著者
長岡 信治
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.97, no.3, pp.156-169, 1988-06-25 (Released:2009-11-12)
参考文献数
28
被引用文献数
4 2

The Kikai caldera volcano located under water in East China Sea is one of the most gigantic calderas in southern Kyushu. At the caldera, a violent eruption occurred from the submarine vent, at ca. 70-80 ka. The eruption is interpreted to have been phreatomagmatic throughout. Each eruptive phase of the eruption sequence generated its own characteristic deposits. The sequence of the events can be summarized as fallows ; (1) a small phreatomagmatic eruption, which generated the fine grained ash including accretionary lapilli, (2) the catastrophic pyroclastic-flow eruption, which formed a large-scale pyroclastic flow (the Nagase pyroclastic flow), two pyroclastic surges (Nishinoomote-1 member : Ns-1, Nishinoomote-3 member : Ns-3), and a wide-spread co-ignimbrite ash fall (Nishinoomote-2 member : Ns-2).The Nagase pyroclastic flow came down from the rim of the caldera, and entered the sea. Then, the flow body, which included a large amount of large pumice blocks and heavy lithic fragments, was disintegrated as gas-particle flow by violent phreatomagmatic explosions, or continued subaqueously as water-supported mass flow. Dilute and fine-particle-rich pyroclastic surges, probably with a density much less than that of water, 1.0 g/cm3, generated off the top or head of subaerial Nagase pyroclastic flow. They could cross on the smooth surface of the sea, becoming water-cooled, vaporish and depleted in large clasts which dropped into the sea. Eventually, the cool and wet pyroclastic surges attacked the islands around the caldera, and deposited as Ns-1 and Ns-3.Ns-2 co-ignimbrite ash fall, composing of glass shards were generated from the upper convective part of the eruption column of the Nagase pyroclastic flow. Included accretionary lapilli indicate that the eruption column was very moisture because of much sea water flash-out subaerially for very violent explosions from the submarine vent. Ns-2 is probably correlated with the Kikai-Tozurahara ash which was found in central Japan more than 500 km off the source.
著者
米倉 伸之
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.77, no.1, pp.1-23, 1968-02-25 (Released:2009-11-12)
参考文献数
34
被引用文献数
14 10

The Kii peninsula, located on the Pacific coast of central Honshu, is fringed with coastal terraces in its southern part, while its east and west coast are characterized by ria shorelines. The purposes of this paper are to clarify coastal development of the peninsula, especially the history of vertical changes in sea level in its coastal region, and secondly, to examine the relation between the mode of Quaternary crustal movement (especially its vertical component) and recent crustal movement associated with contemporary seismic activity.Coastal terraces and an interpretation of their developmentCoastal terraces, developing along the southern coast of the Kii peninsula (Fig. 2), are mostly rocky abraded terraces covered with thin marine beds. In the northern part of the east coast (north of Shingu) and at the mouths of some rivers, however, some marine terraces are composed of marine sediments overlying fluvial beds. The coastal terraces are classified into two levels of the high and low terraces in the surroundings of Shingu, where they are typically developing (Fig. 4). Both levels of the terraces are further subdivided into some sublevels which are dentoted by H1, H2, H3 and H4 in the high terraces and L1, L2 and L3 in the low terraces respectively in the descending order. The height of each sublevels at Shingu are 125, 113, 95, 61, 49 and 39 meters above sea level. The highest terraces H1 are dissected into narrow hill ridges, overlaid by round gravel beds more than 20 meters thick. The H2, H3 and H4 terraces are rocky abraded ones. The most extensively developing terraces Li are composed of marine sediments underlaid by fluvial beds more than 40 meters in thickness. The L2 and L3 terraces are marine or fluvial terraces which have been formed cutting down the L1 terraces. Alluvial plains (denoted by A) do not so extensively develop along the coast, except at the mouths of the rivers. Judging from some boring data, alluvial formations at the mouths of the rivers are more than 30 meters thick (Fig. 3). From these facts it is concluded that the Hi and Li terraces and alluvial plains are depositional surfaces composed of thick marine and fluvial deposits, which are filling submerged fluvial valleys and, therefore, that the coastal region, being interrupted by temporary submergence, has been emerged in precess of the formation of the coastal terraces. The other terraces H2, H3, H4, L2 and L3 have no feature indicating submergence and, therefore, are considered to have been formed in process of emergence. Judging from the thickness of deposits, amplitude of submergence during the later two periods is considered to be greater than that during the previous period.As to the process of formation of alluvial plains, rapid submergence of this period is regarded in another regions as being largely due to eustatic rise in sea level caused by world deglaciation from the results of the researches on submerged topography, marine topography, succession of alluvial formations, absolute age determination of alluvial marine beds, climatic changes and etc. The process of formation of alluvial plains in the Kii peninsula is inferred to be the same as in another regions from the submerged fluvial valleys and the thickness of alluvial formation and, therefore, the submergence during the last period was caused by eustatic rise in sea level.
著者
高木 圭介 青池 寛 小山 真人
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.102, no.3, pp.252-263, 1993-06-25 (Released:2010-10-13)
参考文献数
42
被引用文献数
2 2

