著者

高木 菊三郎

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.52, no.12, pp.577-588, 1940-12-15 (Released:2010-10-13)

被引用文献数

1 1

著者

速水 格

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.83, no.3, pp.157-171, 1974-06-25 (Released:2009-11-12)

参考文献数

37

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The phylogeny and mode of evolution of a Liassic oyster Gryphaea have been studied by many European and American outstanding evolutionists since TRUEMAN's (1922) classical interpretation of its morphological change. For these fifty years many useful ideas and methods in the study of morphogenesis, biometry, autoecology and functional morphology were introduced into paleontology through this material. In this report some of these works are reviewed and evaluated, and what is suggested to general evolutionary paleontology is considered.

著者

増田 富士雄 藤原 治 酒井 哲弥 荒谷 忠

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.110, no.5, pp.650-664, 2001-10-25 (Released:2009-11-12)

参考文献数

48

被引用文献数

14 17

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Relative sea-level changes and variations in shoreline progradation rate over the past 6000 years are elucidated from the elevations of the beach deposits, their 14C ages, and the geographic position of each locality on the Kujukuri strand plain, Pacific coast of the Boso Peninsula, central Japan. These past sea-levels were +4 to +6 m above the modern sea-level (high-stand stage) at 6000 to 5300 calendar years B.P., +3 to + 4.5 m (stable stage) at 5000 to 3500 years B.P., declined from +1.2-+ 3.4 m to + 0.5-+ 2.1 m (falling stage) at 3300 to 2250 years B.P., and were-1.5--1 m to + 1 m (stable stage) from 1650 to 250 years B.P. ; sea level is 0 m at present. The detailed changes revealed by this new method strongly imply the existence of several rapid uplifts (0.4 to 1.2 m per event) at 5100 to 5500 years B.P., 3400 years B.P., 2400 years B.P., 1650 years B.P., and 0-250 years B.P. The uplifts were co-seismic, because the speed of occurrence seems to have been high, and the events are generally associated with so-called “tsunami deposits.” Co-seismic uplift in this region has not been reported previously from historical records or geological evidence.

著者

神保 小虎

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.2, no.3, pp.116-121, 1890-03-25 (Released:2010-10-13)

著者

奥山 喜久夫

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.775-781, 1989-12-05 (Released:2009-11-12)

参考文献数

6

被引用文献数

1

著者

平賀 岳彦

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.108, no.2, pp.110-121, 1999-04-25 (Released:2009-11-12)

参考文献数

27

被引用文献数

3 3

---

Grain boundaries in metamorphic rocks were obsereved by high-resolution transmission electron microscopy. Most of the boundaries have no distinct secondary phases at a nanometer scale. Not only between the same kind of minerals but also between different minerals, mineral grains are directly connected at their boundaries, showing that the structual width of the grain boundaries is less than 0.5nm. Grain boundaries with spheroidal voids are observed in the boundaries. It can be concluded that this type of grain boundary is formed by the healing of microcracks. Low-index plane boundaries are well-developed, showing that this type of boundary has low grain boundary energy. Local structural modification, indicating the segregation of impurities, in grain boundaries is observed. The present results show that the actual width of the grain boundary in rocks is too small for the grain boundary diffusion to be as the dominant transport mechanism.

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

古川 雅英

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.102, no.7, pp.868-877, 1993-12-25 (Released:2009-11-12)

参考文献数

25

被引用文献数

15 12

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Extensive field survey on natural radiation was conducted from 1967 to 1991 over Japan Islands. The average level of the islands was found to be 79.7 nGy/h. In general, Southwest Japan has higher level compared with the Northeast one. From the data obtained at 1304 sites of the survey, a contour map of natural radiation level in the islands was made by simple interpolations, and a geological interpretation on the distribution of the level was done by comparing the contour map with geological information. The islands were divided into four areas according to whether the level is higher or lower than the average level. The boundaries between these areas were found almost exactly coincide with major geo-tectonic lines. This feature suggests that natural radiation level is controlled mainly by distribution of granitic and volcanic rocks.

