The Kanto Plain, the hinterland of the Tokyo metropolitan area, is the largest plain in Japan and is characterized by marked marine and fluvial terrace levels that developed during Marine oxygen Isotope Stage (MIS) 5. Late Quaternary topographical changes to the plain have been controlled by concurrent tectonic activity and glacio-eustatic sea-level changes. The shoreline at the maximum transgression of MIS 11, 9, 7, 5 and 1 is reconstructed based on the distribution of marine sediments revealed by many geologic columnar sections and marine terrace surfaces. A comparison of the magnitudes of the last five full-interglacial transgressions above shows that magnitude decreased over the long term. This is due probably to changes in the tectonic regime in the Kanto basin, from subsidence to uplift along with the northward migration of the depositional center, probably associated with changes in the motion of the Philippine Sea Plate and the collision with the Izu peninsula. The marine transgression has also been controlled by fluvial processes, especially in the north-western part of the plain because of high sediment inputs from the Tone, Ara, and Watarase rivers. Aggradation coupled with regional uplift since MIS 5.4 limited the MIS 1 marine transgression within the incised valley formed during MIS 2. As a result, the Paleo Tokyo bay, which was connected directly with the Pacific Ocean, disappeared. Instead, a large shallow submarine area of about 10,000km2 emerged. The northern part of the present Tokyo bay is still subsiding and large volumes of water and sediments have been concentrated in the bay area during the Holocene. Such natural environmental conditions enable supplies of natural resources, such as fresh water, fertile soil, and flat land for the development of greater Tokyo.
This paper reconstructs the paleogeography of the Tokyo and Nakagawa Lowlands, Kanto Plain, central Japan since the Last Glacial Maximum (LGM) on the basis of 18 sediment cores and 467 radiocarbon dates obtained from the lowlands. Fourteen sedimentary facies were identified from the sediment cores, and paleo water-depths were calculated from sediment accumulation and sea-level curves in the lowlands. The paleogeography of the Tokyo and Nakagawa Lowlands has been controlled by basement morphology, tidal currents initiated by the basement morphology, and migration of the Tone River. Due to sea-level lowering from Marine Isotope Stage 5 to LGM, the Nakagawa and Arakawa Valleys were formed under the lowlands. The last deglacial sea-level rise caused the valleys to change from braided river to meandering river, tidal flat, tidal river, and tide-influenced bay environments. During the Holocene sea-level highstand, the tide-influenced bay in the Arakawa Valley was first filled with Tone River sediment input, and after the migration of the main stream of the Tone River from the Arakawa Valley to the Nakagawa Valley at 5 ka, the tide-influenced bay in the Nakagawa Valley was filled up. The tide-influenced bay in the Nakagawa Valley at 5 ka can be regarded as a tide-dominated delta, and its regime changed from tide-dominated to river-dominated after the bay filled up at 3.5 ka.
Chuseki-so is a valley-fill deposit that originated during the latest Pleistocene and Holocene, spanning the period from the last glacial maximum (LGM) to the present. This study describes its stratigraphic framework and sedimentary processes with particular reference to Holocene sea-level change, in order to understand its topographic and depositional evolution and geotechnical properties in the Tokyo and Nakagawa lowlands. Chuseki-so has been subdivided into two formations: the Nanagochi Formation and the Yurakucho Formation in ascending order. These formations can be clearly distinguished on the basis of physical properties: the Yurakucho Formation is very soft with a high water content, while the Nanagochi Formation is slightly compacted with a lower water content. Previous studies have suggested a falling sea level at the final stage of the Nanagochi Formation, corresponding to the Younger Dryas cold event. This led to the emergence of the deposits and physical differences due to escaping of pore water. However, recent studies seem to suggest that the sea level did not fall during the Younger Dryas. During the main stage of the Nanagochi Formation, from the end of the LGM until the Younger Dryas Event, sea level rose quickly, particularly during the Bølling/Allerød interstadial (B/A), and rapid sedimentation characterized by alternations of sand and mud occurred under a warmer and wetter climate. A new idea proposes that at the stage of the Nanagochi Formation, pore water of mud layers escaped easily into sandy layers immediately after sedimentation (Tanaka et al., 2006). The Yurakucho Formation, on the other hand, consists of very soft marine muddy and partly sandy sediments, and the main stage is thought to have been formed by rapid sedimentation in an inner bay environment during the Holocene. Topographic and paleogeographic changes closely related to the Holocene Jomon Transgression, have been investigated by reconstructing the migration of oyster reefs, which are good markers of paleoshorelines, throughout the wide inner bay during the rising stage of sea-level. After the highest stage of sea level, or thereabouts, the delta front began to prograde rapidly toward the south with the delta front migrating about 30-40km, reaching the present northern Tokyo Lowland 2000-3000 years ago. The NE Japan Megaearthquake of March 11, 2011, damaged large coastal areas of eastern Japan due to liquefaction and related phenomena. The northeast coast of Tokyo Bay in the Tokyo Lowland was damaged severely. Liquefaction occurred throughout the reclaimed land, which is composed of soft sandy soil dredged from the bay. The reclaimed sandy soil is underlain by new sandy delta front deposits and very soft thick marine valley-fill clays. It is essential to clarify the mechanisms of liquefaction in relation to the properties of soft ground (Chuseki-so), and the effects of shaking caused by strong earthquakes and subsequent sloshing for a long period. (View PDF for the rest of the abstract.)
