著者

古村 孝志

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.123, no.4, pp.434-450, 2014-08-25 (Released:2014-09-01)

参考文献数

28

被引用文献数

2 9

---

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

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 49

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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

増田 隆一

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.105, no.3, pp.354-363, 1996-06-25 (Released:2009-11-12)

参考文献数

30

被引用文献数

1 1

---

The richness of fauna diversity is seen on the Japanese Islands, which range through 3, 000 km. When and from where have theanimals immigrated to Japan? Also, for understanding natural history of the Japanese Islands, it is very importantto investigate evolutinary history and origin of animals distributed on each island. While the evolutionary questions have been paleontologically and geologically examined so far, some of them are still unclear. Recent development of molecular phylogenetic study provides a reliable sight to understanding evolutionary history and origin of species. Advantageously, molecular study can analyze phylogeny of living species without fossil data.In Japan, two wildcats, the Iriomote cat and the Tsushima cat, live on the Iriomote Island and the Tsushima Island, respectively, where land bridges between the Asian continent and the Japanese Islands were present in the past time. To know evolution of these wildcats gives us useful information for better understanding of natural history of the Japanese Islands. Moreover, because both the wildcats are now endangered, it is now strongly needed to understand phylogenetic status of them for conservation and management. For the reason, we investigated molecular phylongeny of the two wildcats based on mitochondrial DNA sequences, and revealed that both the wildcats are very closely related to the leopard cat Felis bengalensis, which is widespread throughout Asia. Furthermore, from the DNA data, the Iriomote cat and the Tsushima cat were estimated to have diverged from the continental leopard cat approximately 200, 000 and 100, 000 years before present, respectively. The dates estimated by molecular data were in concordance with formation dates of the Ryukyu Arc and the Tsushima Island, respectively. These results suggest that geographic barrier has led fixation of some unique morphological characters into each cat population, while the two Japanese wildcats are still genetically close to the continental leopard cat.

著者

重村 利幸

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.105, no.4, pp.448-458, 1996-08-25 (Released:2009-11-12)

参考文献数

13

---

Iwo Jima is small volcanic island located isolate in the Western Pacific Ocean about 1, 250 km south of Tokyo. The island with the area about 23 km2 is covered with considerably dense vegetation and no rivers exist in this island. Thus, the source of sediments to nourish the beach seems to be quite limited in this island. Further, the coast is always subjected to rough seas because it is fully exposed to the open sea. Nevertheless, the coast is mostly rimmed by sandy beaches with about 200 m in width and the shoreline is still advancing toward the sea which increases the beach area year by year. Many scientists have pointed out that this increase might be caused by an unusual upheaval of this island exceeding 30 cm per year. However, no quantitative analyses have been done yet on this unique phenomenon that prevents from constructing any port facilities in this island. Thus, the author intends to investigate the features of the long-term variation of the area which have occurred in Iwo Jima during the past seven decades and to examine its variation mechanism.To investigate the features of the long-term variation, analyses are conducted on the seven maps and charts of Iwo Jima which have been published in the period from 1911 to 1981. The analysis reveals that the island has kept increasing its area over the past seven decades although the increasing rate has changed drastically in 1952 from 30, 000 m2 to 90, 000m 2 peryear and that the increased area has reached roughly 4.3 km2 in total during the sevendecades. This change of the increasing rate well corresponds to the fact that the upheaval rate of the island has changed in 1952 from roughly 10 cm to 30 cm per year which has been found by Kosaka et al. in 1979.To examine the effect of the upheaval on the features of the long-term variation of the area, calculation is carried out by giving the various values of imaginary upheaval to the bathymetric data shown on the chart covering the sea around Iwo Jima published by the Hydrographic Department of Japan Maritime Safety Agency in 1981. The result of the calculation indicates that the upheaval of 10 cm per year corresponds to the increasing rate of the beach area roughly 43, 000 m2 per year and the upheaval of 30 cm per year corresponds to the increasing rate of the beach area roughly 130, 000 m2 per year when the foreshore slope is assumed to be 1/20. These values are surely the ones before the beach is transformed by the action of the ocean waves. However, comparison of these values with the ones found through the former analyses indicates that the calculation has provided quite a reasonable estimation of the increase of the beach area. These findings clearly indicate that the continuous upheaval is the governing factor to cause the increase of the beach area in Iwo Jima.

