著者
鈴木 毅彦
出版者
公益社団法人 東京地学協会
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.122, no.6, pp.1088-1098, 2013-12-25 (Released:2014-01-16)
参考文献数
40
被引用文献数
1 or 0

This paper reviews historical volcanic disasters that have affected the Tokyo Metropolitan area and its surroundings, central Japan, and discusses the dangers of volcanic disasters occurring in future. The 1707 (Hoei) eruption of Fuji volcano, the 1783 (Tenmei) eruption of Asama volcano, and the so-called Kanto Loam, volcanic soil deposits containing large quantities of Holocene to Pleistocene fall-out tephras, suggest the potential hazards that originate from volcanic activities. Small to moderate eruptions (VEI 1 to 2) of Asama volcano have resulted in minor ash falls in and around Tokyo every one to two decades. It is most likely that Asama volcano will generate minor ash falls in the near future. Volcanic disasters caused by larger but rare eruptions of VEI 4 to 5 are considered, referring to the 1707 (Hoei) eruption of Fuji volcano, and measures and predictions for the next eruption of Fuji volcano. In this paper, volcanic disasters affecting Tokyo in the near future are not only those caused by ash falls but also those caused by lahar along the Tone, Edo, Sakawa, and Sagami rivers related to Asama, Haruna, and Fuji volcanoes, because the landform developments of these areas in Holocene and historical disasters suggest that these drainage basins have the potential for lahar disasters. In addition, more severe eruptions of VEI 6 to 7 are considered for their impacts and frequencies referring to geological records of air-fall tephras and/or pyroclastic flow deposits such as VEI 6 Hakone-Tokyo tephra (ca. 66 ka) and VEI 7 Aira-Tn tephra (ca. 29 ka).
著者
小山 真人
出版者
公益社団法人 東京地学協会
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.108, no.4, pp.346-369, 1999-08-25 (Released:2009-11-12)
参考文献数
88
被引用文献数
4 or 0

There remain many challenges in using historical documents to reconstruct a reliable history of earthquakes in Japan. Previous catalogs of historical earthquakes in Japan are not conclusive and contain uncertainties about date, hypocenter, magnitude, and tectonic interpretation of each earthquake. There is no database of digital texts of historical documents, which describe each earthquake. Since the density of historical records in Japan is temporally and spatially heterogeneous, seismologists should carefully remove apparent changes of earthquake frequency, which are caused by the heterogeneity of record density. There is, however, no detailed database of the density variation of historical records. The number of researchers, who are interested in historical earthquakes, is small.The situation stated above is caused mainly by the multi-disciplinary character of historical seismology. Japanese seismologists, who usually have little knowledge of history and classical literature, are not qualified to read a historical document and evaluate its reliability.The environment for research on historical seismology is, however, getting better. Japanese historians have published and are still publishing many historical documents, sometimes with translations into modern language. Evaluations of the reliability of each document can easily be done by referring to historical dictionaries or other databases. All these publications and information are available in many libraries. It is now easy and stimulating for many seismologists to read, evaluate, and interpret historical documents.
著者
小山 真人
出版者
公益社団法人 東京地学協会
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.111, no.2, pp.222-232, 2002-04-25 (Released:2009-11-12)
参考文献数
73
被引用文献数
5 or 0

