South Kanto District Research Articles

Earthquake Studies and the Earthquake Prediction System in Japan

At 11h58m of Sept. 1, 1923, the coastal area of the south Kanto District, including big cities such as Tokyo, Yokohama, Kamakura and Atami, was violently shaken by a big shock. Disastrous vibrations continued for a couple of minutes. Immediately after the earthquake, fires arose at 163 points in Tokyo and about 3,800 ha were burnt to ashes. About 316,000 houses, 70% of all the houses in Tokyo, vanished in the fires. In Yokohama, fires arose at about 60 points and burnt about 950 ha and 60,000 houses, that is, 60% were destroyed by fires. Sum of the dead and the missing was 142,807, about 80-90% of whom were killed by fires. The most tragic event took place in the Hihukusho-yard (6 ha), which is located in downtown Tokyo. Many people who escaped from fires and other injuries gathered in this place with the minimum of personal property necessary for daily life. Fire was started on this meager assemblage and finally, 44,030 persons – almost all who gathered there – were burnt to death.

ニホンアカガエルの個体群動態と圃場整備、耕作放棄、復田の関係 -南関東における事例-

ニホンアカガエルの個体群動態と圃場整備、耕作放棄、復田の関係 -南関東における事例-

Detailed horizontal crustal movements associated with the 1923 Kanto earthquake as deduced from network adjustment of old data including the third order triangulation stations

The crustal movements associated with the 1923 Kanto, Japan, earthquake of magnitude 7.9 are deduced from the results of network adjustments with the pre-seimic and post-seismic geodetic data including up to the third order triangulation. The average spacing of the third triangulation is 4 km of higher density as compared with the first order triangulation and the second order triangulation, so we can expect to find much more detailed behavior of the released crustal strain. The main rupture of the 1923 Kanto earthquake that had occurred in the Sagami Bay has been estimated from the crustal deformations deduced from repetition of the first order triangulation and the second order triangulation, while the detailed deformations associated with the secondary faulting are deduced for the first case from the repeated third order triangulation. We find evidence for the secondary faultings in the south Kanto district, Japan, mainly from the pattern of the released strain with the order of 10 −4 and some of them are associated with the surface breakages. We carefully investigate whether these secondary faultings had originated with fracturing at the depth or not. We cannot find any evidence for deep fracturing, though we find remarkable high earth strain release along some secondary faultings. This is mainly due to the fact that the area of high released strain is very limited and narrow along the direction perpendicular to the faulting.

南関東の完新世火山灰土壌の母材 一次鉱物組成と元素組成による判定

Volcanic ash soils occur widely on South Kanto district. The surface soil has black color and high humus content. The parent material is derived from Holocene volcanic ash. There are some buried humic horizons resulted from a succession of volcanic ash accumulation and plant residue supply leading to the accumulation of soil organic matter or humus. It has commonly been considered that Mt. Fuji supplied parent material of those soils.A huge amount of Holocene volcanic ejecta (tephra) with a large particle size accumulates at the eastern foot of Mt. Fuji, Tephrochronological studies enabled us to identify each scoria layers.However, little information of the tephrochronological study is available for soils formed in the region away from Mt. Fuji, such as on Musashino Plateau. A small amount of volcanic ash accumulation with a small particle size, compared with those in the eastern piedmont region close to Mt. Fuji promoted soil formation in this region, it is difficult to identify the characteristic tephra on Musashino Plateau on the contrary to the tephra of piedmont region which can easily be identified. Furthermore, it is assumed that more than one volcano in addition to Mt. Fuji such as Mt. Asama and Mt. Haruna supplied volcanic ash with a different rock type from that of Mt. Fuji in Musashino Plateau region. It is important to claryfy to what extent other volcanoes contributed in south Kanto district as a source volcano of the parent material of soils.In this paper, we compared morphological features, primary mineral composition, and element composition of three profiles which were locatd in Fujisawa City, Kanagawa, Fuchu City, Tokyo, Hidaka Town, Saitama respectivery. They are located almost in the same ditsance but the different direction from Mt. Fuji.At first, we directed our attention to buried humic horizon which was often identified as a horizon where it covered the archaeological site and remnants of the Nara-Heian period in Fuchu City, Tokyo. The horizon has some morphological and mineralogical characteristics; most blackish color in the solum, fine granular structure, and high olivine content. We gave the horizon tentative name as H-1BB (Holocene 1st Black Band).The results are summarized as follows:(1) On the basis of primary mineral composition, following were concluded: The dominance of heavy minerals and high content of olivine indicated that, parent material of Fujisawa pedon was supplied from Mt. Fuji. The dominance of light minerals, high content of non-colored volcanic glass and clinopyroxene, parent material of Hidaka pedon was supplied from Mt. Asama, Fuchu pedon indicated the transitional character between Fujisawa and Hidaka.(2) H-1BB is identified morphologically in all profiles based on relatively strong blackish color and structure. There was high content of transparent olivine in this horizon in every pedon. Because Fujisawa pedon indicated the highest olivine content, We assumed that H-1BB in three pedons were derived from more or less the same parent material erupted from Mt. Fuji.(3) From the viewpoint of chemical classification of volcanic rock, Si contents themselves, didn't directly reflect primary mineral composition of these soils. However, the profile which abounded in the parent material from Mt. Fuji had high Ca and Sr content. H-1BB in three profiles had high Ca and low K proportion in base element.