著者

石橋 克彦

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.108, no.4, pp.399-423, 1999-08-25 (Released:2009-11-12)

参考文献数

75

被引用文献数

12 18

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A large volume of historical documents in Japan show that great subduction earthquakes have repeatedly occurred along the Suruga-Nankai trough off southwest Japan since A.D. 684 with an interval of 100-200 years. They occurred as pairs of M8 events, one in the eastern half (Tokai earthquake) and another in the western half (Nankai earthquake), as was the case for the 1854 Ansei earthquakes, while sometimes occurring as single giant events like the 1707 Ho'ei earthquake. Although the space-time pattern of their recurrence is the best-known in the world, we should study more past events in order to understand the tectonophysical bases of their recurrence. In this respect I review the present understanding of historic Tokai and Nankai earthquakes and discuss related problems from the viewpoint of historical seismology. In this paper, the first of the three in all, I review the events until the early half of the 14th century. The keys to identifying older events are strong ground motion and damage in Kyoto, Nara, and Osaka, those in wider area of southwest Japan, tsunamis along the Pacific coasts of southwest Japan, typical coseismic vertical crustal movements of the Kochi plain, the Muroto and Oma'ezaki points, and the Ise and Suruga Bay coasts, temporal inactivity of specific hot springs, and aftershock activities recorded in Kyoto. The 684 Hakuho earthquake was definitely a Nankai event, and possibly included a Tokai event simultaneously (possibly Ho'ei type). The 887 Nin'na earthquake was also a definite Nankai event and was probably a Tokai event as well (Ho'ei type). The 1096 Eicho earthquake was clearly a Tokai event, but the following 1099 Kowa earthquake has some discrepancies that prevent it from being regarded as a M8 Nankai event. It is not clear yet whether great earthquakes occurred or not in the ca. 200 year intervals of 684-887 and 887-1096. It seems probable that great Tokai and Nankai earthquakes took place in the mid-13th century, but a more detailed investigation of historical seismology is required to discover the missing event.

著者

中田 高

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.289-298, 1990-06-25 (Released:2009-11-12)

参考文献数

9

被引用文献数

5 5

著者

笠原 順三 阿部 なつ江 荒井 章司 下司 信夫 熊谷 英憲 丸山 茂徳 小原 一成 小原 泰彦

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.117, no.1, pp.1-4, 2008-02-25 (Released:2010-02-10)

参考文献数

17

著者

島田 浩二

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.3, pp.451-465, 2010-06-25 (Released:2010-08-30)

参考文献数

24

被引用文献数

1 1

---

The rate of recent sea ice reduction in the Arctic Ocean exceeds that of global warming. The pattern of sea ice reduction accompanies discontinuous changes. This implies that the Arctic climate system is not only influenced by global warming, but also involves significant positive feedback mechanisms that accelerate the reduction of sea ice. Changes of sea ice area in the Arctic Ocean do not show uniform reductions in both space and time. In the present study, we investigate sea ice reduction in distinct regions and time frames, taking into consideration preconditions for the occurrence of tipping points. The initiation of a positive feedback system is found in strengthening ocean and ice circulations since the late 1990s. The activation of both sea ice and ocean circulation caused ocean warming. This contributed to less formation and growth of sea ice in the Pacific Sector. Consequently, ice reduction actually occurred due to an imbalance between ice melt and formation. The sustaining of strong circulations was a precondition leading to a huge anomalous sea ice reduction in 2007 with a zonal asymmetric distribution. The resultant zonal asymmetry establishes a new atmospheric pattern (dipole pattern) associated with southerly winds in the Pacific sector of the Arctic Ocean. Now, the Arctic climate is governed by a strong coupling system linking ocean, ice, and atmosphere. Multi-disciplinary studies are now required to understand the catastrophic Arctic changes and the fate of the Arctic and global climate.