著者

遠藤 毅

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.116, no.5, pp.593-626, 2007-10-25 (Released:2009-11-12)

参考文献数

47

被引用文献数

2 4

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The Tokyo Lowland situated in the eastern part of Tokyo Metropolis was composed of extensive paddy fields until the end of the Edo era in 1868, then became a major industrial area from the beginning of the Meiji era due to the industrialization project of the early Meiji Government. The purpose of this paper is to clarify the changes in the industrial area in the Lowland since the beginning of the Meiji era, and to investigate the recent use of former factory sites following the relocation and closing of factories since around 1960.In the early part of the Meiji era, the industrial area only occupied the western part of the Lowland, the estuary of the Sumida River, and the junction of the Shakujii River and the Sumida River, and the main factories belonged to the shipbuilding and military industries. Then, over the period of two major wars, Shino-Japanese War (1894-1895) and Russio-Japanese War (1904-1905), the industrial area continued to be enlarged. At the end of the Meiji era, in 1912, many factories were developed along canals throughout the Kohtoh Area. Furthermore, the industrial area was developed throughout the Lowland during the period straddling World War I (1914-1918) and World War II (1941-1945).Although the industrial activity in the Lowland stopped for a several years from the end of the World War II in 1945, it started again with the outbreak of the Korean War, in 1950, and advanced remarkably mainly in the heavy and chemical sectors.However, this extreme industrial development brought to the Lowland the overpopulation and the public nuisances such as air-pollution, noise, and vibration from plants and manufacturing sites. To reduce these problems, the National Government and the Tokyo Metropolitan Government took such measurements as restricting the construction of new factories and strengthening regulations on the operations of factories. The regulations to prevent public nuisances were very costly to factory operators.As a result, the number of factories in the Lowland stopped increasing around 1960, and the number decreased rapidly from 1961 to 1975 because of relocation to other countries or ceasing manufacturing operations.Many former factory sites have been converted to other uses, mainly facilities for citizens such as condominiums, general residential areas, parking lots, schools, and parks. In particular, mainly at the sites of former metal industry, timber, and chemical industries, conversion to residential quarters has been remarkable. Among the new uses changed from former factory sites, the residential use occupies more than 50%.However, the change in land use from former factory sites to a residential quarter has brought problems throughout the Lowland area such the blocking of sunlight to existing houses due to the construction of housing complexes, and soil pollution caused by past industry activity. To reduce the problem of the blocking of sunlight to existing houses, each ward government has regulated the construction of housing complexes, and to reduce the problem of soil pollution, the National Government has promulgated the Soil Contamination Countermeasures Law.In addition to these problems, the extension of the subway network to the Lowland area in recent years has promoted an expansion of the residential quarter. This rapid expansion has led to a shortage of public institutions such as elementary and junior high schools. Furthermore, most of the Lowland area is permanently below sea level due to land subsidence, although the land is protected by extensive dikes and drainage systems to prevent disasters related to high tides and flooding.It is essential to solve these problems related to land use in this Lowland area.

著者

大崎 晃

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.4, pp.615-631, 2010-08-25 (Released:2010-11-08)

参考文献数

43

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This study examines the American whaling industry, which was based in New England and dominated most of the Western oil market in the 18th and 19th centuries. Previous studies by the author examined why the industry was attracted to this region, highlighting the favorable economic environment of the area, which allowed investors to accumulate the necessary funds to launch whaling ventures. Each investment consortium that provided funds to whaling vessels organized limited partnerships with individual vessels. This enabled consortiums to collect funds from a variety of stakeholders other than just businessmen involved in whaling and their families. Crewmembers of the vessel, reflecting the multi-racial society of the United States, were also free to select employers without relying on a conventional employer-employee relationship. Taken together, these factors led to New England attracting funds and labor from both within the region and further afield, and developing into the whaling center of the United States. An additional reason for New England's development as a whaling base was the global expansion of the industry to cover the Atlantic, Pacific, and Indian oceans, with only the Southern Ocean not affected by this growth. Crewmembers were also paid from a ship's profits using the lay system. This payment by piece ratio ensured a minimum payment to crewmembers while guaranteeing a return for investors. The hiring of native islanders as part-time laborers in the whalers' areas of operation also kept expenditure to a minimum, with all of these factors contributing to investors' profits. Against this background, the whaling industry prospered and profited up until the 1850s. In the 1870s, however, the industry started to decline as whale resources decreased and the price of whale oil fell as a result of increased petroleum production. This study examines the details of how whaling voyages were managed during this period and how New England's whaling industry underwent a rapid transformation into a mechanized cotton industry with the owners of whaling vessels switching their investments to the cotton industry at New Bedford, the center of the whaling industry in New England. The rapid change from industrial whaling to the mechanical cotton industry in New England was made possible with the abundant funds accumulated by the whaling industry. The reestablishment of consortiums progressed smoothly as limited partnerships already existed and the labor market adapted efficiently from supplying whaling vessels to supplying female workers for the cotton industry due to the presence of non-conventional employment rules. We can see, therefore, in this region the development from one industry to another was controlled only by market-based mechanisms of American management. This is in sharp contrast to the Japanese fishing industry, which developed from a manufacturing fishery through technological innovation and utilizing community manpower or industrial power as the only motive force under limited investment and labor market conditions.

