著者
河名 俊男 中田 高
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.103, no.4, pp.352-376, 1994-08-25 (Released:2009-11-12)
参考文献数
47
被引用文献数
37 31

In 1771, according to several trustworthy historical records, a huge tsunami named the Meiwa Tsunami washed the southern part of the Ryukyu Islands killing about 12, 000 people. The tsunami attributed probably to a large earthquake generated by a submarine thrust fault along the Ryukyu Trench. The southern Ryukyu Islands consisting of Miyako, Irabu, Shimoji, Tarama, Ishigaki, Iriomote and other small islands are fringed by coral reefs, and large Holocene coralline boulders possibly transported by past tsunamis, are extensively distributed on land and reef flats. Among these boulders, those composed of aragonite by 100% or nearly 100% are reliably dated by the radiocarbon method, and are good evidence for inundation, run-up heights and timing of tsunamis in the past.In order to infer the timing of past tsunamis, we dated samples carefully collected from the uppermost parts of these Holocene coralline boulders and fragments. Based on 65 dates, we restored a tsunami history in the area during the past several thousand years.Most of the coralline boulders we dated are much older than the age of the Meiwa Tsunami about 200 yr BP. Certain periodical distributions of the ages among the boulders suggest that the area had been attacked by huge tsunamis around 600, 1, 100, 2, 000 and 2, 400 yr BP during the last 3, 000 years. Thus tsunamis which brought tsunami boulders on land occurred repeatedly with intervals of several hundred to one thousand years in the study area.The tsunamis occurred around 1, 100, 2, 000 and 2, 400 yr BP were judged from the distribution of boulders of similar ages, that they were generated along the Ryukyu Trench while that of 600 yr BP along the Okinawa Trough.The tsunami about 2, 000 yr BP is most reliably restored among the past tsunamis in the area and is named the “Okinawa-Sakishima Tsunami”. This tsunami attacked an extensive area from Miyako to Ishigaki islands and transported many huge tsunami boulders such as “Tsunami Oishi” deep on land.
著者
相馬 清二
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.84, no.4, pp.204-217, 1975-08-25 (Released:2009-11-12)
参考文献数
8
被引用文献数
3 2

A violent fire-tornado has broken out at the former site of the Army Clothing Depot in combustion caused by the destructions of the Great Kanto Earthquake at 3.30 p.m. September 1, 1923. By this fire-tornado, about 40 thousand people have been burned to death in a moment. An interpretation that the fire-tornado has been caused by passage of a cold front has generally dominated academic circles in this field up to date. According to the detailed reexaminations, this interpretation included many inconsistencies. We could obtain recently the various data with respect to the fire-tornado which occurred in the last several years. From these data, it became clear that the fire-tornadoes often have occurred near a combustion area.An example which was the most similar with the fire-tornado at the former site of the Army Clothing Depot was found out in the combustion of Wakayama City by an air raid on July 9, 1945. 748 persons have been burned to death by the fire-tornado in this city.Finally, our study leads to the conclusion that the fire-tornado at the former site of the Army Clothing Depot was caused by the widespread great fire following the Great Kanto Earthquake. There was a cold front in northwestern direction of Tokyo, but it did not play the leading role for formation of the fire-tornado.
著者
山川 修治
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.102, no.2, pp.183-195, 1993-04-25 (Released:2010-11-18)
参考文献数
60
被引用文献数
1 1

Natural disasters in Little Ice Age (c. 1550-1850) and its climatic variations which formed a significant background for the disasters are mentioned. Cool summers due to a prevailing Okhotsk High were characteristic of this period. Cold winters, summer heavy rains and unstable atmospheric conditions also constituted essential features of Little Ice Age. Aerosol ejected by a series of major volcanic eruptions partly prevented solar radiation from reaching the earth' s surface (parasol effect), which resulted in cool and unusual weather. Agriculture, in particular, was vulnerable to bad weather damage. As a result of decreasing direct insolation, sometimes together with lowering temperature and/or locally increasing precipitation, seven major famines occurred during the Edo Era (1663-1868) which approximately coincided with Little Ice Age. Several cases in this paper suggest that natural environment in Little Ice Age was marked by both volcanism and weakened solar activities which “Maunder Minimum”(1645-1715) represented.
著者
西上 欽也 田所 敬一 永井 悟 水野 高志 加納 靖之 平松 良浩
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.111, no.2, pp.268-276, 2002-04-25 (Released:2009-11-12)
参考文献数
21
被引用文献数
1