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 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

矢嶋 澄策

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.72, no.4, pp.178-188, 1963-08-30 (Released:2009-11-12)

参考文献数

10

被引用文献数

1

---

Survey of old records concerning the publication on mercury provides a very useful means to trace exhausted and forgotten mercury mines and thus complete the gaps in the distribution of mercury are deposits in the Japanese segments of the Circum-Pacific metallogenetic zone.The discovery of a mercury mineral in 198 A. D. is described in “Shoku Nihonki”, the earliest literature referring to the Japanese mercury. The mining of the metal in record, however, dates as far back as the Asuka period (600 A. D.) and a peak production was once reached during the Heian period. In those early days, mercury was used principally for medicine, amalgam base for gold-gilding on Buddha statues, and ship paint. The mining was operated throughout Japan by the “Niu” tribe, an ethnic group which was once headed by “Niuzuhime” in Yoshino region and later spread out searching for new deposits. “Niu” means cinnabar and also red color in Chinese. For this reason, villages in former mercury-producing district often bear the name of Niu and have shrines called “Niu Jinja” which are dedicated to Niuzuhime.It is also reported in literature that the search for mercury was also conducted by “Yamabushi”, a group of itinerant buddhist priest who travelled around the country carrying the will of Kobo-daishi, the famous founder of their sect.Although the occurrence of mercury in “Hitachi” and “Dewa” districts was mentioned in ancient literature, no mercury deposits have been discovered geologically in recent years. The mercury contents of reddish clays from these localities, however, are high enough to be indicative of the presence of such are deposits.

著者

稲崎 富士 太田 陽子 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.123, no.4, pp.401-433, 2014-08-25 (Released:2014-09-01)

参考文献数

52

被引用文献数

4 9

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River improvement works in the Kanto Plain have long history of over 400 years. Rice-paddy development in the Kanto Plain was significantly delayed compared to that in other plains including the Osaka Plain and the Nobi Plain. This was because of the difficulty in constructing an irrigation and drainage network, and protecting rice paddies from inundation. Therefore, the Tokugawa government started river improvement works in the Kanto Plain just after Tokugawa settled in Edo in 1590, and the works continue now. The flow of the Tone River into Tokyo Bay was originally blocked by uplands, but was finally rerouted to Choshi, 90 km east to the original mouth. The Ara River was also rerouted to the south. The purpose of rerouting was not only to reduce the risk of flooding in Edo city, but also to develop a waterway network for rice paddies to expand in the central Plain. Moreover, rerouting and connecting the Tone River with the Edo River enhanced the inland waterway transportation network of the Kanto Plain. Accordingly, a number of riverside towns, or Kashi, grew as nodes of the network. Small sailing ships and flatboats were the major conveyors of products. During the Meiji era, which followed the Edo period, canals were constructed and steamboats were introduced to replace sailing ships, at a time when water transportation was peaking. The inland waterway soon began to be replaced by present-day economic transportation systems such as rail and road. The central Kanto Plain was featured consistently by a subsiding basin through the Quaternary. Although the entire Plain was uplifted, Tokyo Bay, at the center of the Kanto Plain, sank over 1000 m. In contrast, the outer margin of the plain was uplifted 50 to 1000 m. Choshi, at the mouth of the present Tone River, is in the uplifted area. In contrast, the lower reaches of the River are at the northern extent of the subsiding basin. This is the reason why back swamp lakes or an estuary such as Kasumiga-ura and the ancient Katori-no-umi were formed in the area. Such crustal movements in the Kanto Plain continued throughout the Quaternary. Hydration and dehydration of two plates lying beneath this region were the driving force. Hydration of mantle peridotite underneath the Kanto Plain due to dehydration of the underlying Philippine Sea Plate (PHS plate), subducting from south to north 30-60 km deep (2 cm/year), causes volumetric expansion particularly at the marginal zone of the overlying plate (North American (NA) Plate). Serpentinized peridotite, produced above the Pacific Plate (PAC plate), which subducts from east to west under the PHS and NA plates, expands like popcorn and results in uplifting of the Boso Peninsular (non-volcanic outer arc) . In contrast, the sinking of the Tokyo Bay area is explained by the overlapping of the fore arc basin towards the PHS and PAC plates. From the viewpoint of tectonics, the sinking belt including Tokyo Bay is in a physical field where a sedimentary basin formed under tensile stress in the NE-SW direction. Small mantle convection caused by serpentinization of the uppermost mantle beneath the fore arc is the key to understanding the tectonic setting of the Kanto Plain.

