Central Japan Sea Research Articles

Diatoms Flora found on Fossil Cetacean Jaw (Ziphiidae) from the Yamato Bank, the Central Japan Sea

Diatoms Flora found on Fossil Cetacean Jaw (Ziphiidae) from the Yamato Bank, the Central Japan Sea

大・中地震の観測記録に見られる短周期波について

The origin of high-frequency waves superposed on low-frequency waves was investigated using observed seismograms at hard rock sites. Two sets of data were used in this paper. One is the seismograms of main shocks and foreshock or aftershock sequences (M2.0-5.2) which occurred in the Kinki district, Japan. The other is the seismograms of the aftershocks of the Central Japan Sea earthquake (M5.3-7.1). For the earthquakes in the Kinki district, P-wave velocitywaveforms of a couple of events which occurred almost at the same location, were compared in the time domain. It was found that the high-frequency waves superposed on low-frequency waves generated by main shocks are very similar to the waveforms of their foreshocks or aftershocks. For the aftershocks of the Central Japan Sea earthquakes, the high frequency waves of every event had almost the same shape, even if its hypocenter is different from each other. Furthermore, their three component waveforms indicated that the high frequency waves did not arrived from the direction of their sources. It was concluded that almost all the high frequency waves analyzed in this paper reflect the site effect rather than the source process.

East Asian Seas: Hypothetical oil spill trajectories

Hypothetical oil spill trajectories are projected for each monsoon from four locations near Japan: the central and southern Sea of Japan; and the northern and southern East China Sea. Actual and possible maritime jurisdictional boundaries are superimposed on these trajectories. Spills at most sites would soon cross international jurisdictional boundaries and eventually reach a sometimes estranged neighbor's coast.

日本海中部地震発生による三陸沖地震活動の低下と東北日本のブロック構造

Seismicity in the east-off Sanriku region decreased conspicuously after 26 May 1983, when the central Japan Sea earthquake with a magnitude of 7.7 occurred on the west coast of Tohoku, northeastern Japan. The decrease was restricted to shallow seismicity in the depth range of 0-40 km. For deeper earthquakes in the depth range of 41-70 km rate of earthquake occurrence rather increased. Seismicity in the land area of northern Tohoku also decreased at the same time. However, seismic activity in the Oga-Oshika tectonic zone increased remarkably after the central Japan Sea earthquake. Increase of seismicity was also observed in the sea region corresponding to the extension of the Oga-Oshika tectonic zone. We consider that the coincidence of the notable decrease of shallow seismicity in the coupling region between the Pacific plate and the land plate with the central Japan Sea earthquake occurring on the west coast of Tohoku indicates the following tectonic regime. The northern Tohoku district constitutes one rigid-like block and stress fields in the both sides of the block, e. g., stress at the east-off Sanriku subduction zone and that at the active tectonic zone along the coast of the Japan Sea, influence each other. On the other hand the increase of seismic activity in the Oga-Oshika tectonic zone and in its extension after the central Japan Sea earthquake is considered to show that the zone corresponds to the southern boundary of the northern Tohoku block and change of stress is likely to be transmitted along the zone. Seismicity in the Oga-Oshika tectonic zone has a tendency to increase always when large earthquake occurs in the zone or in its extension. It should be noted that decrease of stress in the coupling region between the Pacific and the land plates due to the occurrence of the central Japan Sea earthquake was confined to the north of the Oga-Oshika tectonic zone. Seismicity in the subduction region to the south of the Oga-Oshika tectonic zone did not show any change when the central Japan Sea earthquake occurred. It is interesting to note as well that seismicity in the Pacific coast and sea regions did not change at all by the 1964 Niigata earthquake with a magnitude of 7.5 whose focal region was located on the east coast of the Japan Sea, but to the south of the Oga-Oshika tectonic zone. These facts suggest that the eastern Japan, eastern side to the Itoigawa-Shizuoka tectonic line, does not constitute a single microplate, but is separated mechanically at the Oga-Oshika tectonic zone.

日本海東縁地域の地震活動とメカニズムおよびテクトニクス

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.

Synthesis of strong motions by using an aftershock record as the Green's function

A method to synthesize strong ground motions by using aftershock records as Green's functions was developed to reproduce the high-frequency components of the main shock. We call it a random slip model, and have applied it to three large earthquakes, the east off-Izu Peninsula earthquake of 1980 ( M = 6.7), the off-Urakawa earthquake of 1982 ( M = 7.1) and the Central Japan Sea earthquake of 1983 ( M = 7.7). Seismic intensities at six stations during the earthquakes analyzed here were from I to VI on the JMA (Japan Meteorological Agency) scale. All the aftershock seismograms used as Green's functions in the present study were recorded by velocity-type strong-motion seismographs with a large dynamic range over a wide frequency range. A few main-shock seismograms were recorded by seismographs similar to those mentioned above; the rest were recorded by other types of strong-motion seismographs e.g. the SMAC (strong-motion accelerograph) and JMA strong-motion seismographs. From a comparison of the synthetic with the observed seismograms of the main shock, we conclude the following: 1. (1) Using the record of an adequate aftershock and an adequate source model, we can satisfactorily reproduce any observed seismogram of the main shock. 2. (2) The conditions for an adequate aftershock are that the hypocenter is located in the central part of the main-shock fault, and that the mechanism is similar to that of the main shock. 3. (3) The high-frequency waves contained in the main-shock seismograms are reproduced by the random slip model up to 5 Hz for records at every station. 4. (4) By considering a multiple-shock source model, we can fit the synthetic to the observed seismograms for two large earthquakes with magnitudes ( M j ) larger than 7.