Previous studies have reported many examples of possible mechanical coupling between volcanic unrests and large earthquakes, which occurred around the volcano. This paper reviews these studies and reorganizes the types of mechanical coupling into the following five cases (cases A-1, A-2, B-1, B-2, and C) and 10 mechanisms : In case A-1, in which a large earthquake triggers activation of a volcano, the following four mechanisms can explain their coupling : (A-1-1) an increase in compressional stress, which was produced by earthquake source faulting, squeezes magma up to the surface; (A-1-2) an increase in differential stress (or magma pressure), which was produced by earthquake source faulting, promotes dike intrusion; (A-1-3) increase in tensional stress, which was produced by earthquake source faulting, triggers gas bubbling in magma; and, (A-1-4) dynamic stress change, which was associated with seismic wave, triggers gas bubbling in magma.In case A-2, in which a large earthquake triggers deactivation of a volcano, the following three mechanisms can explain their coupling : (A-2-1) increase in compressional stress, which was produced by earthquake source faulting, chokes a vent or prevents gass bubbling in magma; (A-2-2) decrease in differential stress (or magma pressure), which was produced by earthquake source faulting, prevents dike intrusion; and, (A-2-3) increase in tensional stress, which was produced by earthquake source faulting, drains magma back toward a chamber.In case B-1, in which a volcanic unrest triggers a large earthquake, coupling can be explained by mechanism B-1-1 : change in stress, which was produced by dike intrusion (or pressure change in a magma chamber), promotes earthquake source faulting. In case B-2, in which a volcanic unrest prevents a large earthquake, coupling can be explained by mechanism B-2-1 : change in stress, which was produced by dike intrusion (or pressure change in a magma chamber), prevents earthquake source faulting.In case C, in which a change in plate motion causes a stress change and then triggers (or prevents) a large earthquake (or a volcanic unrest), coupling can be explained by mechanism C : stress change promotes (or prevents) earthquake source faulting or ascending / intrusion of magma.
著者
米地 文夫
出版者
公益社団法人 東京地学協会
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.97, no.4, pp.317-325, 1988-08-25 (Released:2011-02-17)
参考文献数
37
被引用文献数
2 or 7
著者
村岡 洋文 浅沼 宏 伊藤 久男
出版者
公益社団法人 東京地学協会
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.122, no.2, pp.343-362, 2013-04-25 (Released:2013-05-31)
参考文献数
47
被引用文献数
6 or 0

Current geothermal power generation from engineered geothermal system (EGS) technologies has two bottle-necks in practical use: one is that the recoverability of injected water is about 50% or less than that in fracture-dominant regions such as Japan, which inevitably requires replenishing large volumes of injected water throughout the power generation operation, and the other is that the injected water raises pore fluid pressures in crustal rocks, causing induced-earthquakes. This paper proposes a new power generation method, which has the potential to resolve these two bottle-necks using EGS technologies in ductile zones. With this method, an artificial brittle fracture reservoir system is completely surrounded by ductile zones at a temperature exceeding 500°C, the presence of which has already been confirmed at the Kakkonda geothermal field, northeastern Japan. The profitability of this method is highly dependent on the depth of drilling, but this concept could dramatically expand exploitable thermal conduction geothermal resources beyond the brittle zones.
著者
山岡 耕春 中禮 正明 安藤 雅孝
出版者
公益社団法人 東京地学協会
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.111, no.2, pp.185-191, 2002-04-25 (Released:2009-11-12)
参考文献数
9
被引用文献数
2 or 0

The temporal relationship between the interplate and inland earthquakes of Kyushu Island region was investigated. We analyzed this relationship by stacking the temporal frequency of the inland earthquakes with reference to the occurrence time of each major interplate earthquake in the Hyuga-nada region, that occurred between 1900 and 2000. A good coincidence of occurrence between the inland and the interplate earthquakes is recognized. While the tendency of the occurrence after the interplate events has already been pointed out, we found that the inland earthquakes also tend to occur before the interplate events. Abrupt activation of the inland earthquakes within several months before major interplate earthquakes is recognized in addition to the tendency to occurr afterwards. The preceding inland earthquakes may indicate that they are triggered by precursory slow slips around the hypocenters of the Hyuga-nada earthquakes.

53 0 0 0 OA 北海道の豪雨

著者
松本 淳
出版者
公益社団法人 東京地学協会
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.94, no.3, pp.181-193, 1985-06-25 (Released:2010-12-22)
参考文献数
24
被引用文献数
2 or 0

