著者

中西 一郎

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.59, no.3, pp.237-238, 2007-03-15 (Released:2013-08-05)

参考文献数

21

著者

奥澤 保 塚原 弘昭

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.53, no.3, pp.241-253, 2001-03-25 (Released:2010-03-11)

参考文献数

31

被引用文献数

5

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Many small earthquakes occurred in the Matsushiro area in a period from 1965 to 1967. This is called “Matsushiro Earthquake Swarm”. The swarm earthquakes are interpreted as a result of deep ground water intrusion into the focal area. At the hardest time of the crustal movement, water flow from pre-existing springs increased, and many new springs appeared along a hidden fault zone formed by the swarm earthquakes. Some of springs are still alive and have peculiar chemical compositions. The deep ground water must have caused the earthquake swarm. We show the characteristics of the deep ground water through analyzing chemical composition. Our measured data and many published data show that the deep ground water obtains Ca2+ from rocks and loses Na+ during its upwelling migration through the fault zone of Matsushiro Swarm Earthquakes. The data also show that the major components of Cl- and (Na++Ca2+) have a liner relation in concentrations, although every spring water has different concentrations of Cl- and (Na++Ca2+). It is interpreted in terms of mixing deep ground water with shallow ground water at defferent ratios. The concentrations of the major ions of the deep ground water are estimated from both of our measured data and compiled data using the liner relation, e. g., Na+: 310meq/l, Cl-: 330meq/l. Analysis of our measured data also reveals that the small temporal variation in the concentration of Ca2+ in a short period such as several months is attributable to the reaction between the ground water with CO2 and surrounding rocks.

著者

山科 健一郎 中村 一明 福留 高明 佐藤 魂夫 田中 和夫

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.38, no.1, pp.81-91, 1985-03-25 (Released:2010-03-11)

参考文献数

18

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Based on a few tens of photographs, height change of Kyuroku-shima Island, which locates very close to the focal region of the Japan Sea earthquake (M=7.7) on May 26th, 1983, is analyzed. 1) The island and adjacent small islets are considered to have subsided by about 30-40cm associated with the 1983 earthquake. The average offset obtained by 13 reliable photographs is 32±9cm. 2) Precursory deformation and secular change of the height were not found during 1964 through 1982. No detectable deformation was caused by largest aftershock (M=7.1) on June 21, 1983, and other aftershock activity. 3) Coseismic tilt and local collapse were not observed in the island. 4) In some cases, a height can be inferred even from snapshots with the error less than several percents. Crustal uplift and subsidence more than 10-20cm may be possible to be detected photographically at the seashore and lakeside.

著者

伊藤 潔 梅田 康弘 黒磯 章夫 村上 寛史 飯尾 能久

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.39, no.2, pp.301-311, 1986-06-25 (Released:2010-03-11)

参考文献数

16

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The source characteristics of the Japan Sea earthquake, May 26, 1983 (M=7.7) is inferred from the seismic observation system with wide-frequency and large-dynamic range at the Abuyama Seismological Observatory. The duration of oscillation of the long-period low-gain seismogram (T0=25s) is much longer than those of other earthquake with nearly the same magnitude and nearly the same epicentral distance, which implies that the earthquake is a multiple shock. The relationship between a multiple shock and duration of oscillation is more clearly indicated in the figure of double amplitude envelope to eliminate the difference in amplitude by magnitude and focal mechanism. This simple method is applicable to detect multiple shocks in seismograms at one station, especially in historical seismograms with a few instrumental records.Seismograms of the main shock of the Japan Sea earthquake recorded by Wiechert seismographs and those of middle-period (T0=10s) low-gain velocity seismographs show a clear onset of the second event at about 22 seconds after the first arrival. Since no such second arrival is seen on the seismograms of the aftershocks at the same station, the phase is not a crustal phase but a P-wave arrival of the second event of the main shock.The main shock recorded by the middle-period low-gain velocity seismograph contains more complicated high frequency waves than the largest aftershock. This indicates that the rupture process of the main shock is much complicated compared with that of the aftershock. Further, comparing the spectrum of the first event of the main shock with that of the second event, the average amplitude at a low frequency (5-10 s) of the first event is smaller than the second event, while that at high frequency (1-2s) is larger than the second event. This suggests that the main shock is composed of double events of different rupture type; the rupture of the first event is smaller and radiated much high frequency waves than the second event.