On two characteristic frequencies of acceleration spectra: Patch corner frequency andfmax

AbstractWe examine two issues related to the radiation of tectonic earthquake sources: (i) the complexity of the shape of observed source spectra, and (ii) the effect of recording site characteristics on the cut-off frequency fmax of acceleration spectra beyond which spectral amplitudes diminish sharply.We compute the far-field spectra of the 1979 Imperial Valley and the 1984 Morgan Hill earthquakes from the slip functions that were inferred from detailed inversion studies of strong motion and teleseismic data of these two California strike-slip events. These spectra, along with data from the 1983 Central Japan Sea earthquake sequence (main shock and aftershocks), demonstrate the existence of the so-called patch corner frequency, which is related to the scale-length of heterogeneities of the fault plane. Such a frequency is a distinct feature of the source spectrum of the specific barrier model of Papageorgiou and Aki.By analyzing accelerograms of the San Fernando earthquake of 1971, we found that fmax determined from records at soil stations is nearly the same as that determined from records at hard rock stations.

1983年日本海中部地震に伴う世帯レベルの被災・復旧過程に関する調査

1983年日本海中部地震に伴う世帯レベルの被災・復旧過程に関する調査

日本海に発生する浅発小地震の発震機構

Seismogenic stress field off the western coast of the Tohoku District, north-east Japan, is clarified by means of focal mechanism of small-to-moderate size earthquakes using polarity data of P-wave first motion. Foreshocks and aftershocks of some moderate size events show irregular patterns of direction of the maximum compressive stresses and of faulting style. Exclusive of such foreshocks and aftershocks, the directions of the maximum compressive stresses are concordant with those derived from two major events in the area, the 1964 Niigata earthquake and 1983 Akita-oki (Central Japan Sea) earthquake. East-west direction of the maximum compressive stress is dominant off the western coast of the Tohoku district. The direction gradually changes to northwest-southeast toward the northern edge of the Itoigawa-Shizuoka Tectonic Line.The isolated event of M 5.1 of May 16, 1975, which occurred in the southern edge of the middle part of the Japan Sea Basin appears to be peculiar in points that oean bottom topography in the source region shows no traces of seismotectonic deformation and that the maximum compressive stress of the event points to NNW-SSE, which is different from those of the seismogenic stress field off the western coast of the Tohoku district.

関東農政局管内の地すべりと崩壊

The writers plotted a location of active mass movennent on a 30“×45” mesh map.It depends on an observation of a stereoscopic pair of aerophotos and surficial survey. Mass movement lies on the elevated side of thrust fault, an anticlinal axis of active folding and the margin of a tectonic basin. Such statistics are shown in this paper.Itoigawa-Shizuoka tectonic line is a greatest thrust fault which has been identified with a boundary line between Eurasian and North American plates since the Central Japan Sea earthquake 1983.The division of creep movement and landfall depends on the facies of layer. Clayey soil introduces creep movement and sandy one an accidental fall by heavy stormy rain or the earthquake. Ancient or fossil mass movement marks are found independent of present crustal movement. Artificial land development may lead slope disasters. They may issue from fossil landslides.Deposits of dam sites-Miwa, Koshibu, Yasuoka and Hiraoka-are very large along the Tenryu river, which flows through an upheaval district. An occasional mud flow takes its rise at Mt. Hieda along the Ura river, Nagano prefecture.

1983年日本海中部地震に伴う世帯レベルの被災・復旧過程に関する調査

At the 1983 May 26 Central Japan Sea earthquake (M=7.7), dwelling and agricultural facilities were severely damaged by sandy soil liquefaction in rural town Wakami, Akita prefecture. This paper presents how each household was affected by and responded to the earthquake disaster for a considerable period of time. Waiting for 6 months after the earthquake, a questionnaire survey was performed for all the 2198 households and interviews were made supplementally for the seriously affected 161 households. The questionnaire asks characteristics of family and housing, damage to dwelling, damage to farming, fishing, shops and so on, effects on daily life after the quake, economic impact, and surrounding conditions related to seismic intensity.The results obtained are as follows.1) Distribution of dwelling damage along 20km within the town, distinctly different between the northern and southern parts, is similar to those of seismic intensity and gas pipe damage. Micro topography of dune and coastal plain amplifies the damage.2) Severe damage to rice field in the northern part of the town resulted in decrease of harvest. Damages to farm land and dwelling are economically equivalent and together amount to 60% of annual income there.3) Methods to repair a dwelling depend mostly upon degree of damage, partially upon building age and insurance payment. Reconstruction costs ¥15 million in average and tends to make houses larger than before. However needy families find it difficult to get proper fund for restoration and suffer from insufficient repairing.4) Economic loss is covered far more by public or private loans than by insurance payment. Among ¥2.9 billion of total direct damage to households in the town, only 18% can be paid by insurance. Economical hardship is likely to last for decades.5) It takes from one week to one month to resettle surroundings, to recover gas supply and to return to daily lives. Repair of houses starts later and finishes 4 months after the quake in average. Repair works of dwellings scatter largely from just after the quake to late fall. Some were still waiting for the next spring. Reconstruction takes about 121 days in average and many families stay in temporary housing.