中期更新世に噴出した大町ＡＰｍテフラ群の層位と年代

Five distinctive tephra beds (A1Pm-A5Pm), carrying abundant biotite and quartz crystals, are widely distributed in central Japan. They are here collectively called Omachi APms or APms in short, and are petrographically identified by high refractive indices of hornblende (n2=1.685-1.696) and orthopyroxene (γ=1.728-1.737). One bed of the APms is found as far as 239km from the presumed source, Momisawadake in the Hida Mountains, where it is 4cm thick and composed mainly of silt-sized glass. No proximal deposit made up of coarse pumice fragments is found owing to unfavorable preservation of the mountainous landscapes; however, the eruption style is supposed to be of plinian type based on analyses of grain size along the dispersal axis.Assuming that the accumulation rate of loess soil has been constant at each locality, the age of APms is estimated 0.24-0.33 Ma by the overlying soil thickness. Moreover, in the south Kanto district, one of the APms is intercalated between two well-established marker-tephra: GoP (0.25-0.30Ma) and TE-5 (0.35-0.40Ma). It is very likely that the age of APms is 0.30-0.35Ma. The time interval between A1Pm and A5Pm eruptions is estimated to be less than 0.05Ma, because the thickness of the soil layer intercalatcd between APms does not exceed 106cm at any locality.The Omachi APms are found on the flanks and slopes of many volcanoes in central Japan, such as Azumaya, Yatsugatake, Kusatsu Shirane, and Takahara. They are also found among deposits forming fluvial terraces in north Kanto. The significance of APms as Middle Pleistocene time-markers will be established hereafter.

