24.

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About 33, 000 eruption-type and 30 A-type earthquakes were observed during the whole activity. The analysis of the observed data shows that the eruption activity may be affected by the occurrence of an earthquake (M=6.2), which was located at distance of 60km from the volcano. The m value of the Ishimoto-Iida’s relation for eruption earthquakes is evaluated to be 2.5-2.9 by the three authors. Two authors reported that the relation between number and amplitude differed for large and small amplitude parts, whereas another author adopted a single relation for whole range of amplitude. The discrepancies may be due to the difference in instruments and procedure of analysis. The initial motion was always compression except for a few number of earthquakes. One author estimated the depth of hypocenter to be less than 50m, while another reported that the depth reached down to about 700m and the vent may be inclined to north of the crater. The partition of eruptive seismic energies may have changed with time.

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Eight seismometers (2 : 1Hz, 6 : 3Hz) were set up during two weeks on the western slope of the volcano. Two seismometers were used for monitoring by smoked paper recording and others were fed to an oscillograph or a data-recorder. Most of the recorded earthquakes were those associated with the eruption, the so called, explosion earthquake. The so called “B-type earthquake” was quite few. The slope of the amplitude-frequency relation of explosion earthquakes was found to be 2.5. The authors would like to stress that this value can not be compare directly with that of other volcanic earthquakes which are caused by the fracture of solid medium. Amplitude decrease of initial motion of explosion earthquakes is shown in Fig.6. Large decrease of amplitude of the initial motion implies that the focal depth being extremely shallow. Acoustic wave was recorded well on the seismogram of No.4 as shown in Fig.7. and Fig.10. Initial onset of these earthquakes is mostly poor which indicates relatively shallower origin than other explosion earthquakes as shown in Fig.9. The authors tried to find chronological relation between the “time of eruption” and the “origin time of explosion earthquake”. “Time of eruption” was accurately determined by an infrared radiation meter set up on the summit of the volcano. Simultaneous observation by both infrared and seismic methods had to give us the solution. Unfortunately, since we used two different crystal clocks for both observations, we were unable to make clear the problem. But the time difference of the both onset scatters to some extent. This result implies the difference in focal depth of explosion earthquake for respective eruption.

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Akita-Komagatake Volcano manifested a large number of explosions during the period September 1970 to January 1971, and about 32, 000 explosion earthquakes were recorded by the seismographs installed temporarily at 3 points at the foot of the volcano. In this paper, the mechanisms of the volcanic explosions are investigated by the use of the initial motions of the explosion earthquakes. Though the initial motions of almost all explosion earthquakes were compressional, about 0.6% of them were dilational. The distribution of push and pull of the initial motions at 3 observation points are classified into 6 types. On the basis of these 6 types, it is suggested that the mechanisms of the explosion origins belong to the “asymmetrical push cone type.” The explosion origins are distributed at the depths of 0 to 300m or more below the crater. The origins of the explosions of which initial motions are all push at the 3 points are shallow, while those of all pull ones are deep. Deep focus explosions occurred frequently at the early stage of the activities. The process of a volcanic explosion is surmised as follows : If an explosion earthquake occurs at a depth, a shock starts from there. The velocity of this shock is calculated to be equal to that of the sound wave in the liquid. This shock attacks the material which fill the upper part of the vent, and then a destructive outburst occurs at the crater.

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A portable infrared radiation meter was successfully used to observe the eruption of Akita-komaga-take. Sharp onset of the records yielded the accurate time of the respective eruption. We could classify the type of the eruption based on the records. They are α, β, γ, and δ as shown in Fig.3. Before the eruption, swelling of magma head at the vent was clearly recorded as shown in Fig.5. Simultaneous seismic observation showed that an explosion earthquake is composed of several earthquakes which were caused by the succeeding eruptions. Most of the eruption were accompanied by explosion earthquakes, however, the eruption of type δ is not associated with explosion earthquake as can be seen in Fig.8.

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Rauch-ring was observed, probably for the first time in Japan, during the 1970 eruption of Mt. Komagatake in Akita Prefecture. The explosion accompanied by the ring was registered by an 8mm movie and a tape recorder installed around the crater. The explosion column was white and rose vertically up to some fifty meters from the crater rim. At this height the ring was released from the column, and was about seven meters in diameter with the average ascending velocity of about six meters per second. In order to investigate the mechanism of this extremely rare phenomenon of “Rauch-ring” and the explosion itself, analyses of the distribution of the explosion energy and others, are made on the explosion sound recorded on the tape, and the seismogram observed simultaneously by chance. In addition, an explanation is given concerning the mechanism of the whole volcanic activity of Akita-Komagatake by analyzing the seismological observation.

