著者
中村 一明
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.75, no.2, pp.93-104, 1966-04-25 (Released:2009-11-12)
参考文献数
14

Taal Volcano, situated at SW part of Luzon Island, Philippines, erupted during 28th to 30th, September 1965 after fifty-four years of quiescence.As an example of a magmatophreatic eruption of basaltic volcano, this paper deals with the course and the ejecta of the eruption with special reference to the horizontal blasts, based both on a paper by MOORE et al. and on the writers field observation. Topography of the island and historical records of eruption are examined from the same viewpoint by literature survey.Taal Volcano is an island in Taal Lake and is 25 km2 in area with a summit crater 2 km in diameter. The volcano island is composed almost of pyroclastic materials which are the product of past magmatophreatic eruptions. Sixteen craters are found on the slope of the main flat cone and they are wide in proportion to their height suggesting their origin to be explosive eruptions.No historical record describes quiet effusion of lava flows, but it indicates explosive nature of eruption.Essential materials responsible for the present eruption was titanaugie-olivine-basalt. The eruption continued for about 60 hours and no lava flow issued. The area of about 60 km2 was covered by new pyroclastic ejecta more than 25 cm thick. New elongate explosion crater was opened by the present fissure explosion on the southwestern slope of the main cone.From the base of explosively rising eruption clouds, horizontal blasts spread out in all directions. The blast is turbulent mixture of mud, lapilli, blocks and crept on the surface of the ground with hurricane velocity and near the crater it spilled over a ridge of 300 m in relative height (summit crater rim). But it tended to spread on lower places as it proceeded further away from the crater. Parts of the blast crossed the lake surface over 2 km and caused much damage to the villages at opposite lakeside. The blasts left giant ripples on the surface of the deposits within 2 km from the crater. The wave-length of the ripples decreases regularly from 15 m to 3 m as the distance from the crater increases.The blasts are regarded to be a low temperature pyroclastic flows. The mobility of the flow is thought to be maintained only by the initial formation of a fluidized system. Because, no delayed vesiculation is expected since the essential material is basaltic in composition, and also because the internal turbulence caused by envelopment of cold air is not great enough, since the temperature of the flow was only about 100°C or below. Magmatophreatic explosion through layers of pyroclastics seems to have been a favourable condition for the initial formation of the fluidized system.
著者
中村 一明
出版者
特定非営利活動法人 日本火山学会
雑誌
火山.第2集 (ISSN:24330590)
巻号頁・発行日
vol.25, no.4, pp.255-269, 1980-12-01 (Released:2018-01-15)

Rift zones are characteristic features of Hawaiian volcanoes. They are long narrow zones of flank fissure eruptions but are distinct from ordinary flank eruption sites on stratovolcanoes in that eruptions, and therefore dike intrusions, occur repeatedly at the same general place for a long time and thus cause a considerable amount of lateral spreading. This spreading should somehow be accomodated. Moreover, the stress field should remain the same after accomodation in order for a new dike to intrude in the same orientation. The current spreading episode in Iceland (BJORNSSON et al., 1979) between North American and European plates revealed that the sequence of events in the spreading process is similar to that observed for Hawaiian volcanic activities. This implies that the process of plate separation and accretion is nothing but the activity of rift zones. Constructional plate boundaries may be regarded as composed of a chain of rift zones and associated feeding polygenetic centers. Room necessary for repeated dike intrusion is supplied in the case of spreading centers, by the lateral motion (separation) of lithosphere over asthenosphere. In the case of Hawaii, sliding of the volcanic edifice over a deep sea sediment layer may be the analogous mechanism such as appears to have occurred during the 1975 Kalapana earthquake, as studied by ANDO (1979) and FURUMOTO and KOVACH (1979). Kalapana earthquake had been anticipated by SWANSON et al. (1976) as one of the repeated steps as the east rift zone has continuously dilated. Thus, the primary cause for the long, well developed rift zones of Hawaiian volcanoes may be in the existence of thick enough oceanic sediments serving as a potential sliding plane beneath the volcanic edifices. Lack of rift zones in Galapagos shields which grew over the young ocean floor with rough topography is consistent with this view.
著者
中村 一明
出版者
特定非営利活動法人日本火山学会
雑誌
火山. 第2集 (ISSN:04534360)
巻号頁・発行日
vol.20, pp.229-240, 1975-12-25
被引用文献数
14