This paper briefly reviews the triggering characteristics of injection-induced seismicity. Water injection experiments were carried out in the Nojima fault, southwest Japan in 1997 and 2000 to detect the healing process of the fault zone after being ruptured by the MJMA 7.3 Hyogo-ken Nanbu (Kobe) earthquake in 1995. During the experiment in 2000, ultramicroearthquakes of M-1.2 to 1.0 were induced at about 2.5-4.5 km from the injection point and about 4-6 days after the beginning of injection. This space-time migration can be explained by a 2-D diffusion process of pore water pressure. Permeability estimated near the surface, at about 540-800 m depth, is extrapolated well to a depth of 2-4 km where induced events occurred. Other experiments at Matsushiro, central Japan and KTB, Germany also showed similar space-time relationships of induced seismicity. From observations in the Nojima experiment, we obtained characteristic states that suggest differences in the generating process between induced and normal (tectonic) earthquakes. Our findings are as follows : (1) high-frequency component is richer in the waveforms of tectonic events, and (2) the clustering of hypocenters is more dominant in induced events. Further analyses of these observations will lead to elucidating the generating process of induced earthquakes by increasing pore water pressure.
著者
西村 蹊二 斉藤 祥 谷岡 誠一 門脇 淳
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.86, no.6, pp.346-363, 1977-12-25 (Released:2009-11-12)
参考文献数
20

Detailed and intensive investigations have been carried out on the abrupt failure of submarine slope of a small island, Shin-jima, in Kagoshima Bay, Kyushu on April 1, 1975 by the joint research group of Geographical Survey Institute of Japan, Japan Maritime Safety Agency and Geological Survey of Japan under the sponsorship of Japan Science and Technology Agency.The Shin-jima island emerged from water at the great eruption of adjacent Sakurajima Volcano in 1779 to 1780 (An-ei Eruption) as one of central corns of Aira Caldera (MATSUMOTO, 1933). The island gradually reduced its exposed area by continuous wave erosion to form truncated configuration, being surrounded by broad, shallow, flat tidal bench.Geologically the island is composed of the latest Pleistocene to the earliest Holocene thick loose pumice deposits with intercalations of silt and clay showing inner bay facies. These formations are strongly faulted by eastwest faults. The surface of the island is covered with thin shell bed dominantly composed of Ostrea and andesitic lava. The flat bench around the Shin-jima consists of recent, loose pumiceous drift sand deposits derived from the island by wave erosion.The failure of the slope now in concern occurred at the spit of the southern tip of the island, where thickness of drift sand deposits attains at most about 35 meters according to sonic prospecting and borings at the spit. It was caused by submarine landslide in the thick, loose drift sand deposits. Volume of the slide mass amounts approximately 265, 000 cubic meters.The spit had formed steep submarine slope near critical angle by abundant supply of very loose drift sand from northern part of the island before the failure occurred. Neither sensible earthquakes nor volcanic activities of the Sakurajima which might have given significant effects to submarine landslide are observed and recorded in seismometer around the crucial moment of the failure.Therefore, the cause of the landslide is considered that the increase of load resulted from the deposition of drift sand attaining about 8, 400 tons in volume atop of the slope brought about decrease of safety factor of the slope to make it slide. Wave pressure seems to have triggered the slide because safety factor decreases about 20 % when fluctuation of pressure at the bottom caused by wave agitation is taken into account.
著者
多田 隆治
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.107, no.2, pp.218-233, 1998-04-25 (Released:2009-11-12)
参考文献数
57
被引用文献数
1 1