The climatological characteristics of heavy rainfalls in Hokkaido Island are investigated on the basis of areal frequency distribution of heavy rainfalls (the amount of daily precipitation above 100mm) and geographical distribution of the maximum daily precipitation. The causes of heavy rainfalls and the maximum daily precipitation are investigated on the daily synoptic weather charts, and their geographical distributions are also investigated. Relation between the regional characteristics of heavy rainfalls and the location of the Polar front is discussed. The statistical period is 25 years, from 1973 to 1977 and the data of 256 stations are analysed.As a result, Hokkaido Island is divided into two climatic regions, the one is ‘high frequency heavy rainfall region (HR)’ and the other is ‘low frequency heavy rainfall region (LR)’. The bordering line of these two regions runs approximately from ENE to WSW, from Shiretoko peninsula, via Shiranuka Hills, Ishikari Mountains, Hidaka Mountains, Iburi Mountains to the southern part of Oshima Peninsula.To the south of this line is the HR in which local extremly high frequency heavy rainfall regions are located on the eastern to southern slopes of the mountain ranges. In this region the frequency of heavy rainfalls is more than once per year, and the maximum daily precipatation exceeds 300mm. The main causes of heavy rainfalls here are extratropical cyclones passing to the south of Hokkaido Island from Japan Sea or from Pacific Ocean, most often in August and September. This types of heavy rainfalls are mainly caused by the moist air flow from these cyclones and the effect of orographic rifting strengthens the rainfalls. In these cases the Polar front is usually located to the south of Hokkaido Island.On the other hand to the north of the line, the frequency of heavy rainfalls decreases drastically to less than once per five years. Here the main causes of heavy rianfalls are fronts and tropical cyclones, mainly in July and August. This time the Polar front lies in the northern part of Hokkaido Island and this location corresponds to its mean northernmost position. The region in which the main cause of heavy rainfalls is such northernmost Polar front is classified into ‘frontal heavy rainfall region’. In addition, extremly low frequency heavy rainfall rigion in which no heavy rainfalls were recorded through whole investigated period are discerned.
著者
堀 和明 斎藤 文紀
出版者
公益社団法人 東京地学協会
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.112, no.3, pp.337-359, 2003-06-25 (Released:2009-11-12)
参考文献数
141
被引用文献数
6 or 0

Large rivers with high sediment discharge, such as the Amazon, Ganges-Brahmaputra, and Huanghe (Yellow) rivers, have formed mega-deltas at their mouths. This paper reviews morphology and sediments of mega-deltas and their Holocene evolution in relation to sea-level changes, channel avulsion, and past-sediment discharge. Application of various radiometric dating techniques to delta deposits, especially since the 1980's has made it possible to clarify delta evolution dynamically on 10-to 1000-year time scales. Most of the deltas are located in developing countries, and have environmental problems principally caused by human activities. For the evaluation of current status and human activities in deltas and drainage basins, both natural and anthropogenic changes of deltas should be analyzed from the viewpoints of geology, sedimentology, and their evolution.
著者
小山 真人 早川 由紀夫
出版者
公益社団法人 東京地学協会
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.105, no.2, pp.133-162, 1996-04-25 (Released:2009-11-12)
参考文献数
50
被引用文献数
17 or 0

We reveal the detailed syn-and post-caldera eruptive history of Izu Oshima Volcano, Japan, by tephra and loess stratigraphy. Twenty-four tephra layers, which overlie the slope outside the caldera, show that 24 eruptions occurred since the formation of the caldera (about 1, 450 years ago). These eruptions are separated by 10-200 years clear dormant periods, which can be identified by eolian dust (loess) interbedded with tephra layers. The 24 eruptions can be classified into three types : 1) eruption with scoria and ash falls (12 eruptions), 2) eruption only with scoria falls (7 eruptions), and 3) eruption only with ash falls (5 eruptions). While tephra discharge mass of the type 1 is generally large (1.5×1010 to 7×1011 kg), that of the type 2 or 3 is small (0.6 ×109 to 1 ×1011 kg). The 1986 eruption is classified into the type 2. Debris avalanches, which occurred just before the caldera formation and covered almost all of the Izu Oshima island, demonstrate that the present caldera wall was formed by slope failure of an old edifice. The tephra-discharge stepdiagram, which shows a relationship between time and cumulative discharge volume / mass of magma, shows : 1) the average tephra-discharge rate is constant (92 kg/ s before the N1.0 eruption and 25 kg/s after the N1.0 eruption), showing an abrupt decrease of the rate at about the time of the N1.0 eruption, which occurred about 900 years ago and was the most voluminous eruption for the past 1, 450 years, 2) both before and after the N1.0 eruption, the type 1 eruption shows volume-predictability, that is, the discharge volume / mass of a next type 1 eruption can be predicted, 3) a type 1 eruption should occur sometime in the future again, and when it occurs, the discharge mass of tephra should attain to as much as 2×1011 kg or more.
著者
長谷川 昭 中島 淳一 内田 直希 海野 徳仁
出版者
公益社団法人 東京地学協会
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.122, no.3, pp.398-417, 2013-06-25 (Released:2013-07-08)
参考文献数
60
被引用文献数
1 or 0