南関東におけるプレート間サイスミックカップリングと仮想東京直下地震

南関東におけるプレート間サイスミックカップリングと仮想東京直下地震

Forecasting of air pollution potential in the South Kanto district in Japan

Forecasting of air pollution potential in the South Kanto district in Japan

Areal Distribution of SO2 Pollutants and Local Wind Flow in South-Kanto District

Areal Distribution of SO2 Pollutants and Local Wind Flow in South-Kanto District

南関東地域の中部更新統の層序とその特徴

The Middle Pleistocene formations of the South Kanto district attract many geologists' attention as one of the important Quaternary type successions in Japan, because of wide distribution, extremely thick sequence of aqueous and aeolian deposits and containing a large number of fossils. The geologic structure of the formations in this district, however, was complicatedly deformed during Quaternary, and detail stratigraphy were veiled till quite recently.In this paper, we proposed a preliminary stratigraphic chronology on the Middle to Upper Pleistocene in the district (Table 2), which has been compiled in some recent fruits by several authors (KANTO QUATERNARY RESEARCH GROUP, 1974; MACHIDA et al., 1974; UESUGI, 1970; KIKUCHI, 1977; NISHIMURA, 1980; etc). Some geologic characteristics of the Middle Pleistocene formations are enumerated, too.(1) Some volcanoes situated in the western region produced the enormous volume of pyroclastic fall materials during Quaternary. An almost continuous columnar section of aeolian pyroclastic materials deposited during the past 400, 000 years is compiled from several sections of the Oiso Hill adjacent to the Fuji-Hakone volcanic region, which is more than 250 meters in thickness.(2) Distribution of the Middle Pleistocene aqueous and aeolian deposits in the district is very complicated. Lithofacies of these deposits are extremely variable in a narrow area. Many characteristic pyroclastic key beds are most useful to correlate with these various deposits.(3) The marine deposits of the Middle Pleistocene are 300 meters thick in the Oiso Hill and 600 meters in the Boso Peninsula. They are divided into 8 to 10 stratigraphic units by disconformable boundaries caused by lowerings of the sea level. Formation of each unit corresponds to a stage of glacial eustatic sea level rising.(4) It is generally believed that the South Kanto is one of the rapid upheaval districts in Japan and there are several marine terraces recording the Middle to Late Pleistocene sea levels in the district. We must indicate, however, that there is no graded feature formed during the Middle Pleistocene at least except the Tama Hill area along the west coast of Tokyo Bay. The contemporaneous marine deposits accumulate one after another or intermittently with disconformities at the great part of the coastal area and do not constitute any terraces.

A model for the earthquake cycle in underthrust zones

The time‐dependent surface deformation due to thrust faulting in an elastic plate overlying a viscoelastic half space is examined and compared with geodetic measurements of earthquake‐related crustal movements from Japan. The model fault is two‐dimensional, and the half‐space rheology is that of a Maxwell solid whose instantaneous response to applied loads is purely elastic but which subsequently flows to relieve imposed shear stresses. A characteristic feature of shallow‐angle underthrusting shown by these computations is that buried slip produces predominantly land uplift, while asthenospheric relaxation due to surface faulting results principally in surface downwarping. These distinctive patterns, also reflected in the measured deformation, are particularly useful in formulating a viable model for the entire earthquake cycle. For the model constructed in this way, coseismic faulting in the upper part of the plate transfers a load to the lower lithosphere and asthenosphere, inducing postseismic slip downdip of the seismic rupture and relaxation in the asthenosphere, these transients eventually merging into the steady buried slip and smoother asthenospheric flow that characterize the interseismic phase of the deformation cycle. Aseismic slip and asthenospheric relaxation cause the load supported by the plate‐bounding fault to be gradually transferred back to the shallow locked segment of the fault, effecting the strain buildup for a subsequent earthquake, and the cycle is repeated. No explicit account of plate‐driving forces is made; their net effect is presumed to be the steady buried slip that persists throughout the cycle and provides the energy that drives it. Characteristic features of the observed deformation in southwest Japan, site of the 1946 Nankaido earthquake, and in the South Kanto district, where the great 1923 earthquake occurred, are matched by this model using conventional values of lithospheric thickness (60 km) and asthenospheric viscosity (1021 P, or 1020 N s/2), although the model is not strongly tied to these exact values. Postseismic movements are adequately explained by episodic slip that occurs below the coseismic rupture, is 10–30% of the seismic slip, and acts in the same sense. Superimposed on these movements are the lesser effects of asthenospheric relaxation, which in the South Kanto district contribute significantly to the postseismic vertical level changes because post‐1923 buried slip is comparatively small and involves largely strike slip movements. Interseismic subsidence in both regions is explained well by asthenospheric relaxation and less significant deformation effects due to steady aseismic slip on the lower part of the plate boundary.