Volcanoes are generally classified into monogenetic and polygenetic types. Monogenetic volcanoes erupt only once to form smaller volcanoes, such as maars, pyroclastic cones and lava domes. Polygenetic volcanoes erupt repeatedly from the same general vents (summit or main crater) for up to 10^5 years to form larger volcanoes such as strato-volcanoes (composite volcanoes of Macdonald, 1972) and shield volcanoes of Hawaiian type. Monogenetic volcanoes tend to occur in clusters as flank and post-caldera cones. Some of the clusters are however, independent of polygenetic volcanoes and appear to be equivalent to them. The essential part of the conduit of a monogenetic volcano is inferred to be a simple dike, intruded into a newly formed crack, whereas a long endured pipe-shaped conduit may exist under a polygenetic volcano. The common occurrence of xenoliths in the eruptive products of monogenetic volcanoes may be related to this difference. Various lines of evidence, indicating the existence, depth, shape, volume and internal structure, of magma reservoirs are tabulated. A shallow magma reservoir appears to exist beneath polygenetic volcanoes with one to one correspondence, which is not the case for monogenetic volcanoes. Most flank volcanoes are monogenetic, thus indicating dikes within the polygenetic volcanic edifice. Dike formation is understood as a magma version of hydraulic fracturing. For the dike to intrude and propagate, would require either the increase of differential stress due to a decrease of minimum compression or increase of pore pressure over the sum of the minimum compression and the tensile strength of the rocks. Earthquakes are understood as the generation of elastic waves associated with an acute release of tectonic stress due to faulting. Accumulation of tectonic stress and strain prior to earthquakes is, then, a necessary part of earthquake phenomena in a broad sense, as well as their release after the event. Based on the above-stated understanding, possible mechanical correlations between volcanic eruptions and earthquake occurrences have been studied. Contractional strain around the magma reservoir can cause the squeezing up of magma within an open conduit causing a summit eruption on the one hand, and dike formation resulting in a flank eruption through the increase of pore pressure, on the other. Second boiling triggered by both the magmatic pressure decrease caused by dilatational strain and the dynamic excitation due to seismic waves might have the same effect as contraction. Decrease of minimum compression causing the increase of differential stress leading to dike formation will also contribute to the liklihood of flank eruptions. Both volcanic eruptions and earthquake occurrences can precede each other depending on geographical location in terms of faulting-related stress-strain changes which are calculated by the fault model of earthquakes. Actual possible examples of volcanic eruptions and earthquakes which are allegedly mechanically related are given. In order to demonstrate which mechanism is responsible for the correlation of the two phenomena, continuous strain measurement on and around volcanoes is necessary together with the observation of changes in the level of magma in crater bottoms.
著者
中村 一明
出版者
東京大学地震研究所
雑誌
東京大学地震研究所彙報 (ISSN:00408972)
巻号頁・発行日
vol.58, no.3, pp.711-722, 1984-01-14

富山トラフ以東の日本海東縁の大陸斜面と陸上の瑞穂摺曲帯よりなる日本海東縁変動帯は北米・ユーラシア両プレート間の1~2Ma前以降の収束(力学)境界域であるという考えが説明される.日本海盆東縁と富山トラフ内に点在する凹地を連ねた地帯は沈み込みを示唆する構造を伴うので新生の海溝である可能性がある.
著者
中村 一明
出版者
特定非営利活動法人日本火山学会
雑誌
火山. 第2集 (ISSN:04534360)
巻号頁・発行日
vol.16, no.2, pp.63-71, 1971-12-01
被引用文献数
1 11

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.
著者
中村 一明
出版者
特定非営利活動法人日本火山学会
雑誌
火山. 第2集 (ISSN:04534360)
巻号頁・発行日
vol.14, no.1, pp.8-20, 1969-04-01
被引用文献数
2