DISTRIBUTION OF WINTER PRECIPITATION BROUGHT ABOUT BY THE PASSAGE OF DEPRESSIONS OVER THE JAPANESE ISLANDS

The winter climate of Japan is dominated by the northwesterly monsoon, which brings: abundant precipitation in the form of snow to the regions facing the Japan Sea. Most climatological studies on winter precipitation have concentrated upon the relationship betweenn the heavy precipitation near the Japan Sea and the winter monsoon, but a considerable amount of precipitation is brought about also by the passage of depressions or fronts. The purpose of this paper is to reveal some characteristics of the precipitation brought about by the passage of depressions, especially its areal distributions and the time variation of precipitation intensity. The results obtained are summarized as follows: 1) The depressions which pass over the Japanese Islands in winter may be classified into two types: the Japan Sea Low type, in which depression forms over the southwest of the Japan Sea, moves northeastward, and passes through the Soya or the Tsugaru Strait, andd the Pacific Coast type, in which depression rises over the East China Sea and travels eastward along the Pacific Coast of Japan (Fig. 1). 2) The Pacific side has little precipitation during the passage of Japan Sea Lows. The distribution of precipitation associated with these depressions is similar to that of monsoonall precipitation. Furthermore, it is fairly irrespective to the location of the depression. On.n the other hand, Pacific Coast Lows bring more precipitation to the Pacific side than to the Japan Sea side, and the distribution of precipitation changes as the depression moves eastward. The ordinary monsoonal precipitation pattern disappears during the passage of a. Pacific Coast Low, and then recovers once more after the passage (Figs. 2, 3 and 4). 3) There are conspicuous regional differences in the characteristics of precipitation during the passage of depressions. On the basis of the depression type which brings precipitation and the period of the maximum intensity of precipitation, eight major regions can be identified.as follows (Figs. 5, 7 and 9). On the Pacific side: (a) West of Kanto-more precipitation occurs during the passage of a Pacific Coast Low than during the passage of a Japan Sea Low. The intensity of precipitation increases rapidly as the depression approaches, reaches a maximum just before the arrival of de pression, and decreases with the depression moving away to the east. (b) Inland Honshu-similar to region (a), except that period of maximum intensity of pre cipitation is longer. (c) Tohoku-similar to region (b), except that the intensity of precipitation decreases much more slowly after the depression has passed. (d) Hokkaido-there is little difference in the amounts of precipitation brought by the two types of depression, and either type produces a little precipitation, i, e. this region is not so affected by the passage of depressions. On the Japan Sea side: (e) San-in-more precipitation occurs during the passage of a Pacific Coast Low than during the passage of a Japan Sea Low, but unlike the Pacific side regions, the maximum intensity of precipitation occurs after the depression has passed. (f) Hokuriku-there is little difference in the amounts of precipitation brought by the two types of depression. More precipitation is brought to this region by the winter monsoon than by depressions. (g) Tohoku-more precipitation occurs during the passage of a Japan Sea Low than during the passage of a Pacific Coast Low. Precipitation reaches maximum intensity before a Japan Sea Low arrives; that is, while it is still located over the central Japan Sea. (h) Hokkaido-similar to region (g), except that the absolute amount of precipitation is less than in any other region except the Pacific side of Hokkaido, and there is no distinct period of maximum precipitation intensity.

The grasshopper-warblers, Locustella lanceolata and L. ochotensis, obtained at sea in the central Japan Sea

In August 1963, Umitaka Maru, a training ship of University of Fisheries, made a research cruise in central Japan Sea. Birds and insects observed during this navigation were recorded with meteorological notes.Twenty or thirty of Streaked Grasshopper Warblers Locustella lanceolata were encountered and one was obtained on August 21 in 36°54'N, 134°47'E and one Middendorff's Grasshopper Warbler L. ochotensis was captured on August 22 in 39°13'N, 135°02'E. On these days a low pressure storm of the maximum wind velocity 15m/s passed this sea area, causing strong NE winds at its front and accompanying heavy rains and SW winds south of it.On both days, dragonflies, Pantala flavescens, also passed near the ship. Apparently, the storm had drifted these birds and dragonflies (which were otherwise found near the coast) to the central Japan Sea while they were migrating along the coast.