著者

海野 徳仁 伊藤 喜宏 五十嵐 俊博 長谷川 昭

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.53, no.3, pp.263-268, 2001-03-25 (Released:2010-03-11)

参考文献数

25

被引用文献数

2 3

著者

加藤 研一 武村 雅之

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.49, no.1, pp.75-83, 1996-05-24 (Released:2010-03-11)

参考文献数

27

被引用文献数

1

---

Direction of rupture propagation φ and fault length L for the 1994 Sanriku-Haruka-Oki earthquake are evaluated from the azimuthal dependence in duration of strong ground motion observed at eleven stations along the Pacific coast of Tohoku and Hokkaido region. After defining the strong motion duration D as the time interval between onset of S wave and 85% of cumulative power curve derived from 5 to 10Hz band-pass filtered accelerogram, the azimuthal dependence of D is examined. We find that D systematically shows directivity: D is the shortest at northern part of Tohoku and the longest at southern part of Tohoku and eastern edge of Hokkaido. By making use of the directivity on observed strong motion durations, we apply the method by IZUTANI and HIRASAWA (1987) to deducing φ and L together with VR/β, where VR and β are rupture velocity and S wave velocity. Assuming that rupture initiated from the epicenter determined by the Japan Meteorological Agency (JMA), we obtain L/VR=53, VR/β=0.62, and φ=W9°N (case 1). This result indicates that the rupture was headed toward northern part of Tohoku, and terminated near the western end of aftershock area. Detailed source-process inversion [for example, SATO et al. (1996)] suggests that main rupture nucleated near the center of aftershock area, corresponding to the epicenter determined by Harvard University. We redo the calculation assuming that the rupture initiated from the Harvard epicenter, then resulting in L/VR=28, VR/β=0.71, and φ=W18°N (case 2). This result also indicates that the rupture propagated toward northern part of Tohoku, and terminated at the western end of aftershock area. SATO et al. (1996) suggest that this earthquake consists of threestage rupture process: the first and the second sub-events corresponding to JMA and Harvard epicenter, respectively, and the third sub-event located near the western end of aftershock area at the distance of about 50km from Hachinohe city. The location where rupture terminated in both cases of this study is consistent with the location of the third sub-event, confirming that the rupture extended to the western end of aftershock area.

著者

力武 常次

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.42, no.4, pp.451-466, 1989-12-24 (Released:2010-03-11)

参考文献数

10

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Anomalous phenomena precursory to the 1891 Nobi earthquake of magnitude 8.0 are collected mostly based on the existing results of the enquiry investigation conducted soon after the earthquake. Precursor data amounting to 202 in number become available along with their precursor times. They are rumbling or detonation, anomalous animal behaviour, earthquake light, changes in underground water/hot springs and so on.It appears that the number of earthquake precursor tend to increase at about 100 days before the main shock. About 10 days preceding the shock, the increasing rate becomes much accelerated reaching a frequency peak having a precursor time of 1 day or so. Such a tendency of precursor appearance is much the same as that for the 1923 Kanto, 1944 Tonankai and 1978 Izu-Oshima Kinkai earthquakes although slight differences may be observed. The tendency that the number of precursor decreases as the distance between the epicenter and an observation spot increases is also confirmed.In light of these precursors detected by human sense in association with the Nobi earthquake as well as other large earthquakes in Japan, attention should be paid to precursors of this class along with geophysical precursors in future earthquakes.