著者

海野 進

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.103, no.5, pp.498-521, 1994-10-25 (Released:2009-11-12)

参考文献数

65

被引用文献数

1 1

---

Recent progress in studies on magma plubming system beneath mid-ocean ridges is reviewed. Mid-ocean ridges are not continuous, homogeneous series of crests, but are segmented by various topographical offsets of several orders of magnitude. Such topographical ridge segmentation is a surface manifestation of along-strike variations in the pattern of mantle convection and the supply of magma.Upwelling of the mantle material beneath the spreading center takes the form of a diapir, rather than sheet-like flow. Such diapiric upwelling has an along-axis dimension of several hundred to several tens of kilometers, which corresponds to first-and second-order segments, but some are as small as the scale of third-order segments. Where magma upwelling is intense, hot lithosphere and high magmatic productivity produces thick crusts, resulting in low gravity anomaly.When the spreading rate is low, mantle flow is cooled on route to the bottom of the lithosphere and ceases to melt at depths. Thus magmas produced beneath slow spreading ridges have low degrees of melting. On the contrary, fast spreading ridges are followed by intense mantle upwelling which melts significantly the mantle column from deeper part of the upper mantle up to just beneath the crust.Magmatic budget is so large at fast spreading ridges as to maintain large, steady-state magma chambers along the ridge axes with extensions comparable to second-and third-order segments. Such magma chambers have a very small melt pocket one to two kilometers wide and up to several hundred meters thick, underlain by a large mush of crystals and melt. Cumulate layers on the top of the mush sink and deform to S-shaped or downwarping layers as seen in large layered plutonic bodies in some ophiolites. However, low spreading ridges do not have enough supply of magma but only possess short-lived small magma chambers consisting of crystal mush.Volcanic landforms consist of major topographic features associated with small volcanic edifices : the former is a large shieled volcano several tens of kilometers long and is probably formed by episodic eruptions every several tens of thousand years ; the latter is a small conical lava cone comprising pillow flows which is constructed during a short-period eruption but some are active as long as 1.8 million years which are unequivocally central volcanoes.Rifting episodes at spreading axes depend on the spreading rate and the width of the volcanic zone. Slow spreading ridges such as Iceland have rifting every 100 years, but fast spreading ridges such as East Pacific Rise rift every year or two. Such rifting episodes continue for several years and are associated with multiple dikes with an injection interval of a month up to a year.Diapiric mantle upwelling, structure of magma chambers and volcanic landforms seen in mid-ocean ridges resemble those in some large ophiolites such as the Troodos, Cyprus and the Semail, Oman, and post-Teritary volcanism in Iceland.

著者

瀬野 徹三 丸山 茂徳

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.141-155, 1985-06-25 (Released:2010-10-13)