Migration of crustal deformation

Abstract Observations on the migration rates of crustal deformation, as recently discovered in several tectonic areas, such as the south Kanto and central Tohoku districts, Japan and the West Cordillera Mts., Peru, has opened up a new opportunity for the study of crustal dynamics. Briefly, these examples from coastal areas are characterized by migration landwards with a velocity of about 10–100 km/yr. This agrees well with the velocity of mIgration of seismicity as previously known. Dispersion and dissipation of the deformation waveform are also noted as characteristics. Simple extrapolation of the migration path back toward the ocean may locate a possible origin of the event. In the case of the south Kanto district, for example, the deformation front seems to have originated in the early 1950s from the vicinity of the junction of the Japan and Izu—Mariana trenches. The deformation front in the central Tohoku district, on the other hand, is thought to have originated in the northern part of the Japan Trench in the late 1960s. One may suppose that either a repeated irregular aseismic plate motion generates the deformation events, or that it results from a periodic seismic slip at a plate boundary. In the latter case, the 1953 Boso-oki and the 1968 Tokachi-oki earthquakes might be suspected of generating the deformation fronts in the south Kanto and central Tohoku districts respectively. As Scholz speculated, the migration of a deformation front might trigger earthquakes, if it hits areas of high seismic potential. Studies of migration events can contribute significantly to earthquake prediction studies.

On Accumulation and Release of the Earth's Strain in the South Kanto District

On Accumulation and Release of the Earth's Strain in the South Kanto District

The behavior of a convergent plate boundary: Crustal deformation in the South Kanto District, Japan

The northwesternmost part of the Sagami trough, a part of the Philippine Sea‐Eurasian plate boundary, was ruptured during the great South Kanto earthquake in 1923. Very extensive and frequent geodetic measurements of crustal deformation have been made in the South Kanto district since the 1890's, and these constitute the most complete data set on crustal movements in the world. These data were reanalyzed and interpreted and according to our interpretation indicate the following sequence of events. The coseismic movements were due to oblique thrust and right lateral slip of about 8 m on a fault outcropping at the base of the Sagami trough. This was followed by postseismic deformation resulting from reversed afterslip of 20–60 cm that occurred at an exponentially decaying rate in time. The interseismic deformation is produced by steady subduction at a rate of about 1.8 cm yr−1. During subduction the top 10–15 km of the plate boundary is apparently locked, while deeper parts slip aseismically at an irregular rate. No significant precursory deformation was observed. The recurrence time for 1923 type earthquakes is 200–300 years. The Boso and Miura peninsulas are broken into a series of fault‐bound blocks that move semi‐independently of the surrounding region. The subduction zone itself, where it is exposed on land, is shown to be a wide zone encompassing several faults that are active at different times.

Artificial Ground Movement in the South Kanto District

As a various kind of the artificial ground movements are observed in the south Kanto district, we have described only the artificial ground movements with the developments of the ground water resources and the natural gas resources (natural gas bearing salt water) in this paper.The stratum-deformation patterns caused by the pumping action are different according to the geological age of the stratum. The Holocene deposits have shown generally the irreversible consolidation with the draw down of ground water pressure by the pumping discharge.The Pleistocene deposits, however, have been contracted under the decrease of ground water pressure by the over pumping discharge but have been rebounded slightly under the recovering of the ground water pressure by the pumping control.On the other hand, it has been observed that the pulse of the ground water pressure has been annually by the change of pumping discharge of the wells for the agricultural use and etc., and has been conveyed undulatingly to a distance. Therefore, it seems that the ground movements have been carried undulatingly by the undulative change of the ground water presures.