Examples are presented in which arrangement of lateral and post-caldera cones indicates the probable regional stress field in the late Quaternary. Izu-Oshima, Hakone and Fuji are among the larger stratovolcanoes to the southwest of Tokyo, the former two having collapse calderas on their summit. Parasitic and post-caldera cones and craters of the three volcanoes are following a trend of similar direction, some of them being produced by fissure eruptions on their flank. The zones trend in the direction of about N 35° W at Fuji, N 45° W at Hakone and N 30° W at Oshima. Dikes are also found in these zones running mostly parallel to them. The trend of fissures of fissure eruptions on the flanks of Fuji and Oshima is also similar to the zones of recent activity. Because sites of flank eruption are regarded as points where radially formed dikes around the central magma column have penetrated the flank, the above described distribution of craters in these volcanoes would indicate a concentration of radial dikes in a specific direction at the three adjacent volcanoes. Considering that dikes are fossil tension cracks formed perpendicularly to the axis of the minimum principal compressional stress, the concentration of parasitic craters can be explained by the stress field caused by the pressure increase in the magma column superimposed on the preexisting regional field with the maximum principal stress axis in a NW direction. The nearly identical, preexisting stress field in the three adjacent volcanoes suggests that the field is part of a more regional one including the area of the volcanoes. This suggestion is strongly supported by the presence of active, conjugate strike-slip faults in the same general area, i.e. on the Izu-peninsula. The maximum compressonal axis indicated by the faults is again in a direction of about N 30° W and oriented horizontally. The last movement of the fault system was observed in 1930, at the time of the Kita-Izu earthquake, whose magnitude was 7.0.
著者
中村一明
雑誌
東大地震研究所彙報
巻号頁・発行日
vol.58, pp.711-722, 1983
被引用文献数
2
著者
松田 時彦 中村 一明
出版者
The Society of Resource Geology
雑誌
鉱山地質 (ISSN:00265209)
巻号頁・発行日
vol.20, no.99, pp.29-42, 1970-03-02 (Released:2009-06-12)
参考文献数
56

A genetical classification of volcanic clastic deposits is proposed in this paper, which is based on the following four principal criteria:1) place of the eruption by which the material was broughtinto the transporting media : W (subaqueous) or A(subaerial), or O(when it was unrelated directly with the eruption).2) kind of media in which the material was transported to the depositional place : W (water) or A(air) or O(absence of media; this is used for some lava flows and dense landslide deposits).3) place of deposition : W(subaqueous) or A(subaerial).4) mechanism of transportation and settling : F(fall) or R(roll) or T(turbulent flow).Any volcanic clastic deposits might be designated as WWW-T, AAW-F and so on, by putting the result according to the four criteria in a descending order. For lava flows(L), the same principle of classification is applicable, e. g., as WOW-L (or WWW-RL, when it is necessary to indicate rolling(R) mechanism of the emplacement in W media). By the use of O in the first term, this classification also applies to normal clastic sediments, e.g. OWW-T for turbidity current deposits.Discriminating features for each kind of water-laid volcanic clastic deposits are described and several examples are presented.The proposed classification concerns mainly with a sedimentary body as a whole, and not with. descriptive features of particles. Parallel usage of the classic descriptive terminology with the proposed one is recommended, as AAW-F tuff, WWW-T tuff breccia, or tuff breccia (WWW-T), etc.,
著者
中村 一明 笠原 慶一 松田 時彦
出版者
東京大学地震研究所
雑誌
地震研究所研究速報
巻号頁・発行日
vol.8, pp.73-90, 1964-09

昭和39年6月16日新潟地震調査概報
著者
森本 良平 村井 勇 松田 時彦 中村 一明 恒石 幸正 吉田 鎮男
出版者
東京大学地震研究所
雑誌
東京大学地震研究所彙報 (ISSN:00408972)
巻号頁・発行日
vol.44, no.1, pp.423-445, 1966-07-25

Geology of the main seismic area in and around the town of Matsushiro, the northern part of Nagano Prefecture, central Japan, is investigated by field and literature surveys for the better understanding of the earthquake-swarm which is now taking place. The Matsushiro earthquake-swarm started at the beginning of August, 1965. Since then, the local seismicity has become more active with occasional rise and fall.
著者
山科 健一郎 中村 一明 福留 高明 佐藤 魂夫 田中 和夫
出版者
公益社団法人 日本地震学会
雑誌
地震 第2輯 (ISSN:00371114)
巻号頁・発行日
vol.38, no.1, pp.81-91, 1985-03-25 (Released:2010-03-11)
参考文献数
18

Based on a few tens of photographs, height change of Kyuroku-shima Island, which locates very close to the focal region of the Japan Sea earthquake (M=7.7) on May 26th, 1983, is analyzed. 1) The island and adjacent small islets are considered to have subsided by about 30-40cm associated with the 1983 earthquake. The average offset obtained by 13 reliable photographs is 32±9cm. 2) Precursory deformation and secular change of the height were not found during 1964 through 1982. No detectable deformation was caused by largest aftershock (M=7.1) on June 21, 1983, and other aftershock activity. 3) Coseismic tilt and local collapse were not observed in the island. 4) In some cases, a height can be inferred even from snapshots with the error less than several percents. Crustal uplift and subsidence more than 10-20cm may be possible to be detected photographically at the seashore and lakeside.