著者

草野 富二雄 吉田 明夫

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.43, no.4, pp.459-469, 1990-12-24 (Released:2010-03-11)

参考文献数

20

---

Aftershock activity of the 1987 Chiba-ken Toho-oki earthquake (M6.7) is investigated using JMA data. It is found that aftershocks during two weeks just after the main shock occurred mostly in a region to the east side of the fault plane which is nearly north-south direction with a steep dip to the east. However, aftershock activity in the area to the west side of the fault plane became high since the beginning of January 1988. The contrast between spatial distribution of aftershocks in December 1987 and that after January 1988 is conspicuous. The later activity was concentrated to a rather small area and the largest aftershock occurred on 16 January in the active region. The mechanism of the largest aftershock was reverse type in contrast to the mechanism of the main shock which was strike slip type. Further, pattern of temporal decrease of aftershock activity deviated notably from the Omori's formula when the later activity was started. All these characteristics suggest that most earthquakes which occurred in the region to the west of the fault plane of the main shock after January 1988 are not the so-called aftershocks in a narrow sense, but that they represent an appearance of a new fracture, which occurrence might be caused by the stress concentration due to the fault motion of the main shock. The phenomenon that aftershock activity in the either one side against a fault plane is higher than that in the other side is frequently observed, even for fault motions of strike slip type. It is interesting to note that seismicity before the main shock was also asymmetrical, i. e. it was active in the region to the west of the fault plane of the 1987 earthquake. The seismicity in the recent one year also seems to be active in the west region. These features may show that the western block to the fault plane has taken a positive part in the accumulation process of stress in the focal region of the 1987 Chiba-ken Toho-oki earthquake.

著者

伊東 敬祐

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.50, no.appendix, pp.157-167, 1998-03-31 (Released:2010-11-17)

参考文献数

40

---

Earthquakes are complex phenomena. The notion of self-organized criticality (SOC) is recently used to explain various kinds of seismological relations expressed as power-laws, such as the size-distribution, the fractal spatio-temporal distribution and the decay of aftershocks. It is often claimed that earthquakes are not predictable if they are critical phenomena It is true that they are not predictable if the earth is at exact criticality. The notion of SOC. however, is not static but is dynamic. The crust gently approaches criticality and breaks down to generate a large earthquake. There are a variety of SOC models of earthquakes. In all of them, most of small and intermediate earthquakes occur while the system is at subcritical state approaching the exact criticality. Large earthquakes mostly occur near the criticality or at the supercritical state. Therefore, large earthquakes differ from small and intermediate ones even in the statistically scale-free SOC models. Large earthquakes in nature also are expected to be unusual events occurring at critical or supercritical states at which physical properties are extemely abnormal. Different SOC models exhibit different precursor phenomena before large events, as observed precursors differ from event to event. There cannot be universal methods to predict every large earthquake, but individual large earthquakes can be expected to have some precursors. It may be careless to reject observed precursors on a reason that they were not always observed before large earthquakes.

著者

多田 堯

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.29, no.2, pp.117-126, 1976-05-15 (Released:2010-03-11)

参考文献数

17

被引用文献数

1

---

A fault model of the 1974 Izu-Oki Earthquake was studied by the vertical crustal movement and the distribution of the after shocks.An assumed fault type is the right lateral pure strike slip, and the fault parameters are as follows, length=15km, width=8km, dip angle=75°, and dislocation=130cm.The stress field in the southern part of the Izu peninsula deduced from this fault motion is the north-south directional compressive field. This stress field represents that the Izu peninsula is compressed from south to north by the movement of the Philippine Sea Plate.