A synthesis of geological, geomorphological, and petrological data was made to reconstruct the tectonic evolution of the collision zone between the Honshu and Izu-Bonin Arcs during the middle to late Miocene time. The collision zone is composed of four allochthonous terranes, which are overlaid by syn-collisional trough-filling deposits. The four terranes, Izu, Tanzawa, Misaka, and Koma Terranes, were originated in the Izu-Bonin volcanic arc and have in turn collided with and accreted into the Honshu Arc at about 1, 5-3, 11, and 15 Ma, respectively. The geomorphology of the northern part of the Izu-Bonin Arc is characterized by two N-S trending ridges, Shichito-Iwojima and Nishi-Shichito Ridges, and Nishinoshima Trough between the two ridges. We interpret the Nishi-Shichito Ridge as a remnant volcanic arc of early Miocene age. The Nishi-Shichito Ridge was left by backarc rifting, which occurred between the Nishi-Shichito and Shichito-Iwojima Ridges during 15-10 Ma. The backarc rifting resulted in the increase of downdip angle of the Pacific plate slab and the eastward migration of the Izu-Bonin volcanic front. The rifting also generated along-arc compressional strain, which caused the intraarc deformation of the Shichito-Iwojima Ridge at about 10 Ma. This interpretation well explains the geologic history of each allochthonous terrane in the collision zone between the Honshu and Izu-Bonin Arcs as well as the present distribution of the allochthonous terranes.
著者
片尾 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.248-255, 2002-04-25 (Released:2009-11-12)
参考文献数
9
被引用文献数
1 1

In the Tamba Plateau, an earthquake swarm area in the Kinki district, Central Japan, seismicity was activated just after the Hyogo-ken Nanbu (Kobe) Earthquake (M7.3), which occurred in an adjacent area in 1995. We found that micro-earthquake activities in the Tamba Plateau corresponded to moon phase. Occurrences of micro-earthquakes increased after a new moon and a full moon during 1995 and 1996. Before 1995, such a correlation could not be found. The present study suggests a possibility that the stress change caused by the Hyogoken Nanbu Earthquake made seismicity in the Tamba Plateau sensitive to tidal forces.
著者
安藤 寿男
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.99, no.3, pp.247-262, 1990-06-25 (Released:2009-11-12)
参考文献数
49
被引用文献数
1

The depositional sequence concept was established in newly developed sequence stratigraphy, as an unconformity-bounded stratigraphic unit formed during one complete sea-level cycle. This paper reviews general meanings of “sequence”, the definition of depositional sequences, their hierarchial patterns and recognition, and sequence boundaries problems, from a viewpoint of sedimentary geology based on outcrops and bore-hole samples.Though the word, “sequence” has many meanings generally applied to successive geologic events and processes in chronologic order, a depositional sequence is defined in a special sense, as “a relatively conformable succession of genetically related strata bounded at its top and base by unconformities and their correlative conformities”. The depositional sequence as one of hierarchial transgressive and regressive units (T-R units), has the first- to forth-order operational units, that is, the megasequence, supersequence, sequence and parasequence in descending order. A sequence boundary with a significant hiatus (=unconformity) is formed by subaerial exposure, concurrent subaerial erosion and partly submarine erosion during eustatic falls or low-stand sea level.The latter half of this paper emphasizes the difference between sequence boundaries and ravinement surfaces. The ravinement surface formed as one of diastems or “transgressive surfaces”, is an erosional surface by shoreface retreat during the following transgression after a sea-level fall. In general, it is lithologically more distinct than the underlying sequence boundary. The right recognition of the difference leads correct reconstructions of sedimentary history.
著者
橋本 学
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.298-307, 2002-04-25 (Released:2009-11-12)
参考文献数
20