Since the ice core records from central Greenland revealed the presence and significance of millennial-scale large and abrupt climatic changes, widely known as Dansgaard-Oeschger [D-O] Cycles, it becomes the major objective of paleoclimatological researches to clarify their extent, nature, propagation mechanism, and driving force. Although the ultimate driving force is not yet understood, results of recent studies suggest 1) D-O Cycles are global phenomena, 2) they involve complicated interactions and feedback processes among the subsystems including atmosphere, cryosphere, hydrosphere, and biosphere, and 3) they seem tohave initiated from changes in atmospheric circulation.Catastrophic surges of Laurentide Ice Sheet called Heinrich Events are closely associated with D-O Cycles. Although Heinrich Events are likely to have been caused by free oscillationsof ice sheet growth and decay, they were probably not a cause of D-O Cycles but the events seem to have been phase-locked by D-O Cycles. Results of numerical modelling suggest the presence of multi-modes for global deepwater circulation. Switching among the modes is most likely caused by slight variation in hydrogical cycles which change the fresh water balance between Atlantic and Pacific.
著者
新井 健司
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.104, no.2, pp.267-283, 1995-04-25 (Released:2009-11-12)
参考文献数
17
被引用文献数
1 1

Plio-Pleistocene fluvial sediments are distributed in the hills along the western margin of the Kanto Plain. In this paper, the author reports its sedimentary environments at Hidaka City, Saitama Prefecture, namely, those of Yaoroshi tuff, Hanno gravel and their correlatives.The basement in this area (Chichibu Paleozoic and Mesozoic formations and Takaoka formation) is divided into three zones structurally by Koma-Hongo and Komagawa fault systems. And terrace deformation dipping toward the Kanto Plain is recognized on its surface.Yaoroshi tuff is in fault or unconformity contact with the basement in part of Koma-Hongo fault system. The vertical displacement of each fault system is over 50 m. Yaoroshi tuff is not distributed in west of Koma-Hongo fault system.Hanno gravel, conformably overlying Yaoroshi tuff, is in fault or unconformity contact with the basement. Hanno gravel and its correlatives were formed by old long rivers, of and the valley bottoms were at least 140 m above sea level in Koma River system area and at least 150 m above sea level in Shukuya River system area.Boulders of granite and diorite characterizing Hanno gravel in the south of Tenran-zan (Mt. Tenran) are not found in the investigated area. This fact suggests that the area was in different conditions of sedimentation from the southern area. The ridge directions of Koma Hills and Moroyama Hills had already settled before dissection proceeded.The gravel from Moroyama Hills and the adjacent areas is divided into the lower and the upper layers. The former is correlated with Yaoroshi tuff, while the latter with Hanno gravel.When Yaoroshi tuff deposited, forested stable land had extended widely. And short rivers flowing out from near mountains sometimes flooded and destroyed forests. After Yaoroshi tuff was formed, long rivers transported boulders with sands in large quantities from distant western mountains. These old long rivers kept their main courses to the present as Koma River system and Shukuya River system, and they mixed gravels by gathering currents and forming talus deposits.
著者
Mitsuhisa WATANABE
出版者
Tokyo Geographical Society
雑誌
Journal of Geography (Chigaku Zasshi) (ISSN:0022135X)
巻号頁・発行日
vol.116, no.3-4, pp.387-393, 2007-08-25 (Released:2009-11-12)
参考文献数
20
被引用文献数
3 5

The cabinet office of the Japanese government demonstrated the prospects of future seismic hazards associated with a working model for possible earthquakes in the capital area of Japan. If this assumption is not unrealistic, it is reasonable to use this working model. However, it has already been reported that several active faults may exist in this area. This discrepancy can lead the assessment into unreal issue.I reveal the nature of the Ayasegawa fault located close to the capital area on the basis of geomorphic features. The fault extends in the NWSE direction for at least over 30 km, and the fault trace is linear, which is indicative of lateral movement. There is a graben structure delineated by the fault in the Minuma ward, Saitama City. The vertical component of the Ayasegawa fault is upthrown to the southwest and the average vertical slip rate is 0.05 to 0.1 m/ky. The netslip rate should be much larger than the vertical one, taking lateral movement into account. Although the single vertical offset is assumed to be 0.8 to 4 m, the rupture history of the fault remains unknown.The Ayasegawa fault is an southeastern extension of the Fukaya fault, and is composed of an active fault extending more than 120km through the Kanto Plain across the capital area of Japan. To prepare for a real seismic hazard and to try to reduce damage, we should check the properties and clarify the rupture history of these active faults as an urgent task. Precise local information on these active faults is necessary for motivating people to develop an awareness of disaster mitigation.