著者

武尾 実

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.42, no.1, pp.59-66, 1989-03-25 (Released:2010-03-11)

参考文献数

24

被引用文献数

1

---

The 1974 Izu-Hanto-Oki earthquake is studied in detail using near-field strong motion seismograms. A waveform inversion method is applied to deduce the dislocation distribution and the characteristics of the rupture propagation during this earthquake.This earthquake involved right-lateral strike-slip motion on the almost vertical fault plane with a strike of N47°W. The rupture initiated at the central deepest part of the fault plane and propagated both sides smoothly, as a bilateral rupture propagation. The total source process time is about 11sec. A dislocation larger than 1m occurred in the region ranging from 3km to 10km in depth, and its horizontal span is about 20km around the hypocenter. Except for the southeastern end of the fault plane, dislocation smaller than 0.5m occurred in the shallower region of the fault plane. The average dislocation on the whole fault plane is 1.0m, and the total seismic moment is 7.6×1025dyne·cm. Few aftershocks took place in the area where dislocation larger than 2m occurred during the main event. Surface fractures, associated with this earthquake, appeared in the meizoseismal area. The dislocation distribution seismologically obtained in this study is consistent with the fault displacement along these surface fractures.

著者

橋本 学

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.43, no.1, pp.13-26, 1990-03-24 (Released:2010-11-17)

参考文献数

35

被引用文献数

5 22

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Average horizontal crustal strain rates during interseismic period in the Japanese Islands, except the Hokkaido district, are deduced from precise geodetic survey, which has been conducted since the Meiji era. First, distance change rates between the first order triangulation stations are calculated using the weighted least squares method. In regions which suffered from disastrous earthquakes or volcanic eruptions, data before the events are neglected in the above calculation. Second, strain rates in arbitrary triangular regions are estimated.The estimated strain rates ate less than 6×10-7/yr, which is greater than those derived from seismic moment release rate or displacements of active faults. The Japanese Islands may be divided into several provinces on the basis of disstribution of strain rates. In the Tohoku district, a N-S to NE-SW extension is prevailing, and there is a zone of small strain rates which runs parallel to the Oga-Ojika tectonic line proposed by MOGI (1985). This zone may be a boundary between crustal blocks. In the northern Kanto district, strain rates are very small, while a NW-SE compression is predominant in the southern Kanto district. These facts indicate that compressional forces due to the subduction of the Philippine Sea plate may not be transmitted to the northern Kanto district. There is a zone of large compression which runs from Niigata to Gifu through Matsushiro. These high strain rates show a good correlation with seismic activity in the crust. Although compressional strain rates are prevailing in both the Chubu and the Kinki-Chugoku districts, principal axes abruptly change their directions from NW-SE to E-W at around lake Biwa. The westernmost Chugoku and the northern Kyushu districts are characterized by small extension, which suggests that compressional stress predominant in the Chubu and the Kinki districts may fade out. In the Shikoku district, NW-SE compressional strain rates are prevailing. However, temporal variations in baseline lengths show that the elastic rebound associated with the 1946 Nankaido earthquake did not occur in the western part of shikoku. It is considered that the coupling between the subducting Philippone Sea and the overriding Eurasia plates may vary significantly from east to west. Large N-S trending extensions appear in the central Kyushu district, which is consistent with the spreading of the Beppu-Shimabara graben.

著者

大竹 政和

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.50, no.appendix, pp.83-99, 1998-03-31 (Released:2010-11-17)

参考文献数

77

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This article reviews observational and theoretical studies on seismic gap and seismic quiescence from the view-point of earthquake forecast. “Seismic gap of the third kind” by Y. Ishikawa is also briefly discussed. Seismic gap, which is recognized as an unruptured segment of plate boundary and large-scale geological structure in the present seismic cycle, is a strong tool for identifying the zone of high seismic potential in the near future. However, the concept of characteristic earthquake is not always valid. We need reliable information on the duration of seismic cycle, slip distribution of past earthquakes, and seismic coupling for effectively applying the seismic gap hypothesis to earthquake forecast. Seismic quiescence has been reported prior to major seismic events of a wide magnitude range, from great earthquakes to rock fractures in the laboratory, but the physical mechanism is not yet established, Among the several hypotheses so far proposed, the stress relaxation model based on the laboratory-driven friction law is of particular interest since it theoretically predicts the formation of seismic quiescence as a natural consequence of the physical process of earthquake preparation. For checking the validity of the model, we need detailed studies on the time-space development of quiescence, and focal mechanism change in the surrounding area. In spite of some limitations described above, past studies demonstrate that a combined use of the seismic gap and seismic quiescence is expected to provide a most useful tool to forecast the occurrence of a large earthquake for the time range of a year to several decades.