We attempt to simulate the activity of earthquakes of moderate to large sizes in and around the Japanese islands using a block and fault model, in which slip deficit rates were derived from triangulation and trilateration data covering one-hundred years. The original block and fault model has 104 faults. However, the original fault size is too large to simulate moderate earthquakes. Therefore, we divide each modeled fault into 5 × 5 elements to generate earthquakes as small as M5. In total there are 2600 elements. We assume stress accumulates according to the estimated slip deficit rates. Interaction between fault elements is represented by changes in Coulomb Failure Function (hereafter Δ CFF) induced by the movement of other faults that are determined by the geometrical relationship and the direction of slip deficit rates. When stress reaches the threshold level, accumulated CFF is released by a forward slip and redistributed to surrounding faults according to the CFF changes calculated above. If the redistribution of CFF induces the next rupture, the same process is repeated until there are no more rupturing elements. We assume rupture thresholds to be 2.5 MPa for interplate boundaries and 10 MPa for inland faults, respectively. We simulate seismicity for 10000 years with a time step of 1 year.In this simulation large events that rupture almost all elements of a fault rarely occur. This suggests that strain rates derived from geological data or historical earthquake catalog might be underestimated. Simulated seismicity does not satisfactorily fit the GutenbergRichter's law, because moderate events occur more frequently than small or large events. This suggests that we have to incorporate heterogeneity in the rupture threshold or the size of elements on a fault plane. The correlation between interplate earthquakes along the Nankai trough and inland events in southwest Japan is not clear, but there seems to be a complementary relationship in activities between both regions. Migration of large events along the Nankai trough is occasionally seen in this simulation, but its direction is different from time to time.
著者
古田 俊夫 中西 正男
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.99, no.5, pp.490-506, 1990-10-25 (Released:2010-10-13)
参考文献数
59

The discovery of the magnetic anomaly lineations that can give ages of ocean floor is a very important role for establishment of the plate tectonics theory. The magnetic anomaly lineations also give us information of a history of movements of the oceanic plates. However, the origin of magnetic anomaly lineations still been obscured, that is, we can not clearly answer for the following questions: How thick is the source layer of magnetic anomaly lineations? How strong is the intensity of the magnetic source layer? In this paper we examined the relevant information concerning the magnetization of the oceanic crust from studies of observed marine magnetic anomalies and from rock -magnetism of oceanic basalts to get a goal of these questions in this paper.The skewness parameter that is deduced by precise magnetic anomaly lineations is important to identify marine magnetic anomalies. The magnetic polarity transition width is also important to do, though the parameter associated with this transition width has not almost utilized in the previous works. The anomalous skewness and the skewness discrepancy are often observed over the oceans. These observations might be explained not by a single-layer model but a two-layer model for magnetic source layer. The polarity transition width is defined the width which 95.4 % of the change from normal to reversed polarity occurs within. This width increases monotonically with spreading rates of ridges and/or with ages of ocean floors. This increasing is considered to be a manifestation of a more complicated crustal source consisting of two discrete layers. The analysis of the skewness parameter and transition width strongly supports that the sourc elayer of marine magnetic anomalies has a two-layer structure. The upper layer, consisting of surface lava flows of layer 2 A and possibly the sheeted dike complex, hasdi stinct and approximately vertical magnetic in the vicinity of opposite magn etized region boundaries. The lower layer, consisting of intrusive and gabbroic layers, has the boundaries gradually sloping downward away from the spreading center.Many detailed survey are carried out to reveal the structure of magnetic source layer by the multi narrow and the deep-towed magnetometer near active ridges. Inversion of magnetic layer using results of detailed surveys concluded that the magnetic source layer near the active ridges is less than 1 km in thickness. The polarity transition width of the relatively young layer is narrower than that of older oceanic floor, and the magnetic intensity of the relatively young layers higher (more than 10A/m) than that of older one. These conclusion indicate that the magnetic source layer near the active ridg es consists of a single layer structure. It is thought that the magnetic source layer grows with ages asoceanic crust by results of analysis of skewness and polarity transition width and inversion of magnetic source layer near active ridges.Several previous paleomagnetic studies indicate that intensity of natural remanent magnetization (NRM) of basaltic rocks composing the ocean crust rapidly decreases with ages in the past 10 to 20 Ma, and gradually increases older one. This change in NRM intensity is roughly proportional to the changes in intensity of saturation magnetization of the rocks and possibly due to sea-water alteration (low-temperature oxidation) of the primary ferromagnetic minerals contained in the rocks. NRM of the oceanic rocks is initially thermoremanent magnetization acquired at the time of formation of the oceanic crust. In accordance with progressive oxidation, fraction of TRM to bulk intensity decreases, while that of the secondary magnetization increases.
著者
盛谷 智之
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.87, no.4, pp.181-200, 1978-08-25 (Released:2009-11-12)
参考文献数
40