Recent investigations based on seismic tomography, hypocenter determinations and focal mechanism analyses using dense seismic network data reveal the precise configurations of the Pacific (PAC) and Philippine Sea (PHS) plates subducting beneath the Tokyo metropolitan area. Estimated geometry shows a broad contact zone between the two plates located directly beneath the Kanto plain. The overlap with the PHS plate subducting above it provides the PAC plate with protection from being heated by the hot mantle wedge. Moreover, the fore-arc portion of the PHS plate, before its subduction beneath Kanto, had been cooled by the subduction of the PAC plate from the Izu-Bonin trench. These cause lower-temperature conditions within the two oceanic plates and the upper continental plate beneath the Tokyo metropolitan area. As a result, depth limits of seismic activities within the plates and along their boundaries are anomalously deep. Seismic tomography studies show that the easternmost portion of the PHS slab mantle is serpentinized. The PHS slab may have been torn in two along the western boundary of this serpentinized mantle, with the eastern portion being left behind relative to subduction of the western portion. This is accompanied by the generation of large intraslab earthquakes along the boundary. We need to take these observations into consideration to understand the mechanism generating M7-class earthquakes, which are anticipated to occur in the southern Kanto region with a high probability.
著者
高橋 雅紀
出版者
公益社団法人 東京地学協会
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.115, no.1, pp.116-123, 2006-02-25 (Released:2009-11-12)
参考文献数
13
被引用文献数
21 or 0

The Euler pole position of the Philippine Sea Plate (PHP) relative to the stable Eurasian Plate (EUP) between 15 and 3 Ma can be estimated at around 150°E, 36°N, on the basis of the geological constraint that the intersection of the Izu-Ogasawara Arc with Southwest (SW) Japan has not moved from South Fossa Magna since 15 Ma. The timing of the migration of the Euler Pole to its present location (154°E, 47°N) should have occurred at 3 Ma because the fore-arc basin in SW Japan was once interrupted by the Kurotaki Unconformity at 3 Ma.PHP moves northwestward and subducts beneath SW Japan at a convergent rate of 4 cm/yr. The Izu-Ogasawara Trench (IT) also moves at the same rate as the westward component (ca. 3 cm/yr.) of the PHP motion. Both the trench-trench-trench (TTT) triple junction and the Japan Trench (JT) should migrate westward, because the thick, cold, and sturdy Pacific Plate (PAP) has never been cut by the transform fault at the TTT junction. Northeast (NE) Japan would also move westward because tectonic erosion along JT would not be sufficient for westward migration of the JT. Thus, the present PHP movement causes the westward migration of IT, TTT junction, JT and then NE Japan. This westward motion of NE Japan against the sturdy oceanic lithosphere of the Japan Sea has caused an E-W contraction of NE Japan since northward motion of PHP changed to NW at 3 Ma.It is expected that rifting of the thin, heated lithosphere of the Izu-Ogasawara Arc would reach break-up before the thick, cold lithosphere of the PAP would be torn by the right-lateral transform fault at the TTT junction. Once rifting reaches break-up, the northwestward movement of the PHP would be compensated by back-arc spreading, and this motion would not propagate to the IT, the JT nor NE Japan. Therefore, the present E-W contraction in Japan would cease in the geologically near future when back-arc rifting along the Izu-Ogasawara arc reaches break-up.McKenzie and Morgan (1969) discussed how the TTT triple junction was unstable except under a few uncommon geometrical and kinematic conditions. However, the PHP actually selected this particular Euler pole position at 15 Ma, and the TTT triple junction had been stable for more than 10 m.y. Although the present TTT junction is in an unstable condition, it would become stable again through back-arc basin spreading of the PHP in the geologically near future. Thus, the TTT triple junction offshore central Japan, which controls tectonics of Japan, would be in a stable state in nature.
著者
柳井 修一 青木 一勝 赤堀 良光
出版者
公益社団法人 東京地学協会
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.119, no.6, pp.1079-1124, 2010-12-25 (Released:2011-03-17)
参考文献数
121
被引用文献数
23 or 0