南関東・西南日本外帯の地震性地殻変動区に於ける巨大地震の再来周期

The seismic crustal movement on the south Kanto district and the Philippine Sea coast of southwest Japan is characterised by the chronic (in the interseismic period) and acute (at the earthquake) tiltings with reverse sense. The accumulation of these tiltings in geological time scale explains the topographic features on the coast such as tilted terraces. The value “d”, which is defined as the ratio of the subsidence in the interseismic period to the coseismic upheaval, was expressed by two independent linear functions of recurrence interval T of series of great earthquakes. The coefficients of these functions are determined by the data of precise levellings and the heights of coastal terraces in the late Quaternary period. Then, the d and T can be determined on the cross point of the two lines which represent the above two functions. This scheme (called d-T diagram) was applied to the Seismotectonic areas of the southern Kanto and the outer arc of the southwest Japan. The results of T obtained are almost in concordance with the intervals of great earthquakes inferred from the historical documents and the rates of convergence of the Philippine Sea plate under the southwest Japan. It was also shown that the d-T diagram can be used as a tool of earthquake prediction taking an example of the Tokai region.

岡山県倉敷市地下の地質とけいそう遺がい群集

Core samples from the two boreholes were studied to clarify the sedimentary environment by diatom assemblage analysis: the results are as follows.1. The Holocene sediments are divided into two parts, namely the sand and the silt facies (Hs Hc). It seems that they are synchronous and heterofacies to each other. The upper part of Hs-facies may be correlative with sediments during and after the small regression of Yayoi belonging to Subatlantic age. And lower and middle parts of Hs-Hc-facies may be correlative with sediments during the Jomon transgression.2. Beneath the Holocene, the late Pleistocene sediments are developed to fill up dissected and buried valleys. The uppermost Clay (C) may be correlative with the Nanagochi formation in the Tokyo downtown area and the Nanyo formation in Nobi Plain. The sand and gravel (G-1) may be correlative with the Buried terrace gravel-1 in the Tokyo downtown area. The clay underlying the former (Mc) may be correlative with the Tachikawa Loam in South Kanto district. Carbon date of a sample from the lower part of the alternation of sand, silt and clay (Alt. s-c) shows the value of 24, 260+2, 810 -2, 080y. B. P.3. Some additional discussion on the systematic position of Coscinodiscus temperei, Cymatotheca weissflogii and Tryblioptychus cocconeiformis are made from a viewpoint of morphological features of diatom valves.

Probability of earthquake occurrence as obtained from a Weibull distribution analysis of crustal strain

Probability of a large-scale earthquake occurrence is estimated from crustal strain geodetically detected over an earthquake area. The Weibull distribution function, which is widely applied to quality-control research, is made use of in this paper in the probabilistic treatments of crustal strain. Using the table of ultimate strain presented by Rikitake (1974), a Weibull model representing a statistical distribution of crustal-rupture occurrence time is determined on the assumption that the crust is strained with a constant speed. In the case of the South Kanto District, the associated probability-density function has a maximum at about 84 years after the time when the strain energy accumulation starts.

A Discussion on the measurement and interpretation of changes of strain in the Earth - Earthquake fault studies in Japan

The paper reviews the current status of earthquake fault studies in Japan, taking the work on the off-Honshu tectonics as an example. Within the Japanese tectonic provinces, the belt along the Pacific coast of central and western Honshu is of special interest from the viewpoints of earthquake prediction as well as of global tectonics. Observations in this field are being made most extensively in the south Kanto district as represented by a complex monitoring system surrounding the Sagami Bay. Complemental use of geophysical instruments with geodetic surveys is successful. The basic mode of strain accumulation thus observed seems consistent with the Sagami trough fault model as proposed by Kanamori and Ando in 1971 to explain the focal processes of the great Kanto earthquake of 1923. Comparison of tiltmeter records at the adjacent stations Aburatsubo and Nokogiriyama suggests crustal strains migrating at an extremely low velocity (20 km/ year, apparently), from east to west (Yamada 1972). Nakamura and Kanamori have shown that correlation of the activities of this fault with those of the Oshima volcano and seismicity in the Kii Peninsula, as seen in old records, provides notable evidence for the mechanism of stress transmission in a tectonic unit.

Secular Gravity Changes Observed in South Kanto District

Secular Gravity Changes Observed in South Kanto District

On Accuracy of the Earth Strain through Geodetic Method in the South Kanto District

On Accuracy of the Earth Strain through Geodetic Method in the South Kanto District