Land transformation in Tokyo and its surrounding regions is reviewed, focusing on reclamation and cut-and-fill during the past several decades. The transition of the land transformation area is estimated using “Municipalities Area Statistics,” topographic maps, and geomorphological mapping data of the Geospatial Information Authority of Japan. Before 1965, reclamation sites were located in Tokyo, Kawasaki, Yokohama, and central Chiba. These areas developed as industrial areas with major trading ports. From 1966 to 1975, the peak period of rapid economic growth in Japan, large areas were reclaimed in the Kawasaki, Yokohama, and Chiba-Sodegaura coastal areas. The land reclaimed during these periods, most of which was connected to old land, has mainly been used to site heavy industries. Since 1976, construction of artificial islands has been the dominant form of land reclamation. From 1976 to 1980, reclamation on a smaller scale than the previous period was carried out mostly between Tokyo and Chiba, not to make industrial sites, but to make business, commercial, residential, and leisure sites. Since 1981, there has been very little reclamation, with the exception of the expansion of Tokyo (Haneda) International Airport. In upland areas, artificially transformed land comprises approximately 20% of the total land, and the increase since the 1980s has been small, while a large part of hill areas was transformed into cut-and-fill mosaic forms mainly in the 1960s and 1970s. Most of it was done to supply residential sites that supported population concentration in Tokyo and its surrounding regions. Many cases were of large-scale land development tied to the opening of new railway lines and stations. At present, artificially transformed land accounts for 56% of the total land in the Tama hills. Land transformation affects the natural environment. Reclamation works sometimes cause water pollution and ground subsidence. To mitigate influences on the environment, artificial beaches were made to recover and sustain coastal ecosystems, and provide places where people could relax. Land transformation also significantly influences natural disaster vulnerability. It is obvious through experiences of major earthquakes in the past several decades that reclaimed land is subject to liquefaction damage. Increases of inundation damage to valley-bottom part of hill areas at times of heavy rainfalls have resulted from residential land development. Fill-up ground is easily deformed by the strong motions of earthquakes. Consequently, it is important to understand the history of land, no matter where it is located.
Climate variations in Tokyo based on reconstructed summer temperatures since the 18th century and instrumental meteorological data from the 19th century to the present are discussed. During the Little Ice Age, especially in the 18th century, remarkably cool episodes occurred in the 1730s, 1780s and 1830s. These cool conditions could be a significant reason for severe famines that occurred during the Edo period. Around the 1840s and 1850s near the end of the Edo period, it was comparatively warm which could correspond to the end of the Little Ice Age in Japan. Although there was a low-temperature period in the 1900s, a long-term warming trend could be seen especially in winter temperatures and daily minimum temperatures throughout the 20th century. While annual precipitation has been increasing during the last 30 years, relative humidity has been decreasing. This could result from a saturated vapor pressure rise due to warming and from a loss of water bodies due to urbanization. During the last century, not only warmer conditions but also wetter conditions in summer and autumn and drier conditions in winter and spring were documented by analyzing hythergraphs.
This paper overviews changes in land use and fauna over the past ca. 100 years in and around Tokyo Metropolis. Rapid changes in fauna occurred during the second half of the last century with urbanization. The faunal changes were caused by habitat destruction due to exploitation, habitat change due to urban warming, regime changes in natural and anthropogenic disturbances, and the invasion of alien species. Although a rapid decline of native species diversity occurred during the period, some species have reinvaded or newly invaded urban and suburban areas in the last few decades. Some alien species have also invaded the built environment such as artificial coasts and abandoned coppice forests. Faunal changes continued even after the rapid environmental changes in the last century. Continuous monitoring is needed to detect further faunal changes in the Tokyo area.
The purpose of this study is to review the history of urban water use in Tokyo focusing on two water sources: surface water and groundwater. First, the following four matters are picked up chronologically: expansion of metropolitan waterworks, enhancement of surface water resources, progress of land subsidence, and groundwater pumping. Second, the change of groundwater use and current conservation policies are clarified for some municipalities. Finally, sustainability of urban water use is discussed. Originally, the water source of metropolitan waterworks was surface water. With increased water demand, the waterworks developed surface water resources in areas remote from Tokyo. Despite increased water demand, groundwater pumping was restricted because of serious land subsidence. Industrial water shifted the water source from groundwater to surface water with the construction of industrial waterworks. On the other hand, groundwater use as residential water has been partially maintained in the Tama Region, western Tokyo. Municipalities in the Tama Region promote policies to maintain groundwater use and recharge. For sustainable urban water use, efforts both to avoid a further increase of water demand and to maintain local groundwater resources are necessary. Moreover, in terms of water security at the time of a disaster or water shortage, a sustainable urban water supply system should include both an extensive water supply system and a local system of water supply.