5 0 0 0 熊本の地震

出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.6, no.9, pp.520b-520b, 1894
著者
河村 公隆
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.98, no.6, pp.791-797, 1989-12-05 (Released:2009-11-12)
参考文献数
35
著者
福山 博
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.92, no.1, pp.55-67, 1983-02-25 (Released:2010-10-13)
参考文献数
11
被引用文献数
3 2

Minami-Iwo volcano is located at the southern end of Izu-Bonnin volcanic chain, 1300km south of Tokyo. It consists of three major volcanic edifices; older volcano I (OV1), older volcano II (OV2), and younger volcano (YV). At the boundaries between OV1 and OV2 and OV2 and YV, pyroclastic materials are characteristically observed; scoria fall deposit between OV1 and OV2, and scoria fall deposit and pyroclastic flow deposit between OV2 and YV. Older volcanoes, OV1 and OV2, are wholly composed of lava flows with both aa and pahoehoe surface textures. Younger volcano (YV), on the other hand, consists mainly of scoriaceous agglutinate which made up the steep slope of the main Minami-Iwo volcano. The older volcanoes are cut by numerous dikes (115 all told) which are dominant at the south and southwest sea cliffs. Four specimens collected from OV1, OV2 and dikes show normal thermoremnant magnetic orientation suggesting the oldest age less than a few hundred thousand years. Rocks of Minami-Iwo volcano are ankaramite, olivine basalt, augite-olivine basalt and plagioclase phyric basalt. The petrographic features and the chemical composition show that the rocks of Minami-Iwo volcano are of alkali rock series. The basalt of alkali rock affinity is the first discovery for volcanoes located at the volcanic front in Japan.
著者
町田 洋 新井 房夫
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.88, no.5, pp.313-330, 1979-10-25 (Released:2009-11-12)
参考文献数
49
被引用文献数
45 22

The eruptions of the Daisen Valcano (35°22'N, 133°33'E) were mainly rhyodacitic and of the paroxysmal type, producing several extensive sheets of tephra. The Kurayoshi pumice (DKP, for short), one of the excellent Late Pleistocene markers arising from the Daisen valcano, is rhyodacitic in composition with abundant hornblende and orthopyroxene crystals and relatively small amount of biotite. Its identification can be made from the above mentioned mineral assemblages as well as from the characteristic refractive index of orthopyroxene (γ=1.703-1.708) and of hornblende (n2 = 1.673-1.682) and the specific crystal habit of orthopyroxene. This pumice-fall deposit occurs on marine and fluvial terraces in the San'in and Hokuriku districts facing the Japan sea and extends eastward beyond the Northern Japan Alps to north Kanto plain as a thinner discontinuous layer. Stratigraphic relation with the dated tephra layers in north Kanto indicates that the pumice was probably deposited between about 47, 000 and 45, 000 years ago. That is, this pumiceous deposit is found at the intermediate horizon between Yunokuchi Pumice (UP, slightly younger than 49, 000 YBP) and Hassaki Pumice (HP, 40, 000-44, 000 YBP) in north Kanto, about 500 km far from the Daisen.Daisen Kurayoshi Pumice wonld be particularly valuable for establshing chronological framwork as a fundmental time-marker in arears where no suitable markers have yet been documented. Moranic deposit of the Murodo glacial advance at Mt. Tateyama, Northern Japan Alps, is mantled by this marker and overlies the Raicho-dai pumice-fall deposit, products of the earliest stage of volcanic activity of Tateyama III, which is correlated with the Omachi EPm deposit approximately 60, 000 years old. Distribution of these two unreworked tephras indicates that major valley glaciers had nearly disappeared by the times of these initial tephra falls. The glacial advance at Mt. Tateyama, the most extensive of the advances during Last Glacial age, therefore, apparently culminated between about 55, 000 and 50, 000 years ago. On the other hand, a filltop terrace repesented by Uwadan terrace along the River Joganji flowing from Mt. Tateyama, is covered by DKP and is nearly younger than the pyroclastic flow deposit of Tateyama III. Accumulation of the river of Uwadan stage is, therefore, simultaneous and probably associated with the Murodo glacial advance.
著者
大森 房吉
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.32, no.3, pp.116-124, 1920
著者
齋藤 有 田村 亨 増田 富士雄
出版者
Tokyo Geographical Society
雑誌
地學雜誌 (ISSN:0022135X)
巻号頁・発行日
vol.114, no.5, pp.687-704, 2005-12-25
被引用文献数
3 3