著者

岡野 健之助 木村 昌三

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.49, no.3, pp.361-374, 1996-11-23 (Released:2010-03-11)

参考文献数

45

被引用文献数

1

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We propose a new idea for the crustal movements associated with past great Nankai earthquakes in Shikoku and the surrounding region, Southwest Japan. This study makes clear that the movements are not due to the oblique subduction of the Philippine sea plate, but due to superposition of strong seismic shaking to the uppermost crust in a compressional stress state in the E-W direction. The ground of this idea is as follows: at the time of the 1707, 1854 and 1946 Nankai earthquakes the Kochi plain subsided and the Muroto and Ashizuri peninsulas uplifted, whereas old documents show that the Kochi plain did not subside at the time of the 1605 Nankai earthquake and therefore no uplifting of the peninsulas is inferred because it is based on a set of subsidence of the Kochi plain and uplifting of the Muroto peninsula appearing at the time of the 1707, 1854 and 1946 Nankai earthquakes. This is explained by the reason why the 1605 (Keicho) earthquake was not accompanied with strong shaking of the ground owing to the tsunami earthquake. Next, because the uplifted peninsulas have anticline axes of the N-S direction, from unconsistency in stress direction it is difficult to attribute the uplifting to the subduction in the NW direction of the Philippine sea plate. On the other hand, it is easily explained that the uplifting was caused by the stress in the EW direction enhanced by strong seismic shaking. Tosa bay, which spreads between the Muroto and Ashizuri peninsulas, is characterized by depression. To be able to explain this depression is not by the elastic rebound theory, but by our idea. The undulation in the forearc zone composed of anticlines (peninsulas) and wide depressions (bays), which range alternately along the Japan island arc, can not also explained by the elastic rebound theory. In addition, we consider that the compressional strain variation in the NW direction of the ground surface observed at present in Shikoku does not result from the oblique subduction in the NW direction of the Philippine sea plate, but it is recovering the overdisplacement of the ground surface caused by the coseismic movement (2-3m at the ground surface) in the SE direction of the Muroto promontory by reverse faulting of the 1946 earthquake. Moreover, this study shows that unconsistency in directions of P axes between the 1946 Nankai earthquake and mantle earthquakes presently occurring in Shikoku and its vicinity is succesfully explained by taking account of constraint of the displacement in the direction along the Japan island arc.

著者

高波 鐵夫 村井 芳夫 町田 祐弥 斉藤 市輔 牧野 由美 勝俣 啓 山口 照寛 西野 実

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.57, no.3, pp.291-303, 2005-02-20 (Released:2010-03-11)