Ocean floor manganese nodules have attracted worldwide attention as the possible future resources. However, many problems in the distribution and origin of the nodules remain still unsolved, because of the lack in reliable seabed information. Thus, several leading ocean-oriented countries are now carrying out, or have plans about the systematic research programs on manganese nodules, including both prospectings mainly by industry groups, and scientific studies by the governmental and university groups, in which the results of the latter scientific studies are expected to serve as the guideline for the former prospectings. The representative programs are as follows : In Japan, “Basic researches on the deep sea mineral resources” is being conducted by the Geological Survey of Japan, for the northern Central Pacific Basin, using R/V Hakurei-Maru (1, 821 tonnage), with participation of the NIPR (National Institute for Pollution and Resources) in each cruise on their own program, “Technological study on the development of deep sea mineral resources”. Also, the industries group, DOMA (Deep Ocean Minerals Association), is engaged in a program of “Development of new prospecting technology for manganese nodule deposits”, with the objective areas south of Hawaii Islands of the Pacific.In the United States, there are three categories of research programs, being actively conducted by the groups of universities, Federal organizations, and industries respectively. The first is “MANOP (Manganese Nodule Program) of NSF (National Science Foundation) Seabed Assesment Program, carried out by the interuniversities group. This is now concentrated on the detailed studies of the sea floor to obtain its real physical, geochemical and biological informations by means of the in-situ measurement instruments, deployed in each representative area of different sedimentological condition of the Pacific. The second is “DOMES (Deep Ocean Mining Environmental Studies)” of the Office of the Marine Minerals, NOAA (National Oceanic and Atmospheric Administration), in which the research groups from the Federal organizations such as NOAA and U.S. Geological Survey are in charge of conducting its geological aspects. The third are the programs of the industry groups related to the commercial development of manganese nodules, including prospecting, mining and processing. Four major international consortiums, Deepsea Ventures Group (OMA, Ocean Mining Association), INCO Group (OMI, Ocean Management Inc.), Kennecott Group (Kennecott Exploration), and Lockeed Gr oup (OMC, Ocean Minerals Co.) are now developing their programs, aiming at mining operation tests in the Pacific as immediate objectives.In West Germany, a program, “Researches on manganese nodules”, is actively promoted under the coordination and sponcering of the Ministry of Research and Technology, with participation of the Federal Institute for Geoscience and Natural Resources, universities and industries association (AMR, Arbeitsgemeinschaft meerestechnisch gewinbare Rostoffe), using R/V Valdivia (1, 317 tonnage), for the objective areas between Clarion and Clipperton fracture zones of the Pacific.In France, CNEXO (Centre National pour L'Exploitation des Oceans) and its research group, AFERNOD (Association Francais d'Étude et de Recherche des Nodules océaniques) are carrying out “Rsearches on polymetallic (manganese) nodules”, mostly for the southern Central Pacific Basin.In England, a research group has done recently some research cruises on the manganese nodules and metalliferous sediments for the Indian Ocean.
著者
小泉 武栄
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.98, no.1, pp.73-81, 1989-02-25 (Released:2009-11-12)
参考文献数
7
著者
斎藤 実篤
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.100, no.4, pp.616-627, 1991-08-25 (Released:2010-11-18)
参考文献数
45

Structural characteristics of Recent forearc regions can be divided into three types (shelf type, terrace type, slope type) in terms of the morphology of forearc basins and the topography of basements. The differences in forearc structure seems to correspond with the difference in deformation style of forearc lithospher by subduction. Supply and/or subduction of sediments at trench are secondary factor of forearc deformation. High plate convergence rate leads to widely uplifting of island arc less subsidence of forearc basin (shelf type). Conversely, low plate convergence rate leads to one-sided subsidence of trench side and increasing in undulation of forearc topography (slope type).Sediments from forearc basin can reconstruct the subsidence rate of forearc basin and the vertical movement of island arc and by use of planktonic microfossils and benthic for eminifera which determine the changes in sedimentation rate and paleobathymetry. Thus, forearc basins are tectonic recorder to estimete the arc evolution and the change in subduction styles.
著者
安田 喜憲
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.94, no.7, pp.586-594, 1986-01-25 (Released:2009-11-12)
参考文献数
59
被引用文献数
1 1
著者
鈴木 康弘 渡辺 満久 廣内 大助
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.113, no.6, pp.861-870, 2004-12-25 (Released:2009-11-12)
参考文献数
9
被引用文献数
8 7