Hyperpycnal flow is one type of turbidity current, which is generated at a river mouth when the suspended sediment concentration of river discharge is so high that the density of the effluent exceeds the water of the receiving basin. Generations of this type of flow have become a common phenomenon recently. A lot of hyperpycnal flow deposits are expected to be preserved naturally in stratigraphic records. Hyperpycnal flow deposits (hyperpycnites) might become an innovative factor for turbidite paradigm considering the particularity of the flow, and bring facies analysis towards the next step. In this review paper, the criteria for identifying hyperpycnal flow deposits from strata are proposed by summarizing studies on hyperpycnal flow and its deposits. Typical characteristics of hyperpycnal flow include : (1) vertical succession composed of 2 parts, inversely-graded lower part and normally-graded upper part, (2) internal scour surface, (3) repetitive alternation of fine-grained and coarse-grained layers, or laminated and massive layers, (4) abrupt pinch-out of beds, and (5) inclusion of terrestrial materials such as leaves. Hyperpycnal flow gradually waxes and then wanes to terminate in response to flood conditions, resulting in (1). The internal scour surface (2) developed in accordance with the degree of waxing. Fluctuations of flow velocity and sediment concentration due to changes of river discharge and/or internal waves can occur during the flow event, and lead to repetitive alternation of (3). Because of the fresh interstitial water, the marine hyperpycnal flow might start to lift off after losing suspended sediments to the degree that the density of the flow is exceeded by ambient saline water. This results in (4) at the lifting point. Terrestrial materials referred to in (5) are the result of the fact that the hyperpycnal flow originated from terrestrial floods.<BR>A much wider variety of deposits, which form in one history of a turbidity current, than that expected from the Bouma sequence model, is shown. Exploration of the variety has just started.
著者
田切 美智雄 森本 麻希 望月 涼子 横須賀 歩 DUNKLEY Daniel J. 足立 達朗
出版者
Tokyo Geographical Society
雑誌
地學雜誌 (ISSN:0022135X)
巻号頁・発行日
vol.119, no.2, pp.245-256, 2010-04-25
参考文献数
29
被引用文献数
8 15

Hitachi metamorphic rocks located in the southern part of the Abukuma Mountains, Northeast Japan, distinctively contain meta-volcanic rocks and meta/sheared granitoids. The igneous ages of meta-granite and meta-porphyry from the Hitachi metamorphic rocks were determined by the SHRIMP zircon method. In this paper, we describe occurrence, petrography, and petrochemical characteristics of these studied rocks. Meta-porphyry, with an igneous age of 506 Ma, intrudes into the meta-volcanic rocks of the Akazawa Formation of the Hitachi metamorphic rocks and has a micrographic texture and a spherulitic texture of an igneous origin. Previous studies have already reported an igneous age of 491 Ma for meta/sheared granitoids using the SHRIMP zircon method. Cambrian meta/sheared granitoid samples occur widely as a granitic body in the northeastern part of the Hitachi metamorphic rocks. (A) Meta-granite of the same age (498 Ma) as the sample used for the above dating is found as boulders in meta-conglomerates. The meta-conglomerate, which is found in the Daioin Formation of the Hitachi metamorphic rocks, lies unconformably on a Cambrian meta-granite body. Both meta-volcanic rocks and meta/sheared granitoids have chemical characteristics commonly associated with island arc volcanism. As such, the Akazawa Formation is likely to have originated in the Cambrian era, although we have no SHRIMP age for meta-volcanic rocks of the Akazawa Formation.
著者
長岡 信治 古山 勝彦
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.113, no.3, pp.349-382, 2004-04-25 (Released:2009-11-12)
参考文献数
32
被引用文献数
5 4