参考文献数

21

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Nine OBSs (Ocean Bottom Seismographs) were deployed just before the 2003 Tokachi-oki earthquake (M8.0) in the source region, off Hokkaido island, Japan for the period from 7 August to 21 September 2003. The observation area was located about 50-100km landward of the southern Kuril trench where the Pacific plate is subducted toward Hokkaido island. The main shock occurred on 26 September 2003, just several days after this 50-days observation by OBS. Until this big earthquake, the seismic activity in this area has been extremely low in contrast with the adjacent trench areas. No big earthquake has occurred since the 1952 Tokachi-oki earthquake (M8.2). From the phase-reading data, 186 micro- and ultra micro-earthquakes were located in the regular seismic active zones. The b-value of 0.82 from the earthquakes with magnitude from -0.6 to 2.9 could support the fore-shocks of the 2003 Tokachi-oki earthquake. Several interesting phenomena were found in this study as follows. (1) Many clusters of events took place simultaneously in the surrounding source region after a short quiescence period of several days. (2) An enormous number of curious burst signals, which oscillated for only a few seconds, continued to appear at almost OBSs. In particular, at two OBSs DHH and DK being the shortest distance to the Kuril trench, over 3500 bursts of events with amplitudes of 10-7m/s were counted during the observation period. On the other side, OBS DD, which was close to the epicenter of main event and far away from the trench, around the beginning of September 2003, the rate of occurrence abruptly rose to over twice of the previous rate, which has kept the low rate until then. The start time of activation was almost the same time of the appearance of many widespread clusters of earthquakes. These results will be applied to a research project of the great earthquakes occurring along the trenches.

著者

木下 繁夫

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.50, no.4, pp.471-483, 1998-03-13 (Released:2010-03-11)

参考文献数

17

被引用文献数

4

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Recently, servo-seismometers (negative feedback seismometers) are used for various fields in seismology and earthquake engineering. First, this report explains five fundamental negative feedback seismometers with a displacement or a velocity transducer. Next, composite negative feedback seismometers, such as STS, VSE and CMG seismometers, are explained. Also, new negative feedback seismometers that have two feedback paths, are discussed. Finally, seismometer noise, the representations of instrument noise and seismometer characteristics, and, some problems in borehole observation by using negative feedback seismometers are discussed.

著者

木村 昌三 岡野 健之助

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.50, no.4, pp.461-470, 1998-03-13 (Released:2010-03-11)

参考文献数

28

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The source region of the 1946 Nankai earthquake is examined based on the recent studies of microearthquakes, focal mechanisms and the crustal structure in Shikoku, southwest Japan as well as the aftershock distribution just after the main shock. Although it is generally known that an aftershock distribution is nearly equal to the fault zone and the source area of tsunami, such an agreement has not been recognized in the case of the 1946 Nankai earthquake. In the present study, we point out the misinterpretation about the aftershock area and the focal plane in the previous works, and give a new interpretation indicating good agreement among the aftershock distribution, source area of tsunami, crustal movements, disaster area and fault region. In the previous works, the defect of the observation network in the west of Shikoku and the east of Kyushu was not taken into consideration on the estimation of the aftershock area. Furthermore, the focal plane was not properly estimated owing to misunderstanding of the crustal movements and the source area of tsunami. Since the focal plane exists in the focal layer in the uppermost mantle, we consider that the aftershocks occurred only in this layer and the crustal earthquakes were induced by stress release due to the occurrence of the main shock. Our microearthquake observations show that this layer has a thickness of about 5km from the west of Shikoku to the western part of the Kii peninsula and from the Median Tectonic Line in the north to the Nankai Trough in the south.

著者

太田 陽子 松田 時彦 長沼 和雄

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.29, no.1, pp.55-70, 1976-03-10 (Released:2010-03-11)