The surface faults of the Mid Niigata prefecture Earthquake in 2004 appeared along preexisting active fault traces (lines) of the Obirou fault, as well as the northern part of the western marginal fault of the Muikamachi basin. The vertical displacement of surface faults are within 30 cm, and the three areas with distinct faults can be summarized as follows. At Obirou, in Hirokami village (Uonuma city), the road surface is vertically displaced by approximately 30 cm just along the active fault line, and the waterways located on the fault line are compressed and broken. At Shitakura, in the Horinouchi town (Uonuma city), the surfaces of both the highway and the old road are cracked and vertically displaced by approximately 20 cm. At Aoshima, in Koide town (Uonuma city), the surface faults clearly extend over 500 m in length. Paddy fields, gardens, waterways, roads, and houses are vertically displaced by approximately 20 cm. The facts mentioned above indicate that the Mid Niigata prefecture Earthquake in 2004 (probably the main shock) was caused by the rejuvenation of these active faults.
著者
小山 真人
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.104, no.1, pp.45-68, 1995-02-25 (Released:2010-11-18)
参考文献数
84
被引用文献数
3 2

A multi-disciplinary synthesis was made on the tectonics and seismotectonics in the northwestern Sagami Bay and adjacent areas, where the plate boundary between the Izu-Bonin and Northeast Japan arcs passes and historical M7-8 inter-and intraplate earthquakes frequentlyhave occurred.In a convergent boundary between plates, a displacement between plates is usually accommodatedalong a broad belt of active imbricated thrusts and folds, which develop in a wedge ofan overriding plate. In such a situation, we can easily identify two kinds of plate boundaries: a deformation front and a subduction entrance. The deformation front is a line connecting thethrusts or folds nearest to a trench/trough axis. The subduction entrance is a line connecting theedges of trench/trough filling coarse deposits, which cover a surface of a subducting plate. In Sagami Bay, the deformation front in the overriding Northeast Japan arc is estimated to belocated along the northern and eastern edges of the Sagami Basin, while the subductionentrance of the subducting Izu-Bonin arc is estimated to be located along the southern edge of the Basin.Many geomorphological, geological, geophysical observations need and support the existenceof the West Sagami Bay Fracture (WSBF), an intraplate propagating fracture between the Izu-Bonin outer and inner arcs, proposed by Ishibashi (1988), whereas these observations require slightmodification of the estimated geometry around the WSBF. Frequent dike intrusions in theHigashi Izu monogenetic volcano field cause spreading of the upper crust and probably move the Manazuru “microplate”(MNZ) to the NNE direction against the Izu block. The Tanna-Hirayama tectonic line is proposed as a transform fault, which accomodates the relativemovement between the MNZ and the Izu block. In the MNZ hypothesis, the Kozu-Matsuda fault, located to the northeast of the Izu Peninsula, is interpreted as a deformation front between theoverriding Northeast Japan arc and the MNZ, which is buoyantly subducting beneath the Oiso Hilland Tanzawa Mountains. The proposed new geometry around the WSBF and the hypothesis of the MNZ can explain (1) the crustal structure and tectonic features around the Kozu-Matsudafault, (2) tectonic implication of the “Oiso-type” earthquakes, which are expected to displacethe Kozu-Matsuda fault periodically, and (3) the rapid upheaval of the Tanzawa Mountains since 1Ma.
著者
吉井 敏尅
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.94, no.7, pp.648-655, 1986-01-25 (Released:2009-11-12)
参考文献数
8
著者
湯田 ミノリ 伊藤 悟 内田 均 木津 吉永 伊東 純也
出版者
Tokyo Geographical Society
雑誌
地學雜誌 (ISSN:0022135X)
巻号頁・発行日
vol.117, no.2, pp.341-353, 2008-04-25
参考文献数
21
被引用文献数
2 1