著者

ベンター ヤコブ

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.103, no.1, pp.1-15, 1994-02-25 (Released:2010-10-13)

参考文献数

22

著者

石村 大輔 加藤 茂弘 岡田 篤正 竹村 恵二

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.5, pp.775-793, 2010-10-25 (Released:2011-01-21)

参考文献数

46

被引用文献数

3 4

---

Episodic subsidence events at the east coast of Lake Mikata, Fukui Prefecture, suggesting fault activity in the Mikata fault zone, are recognized from two drilling cores, MK09 core (60-m long) and NEXCO core (100-m long). Detailed lithology including widespread tephra horizons is described for the MK09 core, with 18 AMS-14C dates. The radiocarbon ages and tephrochronology of the core sediments indicate the continuous sedimentary history of Lake Mikata back to about 130 ka. We recognize eight depositional units ranging in thickness from 5 to 10 m in the MK09 core, each of which shows a clear coarsening-upward sequence from clay to gravel beds. These units are also recognized in the NEXCO core drilled 50 m from the MK09 core site by the correlation of tephras and lithofacies. Analysis of sedimentary features suggests that each unit and its boundary reflect a rapid rise of lake level and subsequent progradation of alluvial fans. The mean sedimentary interval of individual units is equivalent to about 10.6 kyr, which is meaningfully shorter than astronomical cycles of climate change (∼20, 41 and ∼100 kyrs). Thus, we interpret these cyclic units to be the results of recurrent co-seismic subsidence due to Mikata fault zone activity. According to this interpretation, we could identify at least seven subsidence events besides the 1662 Kanbun Earthquake. The mean recurrence interval of these events is estimated to be about 7.7 kyr.

著者

荒井 章司 阿部 なつ江

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.117, no.1, pp.110-123, 2008-02-25 (Released:2010-02-10)