Many basaltic monogenetic volcanoes forming groups are distributed on Fukue Island, the largest island of the Goto Islands. The Onidake volcano group, 8×10 km across, is one of the monogenetic volcano groups in the island. It contains eleven monogenetic volcanoes, namely, Daienji, Koba, Nagate, Masuda, Shimosakiyama, Hinodake, Kamiozu, Shirodake, Sakiyamabana, Midake-Usudake, and Onidake, in chronological order. K-Ar and 14C dating results show that these volcanoes were active during 0.5-0.018 Ma. The average interval of monogenetic volcano-forming eruption is fifty thousand to thirty thousand years, while a long interval of about one hundred thousand years exists between 0.25 and 0.15 Ma. By long interval, the group is divided into older volcanoes, Daienji, Koba, Nagate, and Masuda, and younger volcanoes, Shimosakiyama, Hinodake, Kamiozu, Shirodake, Sakiyamabana, Midake-Usudake, and Onidake.At about 0.42 Ma, the activities of the older volcanoes started with Daienji volcano, which produced a lava flow in the north area. It was covered by thick lava flows of Koba volcano at about 0.38 Ma. During or shortly after the activity of Koba volcano, Nagate shield volcano with two small scoria cones became active in the northeastern area. At about 0.30 Ma, in the western area, Masuda volcano erupted to form a lava plateau and two scoria cones.After 0.15 Ma, in the central area, the intermittent activities of younger volcanoes began with lava flows from Shimosakiyama volcano, which was covered soon by Hinodake volcano composed of three lava flows and one scoria cone. Two small cinder cones with a lava flow, Shirodake and Kamiozu volcanoes, erupted on the northern part of Hinodake volcano. At about 0.03 Ma, at the southeastern end, Midake-Usudake volcano, erupted with three cindercone-forming scoria falls and five small plateau-forming lava flows covering an undated scoria cone, Sakiyamabana.At 0.018 Ma, the latest eruption of the group, Onidake volcano, commenced in the central area, the first phase of which was the Abunze lava flow originating from welded scoria falls or a mixture of lava and scoria fall into the lava ponds in craters. Simultaneously, the scoria falls generating Abunze lava flow contributed the formation of Osako scoria cone, which partially collapsed shortly afterwards, probably because Abunze lava carried the parts away. In the second phase, Onidake scoria falls covered a large area of 5×5 km, while Onidake scoria cone was formed in the proximal area. The third phase consisted of five lava flows, Ohma lava 1, 2, and 3 and Onidake lava 1 and 2, all of which erupted from two summit craters of Onidake cone and covered the northern and western foothills of the volcano.
著者
藤岡 換太郎
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.97, no.1, pp.39-50, 1988-02-25 (Released:2009-11-12)
参考文献数
59
被引用文献数
1 1