Views within and from a city make spatial experiments productive. These views make it easier to understand the importance of the image of a city as several urban planners and designers have indicated. On the basis of literature reviews and data gathered from the author's research on views in Tokyo, this paper establishes four phases, namely “foundation,” “creation,” “obscuration,” and “conservation” for views in Edo and Tokyo in an urban transformation context. First, in the foundation phase, this paper focuses on mountain vistas. Block planning in the early Edo period is considered to have been affected by the orientations of Mt. Fuji, Mt. Tsukuba, and a small hill inside the urban area. It also covers views within places of interest that attracted artists, and indicates such undulating landscapes were chosen deliberately. Second, this paper analyzes some views created under modern town planning and urban design systems and techniques, specifically those based on baroque-style urban design in which some vistas of monumental structures such as the Diet Building and Tokyo Station were embedded into the existing urban space. Furthermore, new views created when developing public spaces utilizing existing views or existing monuments reconstruct the spatial order. Third, many views have been obscured in central Tokyo, where building space has become increasingly dense. Furthermore, views of landmarks have also been blocked or restricted by new buildings. Nowadays, there are few Fujimi slopes from which we can command a view of Mt. Fuji. Moreover, vistas and landscapes of traditional Japanese gardens have been artificially disturbed by developments in their settings. Finally, in the conservation phase, Tokyo metropolitan government and local authorities of their respective wards have introduced a system, which we call the “View Conservation Plan,” since the “Landscape Act” was established in 2004. Some vistas of an outstanding monument, landscapes of traditional Japanese gardens, and several local views have at last been recognized to require protection. However, these plans are still at an early stage and we have yet to verify their effectiveness. Furthermore, we must continue to improve views in terms of both theory and management techniques. On the basis of these discussions, this article concludes that we need sophisticated theories and techniques of view management planning particularly to consider how we can create new views, while taking into account existing urban issues, and we need to come up with ideas for taking advantage of views as local resources to raise a shared awareness of the importance of views in our daily experiences.
This study verifies the vulnerability of Tokyo to natural disasters. Landforms distributed in Tokyo are classified into Yamanote upland, Tokyo lowland, Tamagawa lowland and coastal lowland. Among these, the Tokyo lowland has suffered from natural disasters several times because its elevation is low and thick soft soil covers a broad area. River courses of the Tone and Ara Rivers flowing through the Kanto lowland to Tokyo Bay were modified for shipping and for agriculture on the flood plains in the 17th century. Withdrawal of ground water due to industrial use from the end of the 19th century to the beginning of 1970's caused land subsidence in the Tokyo lowland. These artificial changes of natural conditions continue to affect natural disasters and countermeasures in the Tokyo lowland. Although large investments have been made since the 19th century, Tokyo has suffered from a series of natural disasters because improvements to safety from natural disasters led to a concentration of property and people in Tokyo with increased potential for damage. Extensive damages due to flooding river, storm surges and earthquakes are now causing alarm. Serious warnings of the effects of urban development have been given due to vulnerable natural conditions. Irreversible changes to natural and social conditions in urban areas increase vulnerability, and may bring about a catastrophic disaster in the future.
This paper reviews historical volcanic disasters that have affected the Tokyo Metropolitan area and its surroundings, central Japan, and discusses the dangers of volcanic disasters occurring in future. The 1707 (Hoei) eruption of Fuji volcano, the 1783 (Tenmei) eruption of Asama volcano, and the so-called Kanto Loam, volcanic soil deposits containing large quantities of Holocene to Pleistocene fall-out tephras, suggest the potential hazards that originate from volcanic activities. Small to moderate eruptions (VEI 1 to 2) of Asama volcano have resulted in minor ash falls in and around Tokyo every one to two decades. It is most likely that Asama volcano will generate minor ash falls in the near future. Volcanic disasters caused by larger but rare eruptions of VEI 4 to 5 are considered, referring to the 1707 (Hoei) eruption of Fuji volcano, and measures and predictions for the next eruption of Fuji volcano. In this paper, volcanic disasters affecting Tokyo in the near future are not only those caused by ash falls but also those caused by lahar along the Tone, Edo, Sakawa, and Sagami rivers related to Asama, Haruna, and Fuji volcanoes, because the landform developments of these areas in Holocene and historical disasters suggest that these drainage basins have the potential for lahar disasters. In addition, more severe eruptions of VEI 6 to 7 are considered for their impacts and frequencies referring to geological records of air-fall tephras and/or pyroclastic flow deposits such as VEI 6 Hakone-Tokyo tephra (ca. 66 ka) and VEI 7 Aira-Tn tephra (ca. 29 ka).