参考文献数

56

被引用文献数

9 7

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This article reviews interpretations of the geological and petrological nature of the Moho, which is defined as a discontinuity in terms of Vp, with a view to preparing for the Mohole on the ocean floor in IODP. We strongly propose discarding non-seismic terms for the Moho, such as “petrologic Moho”. The nature of the Moho has been controversial for a long time; an isochemical phase transition boundary between gabbro (crust) and eclogite (mantle) was favored for the Moho by some researchers, while a chemical boundary between mafic rocks (crust) and peridotite rocks (upper mantle) is now favored by a majority of researchers. Boundaries between completely or partially serpentinized peridotite and fresh peridotite may be applicable as the Moho at some parts of the ocean floors of a slow-spreading ridge origin. Antigorite serpentinite can be expected to be observed at the lowermost crust if the Moho is the serpentinization front at the stability limit of serpentine. The Moho beneath the Japan arcs can be estimated using mafic-ultramafic xenoliths in Cenozoic volcanics. Peridotitic rocks scarcely mix with feldspathic rocks, indicating that the Moho at that location is the boundary between feldspathic rocks (mostly mafic granulites; crust) and spinel pyroxenites (mantle). Possible fossil Mohos are observed in well-preserved ophiolites, such as the Oman ophiolite. Two types of Moho are distinct in the Oman ophiolite; gabbro-in-dunite Moho, where a gabbro band network in dunite changes upward to the layered gabbro within a few to several tens of meters, and dunite-in-gabbro Moho, where late-intrusive dunites intruded into gabbros. The former is of a primary origin at a fast-spreading ridge, and the latter is of a secondary origin at a subduction-zone setting in the obduction of the oceanic lithosphere as an ophiolite. The gabbro/peridotite (dunite) boundary as the primary Moho forms in embryo as a wall of melt conduit at fast-spreading ridges as well as at the segment center of slow-spreading ridges. The oceanic primary Moho is modified to various degrees by magmatism, metamorphism and tectonism in subsequent arc and continental environments. The gabbro-in-dunite Moho formation in the Oman ophiolite is an embryo of this modification. We expect in-situ sampling across the primary oceanic Moho formed at a fast-spreading ridge through the Mohole of IODP. Ultra-deep drilling at gabbro/peridotite complexes exposed on the ocean floor is indispensable for our understanding of the suboceanic upper mantle. Studies on appropriate ophiolites and deep-seated xenoliths from oceanic areas should complement the Mohole and other ultra-deep drillings to grasp the whole picture of the oceanic upper mantle.

著者

岡田 篤正 松田 時彦

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.101, no.1, pp.19-37, 1992-02-25 (Released:2010-10-13)

参考文献数

42

被引用文献数

11 7

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The Neodani fault to the north of Nagoya, central Japan, is a principal strand of the Nobi fault system. The fault moved primarily in a left-lateral sense during the great Nobi earthquake of 1891 (Magnitude 8.0). The maximum vertical and horizontal offsets are 6 m at Midori and 7-8 m at Naka, respectively. After reviewing previous works, we discuss new geomorphological and geological data bearing on the 1891 and earlier paleoearthquakes in the latest Quaternary: specifically we find that1) The 1891 vertical offset was about 5 m at the western slope of Terayama which is located on the northwestern extension of the well-known Midori fault scarp.2) The subsurface data show that the vertical offset of the base of the alluvial deposits beneath the Midori scarp is about same as that of the ground surface at the scarp, implying that the 1891 event was the first faulting event since the beginning of the deposition of the allu-vium. The radiocarbon date suggests that the alluvium is not older than 1, 000 years in age.3) The lower river terrace, ca. 14, 000 years old, is displaced as 14 m vertically at Terayama in Midori where about 5 m vertical displacement occurred in 1891, and a stream incising the lower terrace on the fault line at Naka offsets ca. 28 m laterally where 7-8 m lateral displacement occurred in 1891. If the fault is characterized by the repeated occurrence of earthquakes of the same size, both the 14 m vertical offset at Terayama and 28 m lateral offset at Naka may be interpreted to be the result of 3 to 4 events occurring at average intervals of about 3, 000-4, 000 years since the formation of the lower terrace. The observation also indicates that the Neodani fault, at least in the central segment including Midori and Naka, has averaged 2m per one thousand years in the left-lateral slip rate in that period.

著者

佐野 貴司 テハダ マリア ルイサ 中西 正男 羽生 毅 三浦 誠一 末次 大輔 利根川 貴志 石川 晃 清水 健二 淸水 祥伽

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.130, no.4, pp.559-584, 2021-08-25 (Released:2021-09-23)