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The new lava erupted in the 1970 activity of Akita-Komagatake comprises volcanic bombs, volcanic blocks and lava flows of hypersthene-augite andesite (V_d→_c type). The mode comprises phenocrysts of plagioclase, augite, hypersthene and magnetite in a groundmass of plagioclase, clinopyroxene, hypersthene, magnetite and brown glass. Rarely anhydrite-bearing xenoliths, consisting of tridymite, anhydrite, augite and glass, are found. These may have been formed by pyrometasomatism of basement rock rich in opaline silica. Petrochemical features of the new lava are discussed by means of various diagrams. It noted that K₂O is very low, though total alkalis are nearly the same as in the older lavas. The new lava plots near the boundary line between the tholeiitic series and calc-alkalic rock series, whereas the older ones fall always in the central part of the tholeiitic series in the MgO-(FeO+Fe₂O₃)-(Na₂O+K₂O) diagram. When plotted in the (FeO+Fe₂O₃)/(FeO+Fe₂O₃+MgO)-SiO₂ diagram, the new lava shows a trend different from that of the older ones. This change may have been brought about by increase of pO₂ as the result of increasing water content in the magma reservoir. If this trend goes further, a pyroxene andesite of Vd type with about 60% SiO₂ might be erupted by this volcano in the future.

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A large number of chemical analyses of the historic and prehistoric lavas are now available. New ejectas (Table 2) and lava flow (Table 3) are very homogeneous in chemical composition as shown by analyses from different laboratories. Compositions of mechanically separated groundmass (Table 4) and glass composing major portion of the groundmass (Table 5, by EPXMA) are also soown. Variation diagram (Fig. 1) indicates that the 1970 magma is more felsic than the prehistoric lavas of the volcano, including Medake from the top of which the 1970 magma was erupted. The variation trends (Figs. 1 and 2) indicate that they are in agreement with the petrography of the rocks that prehistoric and new lavas belong to the tholeiitic (pigeonitic) rock series of KUNO with the 1970 lavas falling very close to the boundary between the tholeiitic and calcalkali (hypersthenic) rock series. Very marked concentration of iron is apparent in the groundmass glass which is quite off the general trend of the Japanese volcanic rocks. Enrichment of Na with respect to K is also pronounced. Plagioclase ranges its size from 2mm down to less than several microns, making the distinction between phenocryst and groundmass rather arbitrary. Few phenocrysts have cores as calcic as An₉₀. Range in composition is generally from An₇₀ to An₅₀, with total Fe as FeO as high as 1.0%. Augite and hypersthene (Table 8 and Fig. 5) occur only as phenocrysts and microphenocrysts. They appear to have ceased crystallizing in the groundmass stage. Ca-poor clinopyroxenes (pigeonite and subcalcic augite) crystallized in the groundmass stage with Wo content from 7 to 23 mol%. There is no well-defined trend in the pyroxene quadrilateral and the Al content varies very mucy haphazardly. These may indicate that Ca-poor pyroxenes crystallized metastably from the magma cooling rather rapidly. Opaque minerals are titanomagnetite (Table 9) with 41-46% ulvospinel molecule. Groundmass magnetite is more Ti-rich than the phenocryst. Xenoliths composed mainly of tridymite with anhydrite and clinopyroxene are found in the 1970 lavas.

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High-temperature quenching experiment was carried out by T. KATSURA in order to determine the liquidus relations of the 1970 magma. The separated groundmass and artificial mixture of the glass composition were heated in the resistance furnace in the controlled atmosphere of CO₂ and H₂. The liquidus temperature of magnetite (Fig. 7) is sensitive to fo₂. In the groundmass composition, plagioclase appears at temperatures 50 to 70℃ higher than that at which pyroxenes appear. In the glass composition, plagioclase and pyroxenes appear at about the same temperature (1100℃), very close to the maximum temperature (1090℃) observed during the eruption. It may be argued that the magma just before the eruption and whose composition is represented by the composition of glass had adjusted its composition to low total pressure and water pressure state to crystallize out plagioclase and pyroxene (and magnetite) concomitantly. On the other hand, the magma represented by the composition of the whole groundmass may have been in equilibrium with crystals under some appreciable water pressure way down in the volcanic conduit. It is suggested that these relations may be used as an effective indicator of water pressure in the magma.

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The content of the volcanic gases were measured for the period of activity of Akita Komagatake from 1970 to 1971. The results are as follows : CO₂ 300～930, SO₂ 0.4～2.5, HCl 8.9～66, HF 3.6～9.4, NH₃ 4.0～26ml/1000l. Presence of Halite (NaCl), Fluorite (CaF₂), and Anhydrite (CaSO₄) were identified in the volcanic sublimates. Relation among the volcanie gases, sublimates, and volatile matters in ejecta was investigated. The origin of volatile matter contained in the magma thrown out during the eruption was also discussed.

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