参考文献数

85

被引用文献数

1 3

---

The models on the tectonic evolution of the Philippine Sea can be divided mainly into two:(1) entrapment of a normal ocean at 43Ma for the West Philippine Basin formation and back-arc basin formations caused by successive landward retreats of the upper Philippine Sea plate, and (2) back-arc basin formations caused by successive seaward retreats of the trench axis. These models are discussed on the basis of the presently available data.The island arc type volcanism at the northern half of the Palau-Kyushu Ridge since 48Ma, and the age-depth relation and chemistry of basalts at Sites 291 and 292 in the West Philippine Basin favor the back-arc spreading origin for this basin. In contrast, the chemistry of basalts at Site 447 and the large distance between the Central Basin Ridge and the northern Palau-Kyushu Ridge favor the entrapment origin for this basin. It is likely that the West Philippine Basin has formed under the unique situation of the ocean basin formation, i.e., with subduction at its northern margin and a spreading center far from this subduction zone.Subduction or collision of the aseismic ridges in and around the Philippine Sea is discussed in terms of the crustal structure of the ridges, seismicity, and onland geology. It is found that all these aseismic ridges except for Izu which has a thick 20-30km crust have been subducted, although many of them have formed a cusp or discontinuity at the trench. Elucidating the times of the bending of the Cretaceous-Paleogene terranes in central Honshu and in Kyushu is critical to discriminate the two models for the Shikoku Basin formation, i. e., the fixed trench model and the trench retreating model.

著者

笠原 順三 佐藤 利典

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.106, no.4, pp.557-566, 1997-08-25 (Released:2009-11-12)

参考文献数

17

被引用文献数

2

---

The 1995 Amami-Oshima-Kinkai Earthquake occurred near the Nansei-Shoto Trench where the upheaval area of the Philippines Sea plate subducts beneath the Nansei-Shoto islands. The main shock was MJMA 6.6 and its largest aftershock was MJMA 6.5. The aftershock distribution for these two events by Yamada et al. (1996) corresponds to two distinct and nearly vertical fault zones. The focal mechanisms obtained by Kikuchi (1996) are consistent to the aftershock distribution.The authors propose that the seamount found beneath the trench-continental-slope indirectly triggered this earthquake activity. If a subducting oceanic plate is normal oceanic denser than an overriding island arc, the oceanic plate should be faulted near vertically priori to the plate subduction by horizontally tensional force due to plat bending. On the other hand, an oceanic plate with seamounts or an oceanic plateau lighter than a normal oceanic plate, might resist to plate subduction due to its small density and delaying normal faultings might occur in the subducting oceanic plate. The delaying normal faultings between a subducting seamount and a preceding normal portion of the oceanic plate can compensate the subduction process. The compressional convergence margin such as the Nankai Trough, however, may not generate such normal faultings due to the nature of stress field.The low seismicity area existing across the trench axis is also seen both in this aftershock activity and ISC hypocenters. This is the same result as those in other regions. This might imply low earthquake potential for this portion of plate interface due to the existence of low density sediments and water contained in the sediments.

著者

林 慶一

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.6, pp.723-727, 1996-12-25 (Released:2009-11-12)

被引用文献数

1 2

著者

日本地学史編纂委員会 東京地学協会

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.110, no.3, pp.362-392, 2001-06-25 (Released:2010-04-23)

参考文献数

132

被引用文献数

2 2

著者

山本 薫

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.98, no.7, pp.911-933, 1989-12-25 (Released:2009-11-12)

参考文献数

35

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Comprehending patterns of raw material utilization for stone implements can be a key to the solution of questions concerning cultural tradition, cultural behavior, culture areas and trade in prehistoric times. The materials of stone implements reflected “knowledge of requirements (specificastion)” of the tool-makers, and this point is exemplified by stone implements of the Middle Jomon period, the Neolithic period in Japan.Patterns of raw material utilization for stone implements, at that time, were different among tool types. Evidence is presented which supports the view that Jomon people intentionally selected raw materials according to their “knowledge of tool requirements” (relating to the size and the shape of the tools). In addition, utilization of raw materials for sometypes of stone tools varied between districts. However, such differences confirmed to the intentional hypothesis.Eight types of stone implements in the Middle Jomon period, arrowheads (sekizoku), drills (ishikiri), points (sentouki, sentoujyousekki), pounding stones (tatakiishi), ground or pitted stones (suriishi, kubomiishi), querns (ishizara), roughly chipped ax-like stone tools (daseisekifu) and polished ax-like stone tools (maseisekifu), have been divided into three categories (1, 2, 3) according to the pattern of raw material utilization.Category 1 : Arrowheads, drills and points. All these are small light thin and sharp pointed tools which were made from thin flakes and hard compact silicious materials, predominantly obsidian or shale. There were two regional spheres of raw material utilization, the shale area and the obsidian area. Near the producing areas of obsidian in Chubu and Kanto districts, Jomon people tended to use this material. However, in the shale producing district, Tohoku district, Jomon people used often shale (silicious shale and hard shale).Category 2 : Pounding stones, ground or pitted stones, and querns. These stone implements are big heavy round lumpish tools which were made from round stones and primarily either andesite or sandstone. In every site in Eastern Honshu, these implements were predominantly made of one of these two rock types regardless of region. In addition, such characteristic materials used for category 1, such as obsidian and shale (silicious shale or hard shale), were rarely utilized for this category.Category 3 : Roughly chipped ax-like stone tools and polished ax-like stone tools. These are the middle weight-thickness implements for which various kinds of raw materials were used, except for obsidian and shale (silicious shale or hard shale). The typical regional patterns of raw material utilization could not be found among the sites.