参考文献数

163

被引用文献数

2 4

---

Large Igneous Provinces (LIPs), such as the Ontong Java Plateau (OJP) in the western equatorial Pacific, provide information on mantle processes and composition, and their formation may have global environmental consequences. The OJP is the largest oceanic plateau and is probably the most voluminous igneous edifice on Earth. Despite its importance, the size, volume, and formation rate of the OJP are not yet well constrained. The maximum extent of OJP-related volcanism may be even greater than currently estimated, because volcanological studies indicate that long lava flows (or sills) from the OJP may have reached the adjacent Nauru, East Mariana, and possibly Pigafetta basins. Moreover, the similarity in age and some geochemistry of lavas from the Ontong Java, Hikurangi, and Manihiki plateaus suggests that they once may have been part of a single LIP (Ontong Java Nui, OJN). If true, the massive volcanism may have covered > 1% of the Earth's surface. The lack of detailed knowledge of the size, age, and composition of the OJP has given rise to various models, such as a surfacing mantle plume head, bolide impact, and fusible mantle melting, but no model satisfies all observational data and no consensus has been reached on its origin. The OJP is divided into the High Plateau to the west and the Eastern Salient to the east. The basaltic basement of the OJP was cored at seven sites during Deep Sea Drilling Project (DSDP Site 289) and Ocean Drilling Program (ODP Sites 289, 803, 807, 1183, 1185, 1186, and 1187) expeditions, but all sites are exclusively located on the High Plateau. In order to examine the true extent of the OJP (i.e., whether the flows in the Nauru, East Mariana, and Pigafetta basins, as well as the Manihiki and Hikurangi plateaus are parts of the OJN), we propose drilling in the Eastern Salient and adjacent basins to recover basement samples. We also propose drilling through the sedimentary section on the Magellan Rise, a small plateau that formed > 20 Myr before the proposed OJN emplacement. Because of its greater age, the sedimentary sequence on the Magellan Rise may preserve ash layers or other chemical tracers that cover the entire eruptive history of OJN. The sediment layers from the Magellan Rise are also useful for evaluating environmental effects of OJN emplacement, including older and younger perturbations related to other LIPs.

著者

木村 政昭

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.105, no.3, pp.259-285, 1996-06-25 (Released:2009-11-12)

参考文献数

53

被引用文献数

76 82

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Compilation of seismic reflection profiles, rock sample analyses and manned and non manned diving observations revealed that land bridges, connecting the Ryukyu Arc to the Chinese continent, have appeared in two stages since the late Pliocene. The first stage occurred before the deposition of Pleistocene Ryukyu Limestone and ceased in the early stages of deposition, while the second was apparent during the middle and late stages of the Ryukyu Limestone. The land bridge in the first stage may have appeared between 2.0-1.0 Ma and is well identified by the unconformity between Shimajiri and Ryukyu Groups before the deposition of Ryukyu Limestone. The land bridges may have appeared two or three times in the second stage during or after deposition of Ryukyu Limestone between 0.4-0.02 Ma. The last land bridge may have appeared during the Würm glacial stage in the late Pleistocene, connecting the Chinese continent through Taiwan, Okinawa and Amami-Oshima to the Kyushu main land. This supports the hypothesis that the Minatogawa Man migrated to Nansei-shoto Islands passing through the land bridge in the Late Pleistocene.

著者

中村 有吾 松本 亜希子 中川 光弘

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.114, no.4, pp.549-560, 2005-12-25 (Released:2009-11-12)