参考文献数

21

被引用文献数

1 1

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Seven steps of marine terraces are well developed on the Ogi Peninsula, Sado Island. The seventh (lowest) one is a raised abrasion bench less than 2m high, emerged at the time of the destructive earthquake of 1802, hence it is named the 1802 terrace. The sixth terrace is about 2-4m high, and probably was formed at the time of Holocene transgression. Higher five terraces (Pleistocene terraces V-I) are well preserved over the most part of the peninsula and have the height of 32-40m, 70-55m, 94-118m, 123-137m and more than 165m, respectively.The height of former shorelines represented by shoreline angle of each terrace shows that all the terraces tilt northward. Generally, the higher the terrace, the larger the amount of tilting. However, the tilt of the lowest two terraces (the 1802 and Holocene terraces) is almost the same, ca. 1.5′. This indicates that the 1802 tilting caused by the earthquake was a first event after the formation of the Holocene terrace of ca, 6, 000 years old and the recurrence interval of the events was more than 6, 000 years. A uniform regional difference of 2m in height between these two terraces is probably interpreted as a superposed result of the eustatic lowering of sea level and a regional uplift during last 6, 000 years.It is possible to estimate the average intervals of earthquakes after the terrace formation by comparing the tilting rate of all Pleistocene terraces with that of the 1802's. Thus, the average recurrence intervals are estimated at about 8, 600 years since Terrace III was formed and about 5, 000 years since Terrace IV was formed. These values are consistent with the interval of more than 6, 000 years which is estimated from tilt of the 1802 and the Holocene terraces. It is concluded, therefore, that the earthquake has taken place repeatedly in a similar manner with a recurrence interval of about 5, 000-9, 000 years during at least last 105 years.Uplift and northward tilting of the Ogi Peninsula at the time of the 1802 earthquake (Magnitude 6.6) is significantly larger than those of Awa-shims at the Niigata earthquake of 1964 (M 7.4), though its magnitude was smaller.This fact and the limited areal deformation by the 1802 earthquake imply that the epicenter of this earthquake was located very close to the coast of the Ogi Peninsula, probably within a few kilometers off Shukunegi. A reverse faulting is inferred to have occurred along a northward-dipping fault plane at the 1802 earthquake.

著者

木村 昌三 岡野 健之助

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.48, no.2, pp.213-221, 1995-08-25 (Released:2010-03-11)

参考文献数

20

被引用文献数

1

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We examine the decreasing of seismic activity which occurred in Shikoku, and the Wakayama and Tanba districts prior the 1946 Nankai earthquake, southwest Japan. Considering the effect of the Second World War, the seismic observation for that period by the Japan Meteorological Agency was not in sufficient conditions. Thus, the idea of seismicity lowering on the area has not been generally accepted, notwithstanding some studies on the subject were previously presented. This study proves however the lowering was not caused by the insufficient seismic observation. We give an explanation for the cause of lowering of seismicity. We also showed that the crustal seismic activity in Shikoku in terms of earthquake occurrence gradually decreased with time after the 1946 earthquake. The decrease in seismicity could be anticipated to continue till the next Nankai earthquake in the above mentioned districts. Accordingly, the lowering of seismic activity is expected as a precursor of the great Nankai earthquake, in particular in the Wakayama district where the usual seismic activity is comparatively high compared to other districts.

著者

和田 博夫 三雲 健 小泉 誠

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.32, no.3, pp.281-296, 1979-09-25 (Released:2010-03-11)

参考文献数

28

被引用文献数

2 3

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Seismicity in the northern Hida region, Japan, has been routinely observed since May, 1977 at telemeter-network stations of the Kamitakara Geophysical Observatory, and about 1500 local shocks with magnitudes greater than 0.5 have been located. (1) The observation reveals high seismicity along the Atotsugawa fault, along the northern Japan-Alps, south of Mt. Norikura and Mt. Ontake, and near Hida-Osaka, with focal depths shallower than 20km. (2) Seismic activity along the Atotsugawa fault is high at the eastern and western portions, with an intermittent zone of low activity, extending over 70km. Epicenters are deviated about 2-3km north of the fault trace, and this deviation together with focal depth distribution suggests a slightly northwestwardd dipping fault plane. All these shocks are confined above 13km, suggesting either that the fault plane extends down to this depth, or that minor brittle fractures do not take place under the depth due to some flow propertities of rock materials there. (3) Nine shocks along the fault show focal mechanisms consistent with right-lateral strike-slip evidenced by geological and geomorphological surveys (MATSUDA, 1966). (4) Heavy damage along the fault region at the time of the 1858 Hida earthquake (M=6.9) appears to indicate that this large earthquake was caused by faulting motion of the Atotsugawa fault. Most of the present seismic activity along the fault might be associated with some readjustments of residual stresses around there.