著者

田島 稔

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.139-150, 1970-06-25 (Released:2009-11-12)

参考文献数

10

---

Recently, crustal movements in the Boso and Miura peninsulas have changed their mode of previous gradual sinking after the Great Kanto Earthquake to an upheaval tendency. The phenomena attracted much notice widely and increased the interest for earthquake prediction. At this opportunity, short sketch and discussion were made about the characteristics of recent crustal activities in the south Kanto district which had mainly obtained by means of geodetic repetition surveys such as levelling, tide observation, triangulation survey, distance measurement and geomagnetic survey. A picture for explaining the outline of the feature of crustal movement and a new geodetic survey planned by G.S.I. were also briefly discussed.

著者

古川 雅英

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.552-564, 1991-08-25 (Released:2010-11-18)

参考文献数

72

被引用文献数

5 11

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The Ryukyu Arc-Okinawa Trough system is one of several arc-backarc systems alongthe margin of the western Pacific and east Asia. Recent results of seismic refractionand reflection studies have revealed that the origin of the trough is the continental rifting, and the trough is in a state of the active rifting stage (e. g. Hirata et al., 1991; Furukawa et al., 1991). Basement rocks and sediments to appraise for the age of the trough, however, have notbeen sampled by deep sea drilling. Then the formation age of the trough was mainlyexamined by the geological studies of the Ryukyu Arc and Okinawa Trough areas andpaleomagnetic stud -ies of the Ryukyu Arc.Geological sequence of the Ryukyu Arc is generally divided into two units, thepre-Miocene basement complex and post-middle Miocene sediments (e. g. Kizaki, 1986). Thenorth and central Ryukyu Arcs represent the geological continuation of the Outer Beltof the south west Japan Arc, composed of the Mesozoic to Eocene sedimentary sequences, whereas thesouth Ryukyu Arc is characterized by high-pressure metamorphic rocks and the Eocenevolcanics and limestone. The geological and structural contrasts between the north-central andsouth Ryukyu Arcs are conspicuous before the late Miocene. These observations suggest that theformation of the Okinawa Trough have been originated since the Miocene, and the Ryukyu Archave been established since then.Paleomagnetic studies for the the Eocene to Pliocene volcanics and sedimentscollected from the Ryukyu Arc and northeast Taiwan suggest that the south Ryukyu Arc rotatedclockwise 19° with respect to the central Ryukyu Arc and Taiwan between 10 Ma and 4 Ma.Paleomagnetic studies attribute this rotation to the opening of the southern part of theOkinawa Trough during 10 and 4 Ma (Miki et al., 1990; Miki, 1991).The acoustic sequence in the trough can be divided into three units. And the eachunit are correlated to the Pliocene to early Pleistocene, middle to late Pleistocene, and late Pleistocene to Recent sediments in descending stratigraphic order. Widespreaddeformation by normal faulting is especially developed in the lowest unit which displays horst andgraben structures. The deformation would have originated at about the Plio-Pleistoceneboundary and formed the prototype of the trough (Furukawa et al., 1991). Synthesizes of the above studies have resulted in interpretativedevelopment of the trough as follows; a major phase of formation of the trough occurred during 10 and4 Ma, and reactivation of the formation of the trough occurred after 2 Ma. Recent studies fortherifting of several continental margins, however, suggest that the intra-arc rifts evolved 10times as first as major intra-continental rifts, and the life time of the rapid rifting is onlyabout 3 to 5 Myr (Yamaji and Takeshita, 1989). The middle Pleistocene reefal limestone dredged fromsome knolls in the Okinawa Trough indicate that the subsidence of the trough was about500-1, 000m after the middle Pleistocene. As a preliminary conclusion, therefore, the age ofthe trough could be estimated to be roughly 2 Ma. The present age estimation is againstrecently proposed clockwise rotation of the south Ryukyu Arc due to the opening of thetrough from 10 Ma to 4 Ma which was reported by the paleomagnetic study.