参考文献数

18

被引用文献数

8 6

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The AD1663 ejecta of Usu volcano, western Hokkaido, northern Japan, have a thickness of approximately 200, cm and are divisible into 7 units on the eastern foot of Usu volcano. The basal deposit, Unit-A, is composed of fine-grained, poorly sorted pumice and ash with well-developed stratification. It contains base surge beds near the sources, and is interpreted as phreatomagmatic in origin. Unit-B is typical in many respects of the product of plinian activity. It is characterized by coarse grain size and wide dispersal, combined with low lithic contents and poor development of internal stratification. This fallout deposit (Usu-b tephra) can be seen in the eastern regions of Usu volcano, and has a thickness of about 4 cm at 200 km from the vent. Unit-C, Unit-E, Unit-F, and Unit-G have characteristics that are satisfactorily interpreted as a consequence of phreatomagmatic activity : fine grain size near source, poor sorting, well-developed stratification and base surge bedforms. Unit-D is typical of phreatic deposits, being composed largely of non-juvenile lithic fragments.According to historical records, the eruption was preceeded by earthquake swarms for 3 days from August 13, 1663. The first eruption (Unit-A) occurred on August 16, followed by an eruption associated with a large amount of pumice and ash (Unit-B) on August 17. The explosive eruptions lasted approximately 20 days.The petrographic properties of the plinian product differ considerably from those of the phreatomagmatic products. The pumice clasts of Unit-B contain abundant glass and little phenocryst (plagioclase, orthopyroxene, and hornblende). The glass has a relatively high refractive index (n = 1.490-1.492) and a low SiO2 content. Orthopyroxene in Unit-B has a relatively high refractive index (γ = 1.735-1.740) and low Mg#. The phreatomagmatic products are rich in massive glass. This glass has a relatively low refractive index (n = 1.475-1.490) and a high SiO2 content, with values differing among horizons. The compositional diversity of the 1663 products indicates the existence of some magma. Presumably, their mixing triggered the 1663 eruption of Usu volcano.

著者

中田 高 島崎 邦彦 鈴木 康弘 佃 栄吉

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.107, no.4, pp.512-528, 1998-08-25 (Released:2010-12-22)

参考文献数

28

被引用文献数

15 14

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This paper proposes a method to identify the directivity of rupture propagation based on the branching features of active fault traces.Direction of ruptre propagation is closely related to strong ground motions and resulting earthquake damage. Therefore, predicting rupture directivity is crucial in predicting strong motions to mitigate earthquake damage. However, the directions of fault ruptures were ascertained only after earthquakes from the observed seismological records and not before the earthquakes.We found an interdependent correlation between the branching direction of the surface ruptures and the direction of their propagation as shown in Fig. 1, from an investigation of recent earthquake fault ruptures such as the 1995 Northern Sakhalin earthquake, the 1995 Hyogoken-nambu earthquake, the 1992 Landers earthquake, the 1990 Luzon earthequake, the 1979 Imperial Valley earthequake, and the 1930 Kita-Izu earthequake. The branching of faults during rupture propagation is regarded as an effective energy dissipation process and could result in final rupture termination.Because patterns of surface traces of active faults are the results of repeated earthquake faulting, the branching of active faults leads us to suggest that the direction of rupture propagation is also predictable before the active faults generate earthquakes in the future.Several active faults with well-defined branching such as the active faults of the strike -sliptype in the Kobe-Osaka area, those in California, and the active fault sysytem in the northern Luzon, Philippines are examined. Branching of the reverse faults in the foot -hills of Darjeeling Himalaya is also shown as an example of active faults of the dip -slip type. This test clearly shows that the direction of rupture propagation, and in some cases the epicenter location, can be deduced from the branching features on the basis of our proposed method.

著者

小暮 岳実

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.120, no.4, pp.585-598, 2011-08-25 (Released:2011-11-10)

参考文献数

31

被引用文献数

2 4

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Many Rivers are concentrated in the central part of the Kanto Plain, central Japan. The Tone River, the largest river, flowed south-southeast before it was artificially diverted eastward at the end of the 16th century. A large meandering paleochannel in the present Arakawa Lowland, which is situated about 30 km south of the present Tone River's course, is thought to have been formed by the former Tone River. However, the upriver part of this paleochannel is indistinct in the Menuma Lowland lying north of the Arakawa Basin. Detailed aerial photo interpretation and observations of sediment samples obtained with hand augers reveal that a paleochannel with a north-south trend in the eastern Menuma Lowland contains volcaniclastic sediments derived from the Tone River catchment. The age of the buried paleochannel, i.e., the former Tone River, was estimated to be around 1,300 years ago based on many archeological survey reports of ancient tombs and other sites located around the paleochannel. The subsequent eastward natural diversion of the Tone River into the Kazo Lowland seems to have been caused by aggradation due to a rapid supply of volcanic sediments and tectonic subsidence in the Kazo Lowland situated in the central part of the Kanto Tectonic Basin.