The use of GIS in education in Japan has not yet been widely diffused, although the computer and network environments of schools have been improved, and teachers have already recognized the characteristics and advantages of this tool in education.<br> Nowadays, GIS has been intergraded into many aspects of our lives. Mobile phones are also basic tools in our daily lives. A GIS application that runs on cellular phones would be helpful in school education.<br> From this point of view, the authors have developed a system called Cellular Phone GIS including a GIS application for mobile phone (hereinafter Cell Phone GIS Application) and its web-based GIS viewer application for PC using Google maps (hereinafter PC viewer), and carried out fieldwork at an upper secondary school using these tools. Data can be input and edited outdoors with the Cell Phone GIS Application. These data can be viewed on both cellular phones and personal computers via the Internet. Students carried out a land use survey in the area around the school with the Cell Phone GIS Application, and examined and presented the results using the PC viewer in class.<br> Students participated actively in the fieldwork with the cellular phone. Through experience of the survey with the tool, they found many new things and learned to adopt multi-dimensional points of view and ways of thinking. Also, this project generated more interest among students in geography classes.<br> The Cell Phone GIS Application provided high school students with a feeling of accomplishment from the fieldwork. Meanwhile, this tool and PC viewer minimized work after fieldwork because users do not have to input and aggregate data again. Therefore, teachers and students can use course hours efficiently. The Cellular Phone GIS can provide an environment in which students are able to receive educational effects from fieldwork.
著者
関谷 溥
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.79, no.3, pp.175-180, 1970-06-25 (Released:2009-11-12)
参考文献数
7

September 1, 1923, southern Kanto was devastated by a severe earthquake. The main shock of the events, which occurred at 11 h 58 m (JST), was felt all over Japan. The highest seismic intensity is 7 in JMA scale at the southern part of Kanagawa, Tokyo and Chiba prefectures. The epicenter is located on the Sagami-nada. Parameters of the main shock given by the Japan Meteorological Agency are as follows ;origin time : 11 h 58 m, September 1, 1923.epicenter : 35° 20' N, 139° 20' Emagnitude : 7.9The earthquake was accompanied by many fore- and after-shocks. The foreshocks occurred in Kashima-nada. It seems that the foreshock activities began in May 1923 and became vigorous in June 1923 before the occurrence of the main shock.The aftershocks occurred in Kanagawa, Yamanashi, Saitama, Chiba, Ibaraki prefectures and near seas, and the general trend in decrease of daily number of the aftershocks can be explained by Omori's generalized formula.
著者
田村 芳彦
出版者
Tokyo Geographical Society
雑誌
地学雑誌 (ISSN:0022135X)
巻号頁・発行日
vol.112, no.5, pp.781-793, 2003-10-25 (Released:2009-11-12)
参考文献数
25
被引用文献数
1 1

Mantle melting and production of magmas in NE Japan may be controlled by locally developed hot regions within the mantle wedge that form inclined, 50 km-wide fingers. In this case, are these hot fingers chemically and/or isotopically different from the host mantle wedge? Forty-four Quaternary volcanoes in NE Japan have been reviewed to evaluate twodimensional strontium isotopic variations, and to infer 87Sr/86Sr contours of the source mantle. The isotopic composition of magma source materials at depth is found to have little relationship with slab depth, suggesting that mantle heterogeneity was established before the flux of fluid released from the subducting slab reached the magma source regions. On the other hand, Miocene Japan Sea back-arc Yamato basin basalts have the same isotopic variation as the Quaternary volcanic arc. Cousens et al. (1994) suggested the possibility that partial melts of sediments, forming at a depth of >200 km may mix with mantle wedge material (87Sr/86Sr0.703), resulting in a magma source component with enriched 87Sr/86Sr of0.705. I suggest that after the cessation of Yamato basin rifting, a MORB-like mantle source (87Sr/86Sr0.703) in the mantle wedge below the Quaternary NE Japan arc was replenished by a fertile mantle material (87Sr/86Sr0.705) through convection induced by the subducting lithosphere. On its way to the shallower mantle wedge (<150 km), the fertile mantle material changes shape from a hot sheet to hot fingers, for reasons not yet fully understood. Thus, the hot fingers, with 87Sr/86Sr of0.705, extend from150 km below the back-arc region towards the shallower mantle (50 km) beneath the volcanic front. A conveyor-like return flow is interpreted to carry the remnants of these fingers to depth, resulting in greater amounts of fertile material being incorporated in diapirs beneath the volcanic front, and smaller amounts incorporated in areas behind the front.