著者

三雲 健

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.99, no.1, pp.18-31, 1990-02-25 (Released:2010-11-18)

参考文献数

58

被引用文献数

3 4

---

Extensive studies on seismic activity and focal mechanism of major to moderate-size earthquakes that occurred along the eastern margin of the Japan Sea and the northern part of the Fossa Magna regions are reviewed and discussed in relation to regional tectonics. The validity of a prevailing hypothesis is also tested from various observations to see if the Northeast Honshu arc is part of the North American plate.Seismicity along the Japan Sea coastal regions extends southwestwards across the Toyama trough in one way, and also in other way to the northern Fossa Magna through a zone near the Shinano river. It has been noticed that the seismicity show temporarily successive properties similar to migration phenomena. Five major earthquakes along the Japan Sea regions have been interpreted as having thrust-type mechanisms, and the depth distribution of aftershocks of two larger events and the dip of submarine active faults suggest an eastward dipping fault plane.The maximum compressive stress derived from the focal mechanisms of more moderatesize earthquakes indicates E-W to ESE-WNW orientations, changing gradually from the northern to southwestern part of the coastal regions. In inland regions west of the northern Fossa Magna, the general trend appears oriented again in the ESE-WNW direction, which is found to be well consistent with the direction of principal compressive strains derived from geodetic triangulation surveys over the last 80 years.All the above evidence suggests that the regions under consideration may be a tectonically active, convergent zone, and might be regarded as a zone of a possible plate boundary. There is no direct evidence, however, suggesting an eastward incipient subduction of the Japan Sea lithospheric plate, from observations of submarine topography and upper crustal structure beneath the regions.Numerical calculations show that the suggested hypothesis could partly account for the observed directions of compressive stress in Northeast Honshu but is apparently inconsistent with those in Southwest Honshu. The results suggest that the Northeast Honshu arc may better be regarded as a “micro-plate” which receives strong compressive stress not only from the westward movement of the Pacific plate but also from the west side. One of possible sources of the driving stress from the west side might be an eastward movement of the “Amurian plate” which is one of micro-plates detached from the Eurasian plate. Another possible source could be bilateral extension of the central Japan Sea region or of the regions covering Northeast China and the Korean peninsula.

著者

青木 久

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.126, no.4, pp.413-424, 2017-08-20 (Released:2017-09-27)

参考文献数

31

被引用文献数

2 4

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To investigate cliff recession processes and rates for the purpose of studying the development of horizontal shore platforms, data taken from a masonry seawall at Ashikajima located on the Choshi Peninsula coast is used. The seawall is composed of artificially cut blocks of Cretaceous sandstone, which is the same rock type as that forming shore platforms in the area. The seawall with a horizontal length of 100 m was built 90 years ago to protect reclaimed land on pre-existing shore platforms. Two sites are selected for this study: Site A without a shore platform in front of the seawall and Site B with a platform. The surfaces of sandstone blocks in the supratidal zone are depressed at both sites; Site B has a more noticeable depression. The depression depth (i.e. erosion depth) after the period of 90 years is considerably larger (more than double) at Site B than at Site A, in spite of Site A suffering direct attacks from waves, irrespective of tidal stages, while Site B experiences low-energy waves only during high tides due to the presence of the horizontal platform. Granular disintegration occurs markedly on the sandstone surface at Site B, but little occurs at Site A. This strongly suggests that salt weathering is prevalent at Site B, reducing the strength of rocks. The moisture content in sandstone blocks at Site B is lower than that at Site A, which implies that Site B is more susceptible to weathering than Site A which is always exposed to waves and tides. It would be reasonable to consider that the seawall at Site A is analogous to a cliff at the initial stage of platform development, while the seawall at Site B is similar to a cliff at its middle stage. Horizontal shore platforms start to develop due to wave action alone, but as they grow wider the importance of salt weathering increases and the combined effects of waves and weathering become crucial to platform development.