Topographic features around Izu Oshima region were summarized with special references to the characteristics relating to subduction of the Philippine Sea plate (PHS) along the Sagami Trough. Several topographic highs having circular contours around Oshima Island may be the results of parasitic eruption of Oshima Volcano. Their trend is almost parallel to the elongation of the linearity of parasitic cones distributing onshore. Along the eastern trench slope off Oshima Island, large scale collapse structures are seen in the submarine topographic map. These are the result of the falling down mostly of pyroclastic materials (tephras) during historic ages. The Oshima Island shows eastward tilting owing to the bending resulted from the subduction of the PHS. Large scale of the collapse structures may be caused by the oversteepening of the eastern trench slope of Oshima Island and soft sediments consisting mostly of pyroclastics including a large quantity of pore water slide down along the trench slope as far as the Sagami Trough.Radial distribution both of saddles and valleys observed northeastern offshore part of the Oshima Island may be lava flows and/or dike swarms derived from the old Okata Volcano whose eruption center was estimated to exist at the northeastern extension of the Oshima Island.About 4 km south off the Oshima Island, large scale of the topographic breakes consisting of caotic highs and depressions are seen in the submarine topographic map. These may be debris flow deposits along eastern slope of Ohmurodashi which consists mostly of the Quaternary rhyolites geologically and chemically similar to those exposed on the Niijima and Kozushima Islands. These are the results of catastrophic phreatomagmatic eruption of the Ohmurodashi such as that of the Myojinsho. Mixture of pyroclastics and lavas will be found from these caotic areas.The upper steam of the Sagami trough shows notable meandering whose direction is almost parallel to that of the movement of the PHS east off Oshima Island and almost perpendicular east off Ohmurodashi. At the latter case, thrust structures may trace along the subduction zone at the landward slope of the Sagami trough. Just landward side of the thrust, curious round topographic highs can be observed. They may be mud volcanoes.Topographic features observed around Oshima Island may best be understood in term of the normal and oblique subduction of the Philippine Sea plate along the Sagami Trough.
著者
中井 順二 駒沢 正夫 大久保 泰邦
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.96, no.4, pp.185-200, 1987-08-25 (Released:2009-11-12)
参考文献数
15
被引用文献数
2 2

Bouguer anomaly map of the Kanto district was compiled. On the whole, gravity anomalies are high in Tsukubasan areas, and low in the central parts of the Kanto plain. Negative anomalies in the Boso peninsula and the Tama hills suggest the basement structures of the sedimentary basins. The Kanto mountains district is bounded on the east by steep gradient of gravity anomaly. The above gravity features can be recognized in the map of the upward continuation. The gravity trend along Narashino, Sakura and Omigawa is more clearly indicated in the map of the first derivative, and the tectonic line covered by sediments can be supposed.In the Izu peninsula and the Tanzawa mountains district, gravity anomalies are rather high and low anomaly zone is recognized between these two areas. The northern border part of this zone correspond to Kannawa fault and Kozu-Matuda fault.Aeromagnetic map was compiled with the data obtained by New Energy Development Organization and Geological Survey of Japan.Generally, the Kanto district is surrounded by intense magnetic anomalies with short wavelength which are distributed along Nasu volcanic zone and Fuji volcanic zone.In the central part of the Kanto plain, weak magnetic anomalies are scattered zonally in the E-W direction. According to the geological and well data, this zone seems to be corresponded to Sambagawa metamorphic belt and these anomalies are presumably caused by the basic or ultrabasic rocks.The Hakone mountains district has sharp anomalies with short wavelengths, and the maximum amplitude of the anomalies reachs 900nT. Low anomaly zones are recognized arround this high anomalies. The source of this low anomalies is not clear, but it is of great interest geologically and geophysically.
著者
鵜川 元雄 藤田 英輔 熊谷 貞治
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.111, no.2, pp.277-286, 2002-04-25 (Released:2009-11-12)
参考文献数
13
被引用文献数
6 8

Continuous seismic observations at Iwo-jima, an active volcanic island belonging to the Izu-Ogasawara island arc, have detected remote triggering of microearthquakes in and around the island. The remote triggering at Iwo-jima is a phenomenon of an abrupt increase of microearthquake activity at the time of a passage of seismic waves from a distant large earthquake. We examined seismograms of a total of 21 earthquakes with magnitude larger than 7 and within an epicenter distance of 3000 km from Iwo-jima. Remote triggering phenomena were found at four events during the period from 1980 to 1993 : the 1983 west off Tohoku earthquake, the 1984 southeast off Kyushu earthquake, the 1993 southeast off Hokkaido earthquake, and the 1993 Mariana Island earthquake. The largest epicenter distance among them was 2009 km. The initial times of triggering coincide with the theoretical arrival times of surface waves and successive occurrences of earthquakes continued for 6 to 15 min, suggesting that dynamic stress or strain caused the remote triggering phenomena at Iwo-jima. As a well-developed hydrothermal system is suggested in shallow depths beneath Iwo-jima, volcanic fluids presumably play an important role in remote triggering.