Median Tectonic Line (MTL) and Fossa Magna (Itoigawa-Shizuoka Tectonic Line) had long been considered to be the most critical fault boundaries controlling development of the Japanese Islands since Naumann (1885) and Kobayashi (1941). After the appearance of plate tectonics, several new interpretations emerged, e.g., sub-surface Benioff plane for the MTL. In this paper, we propose that those tectonic lines, major faults, and Tanakura Tectonic Line (TTL) were formed through a process at micro-plate boundaries during the opening of the Japan Sea in the Miocene. MTL could have been formed along the consuming boundary between the PHS plate and Japan Sea microplate, which has shifted southward to the Nankai trough, accompanying large-scale tectonic erosion. Fossa Magna was formed as a gigantic transform fault with a transtension component in the Medial-Japan Sea when opening was initiated. The eastern and western boundaries of the Japan Sea must be a strike-slip fault, corresponding to TTL to the east, and a newly proposed strike-slip fault called the West Kyushu Tectonic Line, respectively. Fossa Magna, a medial region defined by two NS-trending Miocene parallel faults in central Honshu, defined by Nauman (1885) could be interpreted to be the largest transform fault in the Medial-Japan Sea to offset the spreading axis when the Japan Sea opened. It should be emphasized that large-scale tectonic erosion occurred in front of consuming plate boundaries facing the PHS and PAC plates oceanward during the opening of the Japan Sea. The volume of tectonic erosion is calculated to be 17,581,500 km3, which is equivalent to 2/3 of the present-day Japan arc crust, which is sufficient to reach the depth of the megalith between the upper and lower mantle boundary, even with 10 km thickness of materials eroded and transported along the Benioff zone. Although MTL, Fossa Magna, and TTL are remarkable in the geology of Japan, these young faults never affected the orogeneses of Japan back to 520 Ma, which grew the continental crust of Japan. We propose that microplate boundary processes decreased the volume of the Japan crust.
著者
古村 孝志
出版者
公益社団法人 東京地学協会
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.123, no.4, pp.434-450, 2014-08-25 (Released:2014-09-01)
参考文献数
28
被引用文献数
2 or 0

In the Kanto basin, long-period (T = 6-8s) ground motions develop during large (M > 7) earthquakes due to the resonance of surface waves with thick (> 3000m) sedimentary layers covering rigid bedrock. Such long-period ground motions can cause significant damage to modern large-scale, long natural-period structures such as skyscrapers, oil storage tanks, and long bridges. Therefore, it is important that residents of modern Tokyo metropolitan area cities be forewarned about the potential for such disasters. In the present paper, the development of large and long-duration, long-period ground motions in central Tokyo is investigated by analyzing waveform data from recent large earthquakes obtained from dense seismic networks extending over the Kanto basin. It is demonstrated that the interaction of surface waves with the three-dimensional sedimentary structure beneath the Kanto region causes peculiar and directional dependencies related to the amplification strength of the long-period ground motions. For example, the long-period ground motions resulting from earthquakes occurring in northern Japan cannot develop efficiently in central Tokyo. This might be one reason for the relatively weak long-period ground motions observed in central Tokyo during the 2011 Off Tohoku M 9.0 earthquake compared to those of other M 7-8 class events, such as the 1944 Tonankai (M 7.9) and the 2003 Off Tokachi (M 8.0) earthquakes. The results of computer simulations using detailed subsurface structures and source-slip models for an anticipated Nankai Trough M 8.7 earthquake indicates that the strength of long-period ground motions in central Tokyo can be expected to be at least double than those observed during the 2011 Off Tohoku earthquake.