著者

吉岡 敏和 苅谷 愛彦 七山 太 岡田 篤正 竹村 恵二

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.51, no.1, pp.83-97, 1998-07-03 (Released:2010-03-11)

参考文献数

19

被引用文献数

1 1

---

The Hanaore fault is a right-lateral strike-slip active fault about 48km long in central Japan. We carried out comprehensive surveys including trench excavations on the Hanaore fault to evaluate the seismic risk of the highly populated area, such as Kyoto City, along this fault. Three trenches were excavated on the fault. On the exposure of the northernmost Tochudani trench, a fault cutting fluvial sediments and humic soil beds appeared. The youngest age of displaced sediments is 460±60 14C yBP, and the sediments covering the fault is 360±60 14C yBP. This faulting event may be correlated to the historical 1662 Kambun earthquake. The southernmost Imadegawa trench was excavated on the road in the urban area of Kyoto City. A thrust fault cutting humic soil with pottery fragments of the Late Jomon period (about 3, 500 years ago) was observed on the trench walls. It was difficult to detect the age of the last faulting event due to lack of younger sediments and artificial modifications of the surficial materials. However, the southern part of the fault might not move during the 1662 earthquake because the damage in this area was much less than in along the northern and middle part of the fault. The historical documents recorded that the land along the Mikata fault which is located at the north of the Hanaore fault was uplifted, and the land along the western shoreline of Lake Biwa where is the east of the Hanaore fault was subsided during the 1662 earthquake. This means that the 1662 earthquake might be a multi-segment event caused by these three faults, the Mikata fault, the northern part of the Hanaore fault, and the faults along the western shoreline of Lake Biwa.

著者

江口 孝雄

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.42, no.2, pp.227-230, 1989-06-24 (Released:2010-03-11)

参考文献数

12

著者

岩崎 貴哉 佐藤 比呂志

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.61, no.Supplement, pp.165-176, 2009-07-31 (Released:2013-11-21)

参考文献数

81

被引用文献数

2 5

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Recent seismic expeditions with controlled sources in Japan provided important constraints on the deformation styles and physical properties of the island arc crust and uppermantle. The crustal structure in the Japanese Islands is characterized by an upper part with a large velocity variation (5.5-6.1 km/s), a middle part with a velocity of 6.2-6.5 km/s overlying a lower part whose velocity is 6.6-7.0 km/s. In many cases, most shallow microearthquakes are concentrated in the upper crust and upper half of middle crust while the lower half of middle crust and lower crust are reflective with very low seismicity. The uppermost mantle is characterized by a low Pn velocity of 7.5-7.9 km/s. Several observations on PmP phase indicate that the Moho is not a sharp boundary with a distinct velocity jump, but forms a transition zone from the upper mantle to the lower crustal materials. A detailed structural section across the NE Japan Arc from intensive onshore-offshore profiling in 1997-1998 revealed crustal deformation associated with the Miocene backarc spreading of the Sea of Japan. The backbone range of this arc shows a pop-up structure formed by inversion tectonics due to the present compressive stress regime. Crustal thinning associated with the backarc spreading is very clear west of this pop-up structure where the crust deduces in thickness from 30 to 25 km. A section across the SW Japan arc elucidated the detailed subduction geometry of the Philippine Sea Plate and inland crustal evolution associated with processes of accretion and magmatic intrusion. The Outer Zone south of the Median Tectonic Line is characterized by northward dipping structures of accretionary complexes, while the lower part of the crust in the Inner Zone is quite reflective, probably modified and homogenized due to the magmatic intrusion at Cretaceous time. Clear structural images obtained for arc-arc collision zones in central Hokkaido and easternmost part of the SW Japan arc provides direct evidence of crustal delamination. The structure in Hokkaido strongly indicates that the delamination of the Kuril forearc occurs at its brittle-ductile transition zone.

著者

瀬野 徹三

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.73, pp.1-25, 2020-05-10 (Released:2020-05-26)

参考文献数

143

---

Subduction off Miyagi and Fukushima prefectures, northern Honshu, has been recognized as having a triple-planed structure of seismicity at the deep thrust zone in the 40-60 km depth range. This triple seismic zone is composed of thrust-type earthquakes, down-dip compressional and down-dip tensional earthquakes from top to bottom. At the time of the 2011 Tohoku-oki earthquake, peculiar phenomena such as radiation of short-period seismic waves and pre- and after- slow slips within the asperities of M7 class earthquakes were observed over this thrust zone west of the 2011 main rupture zone. Further to the south, where the Philippine Sea plate is subducting beneath Kanto, a triple seismic zone has also been recognized particularly under southwest Ibaraki prefecture. The thrust-type earthquakes at the top of the triple seismic zone off northern Honshu and beneath Kanto have been believed to be interplate events representing the relative motion between the overriding and subducting plates. I conclude that the thrust-type earthquakes beneath southwest Ibaraki prefecture are in fact within the crust of the subducting Philippine Sea slab, not at the surface because their slip vectors are different from the relative motion between the subducting and overriding plates. Therefore, there would be an aseismic plate boundary above the seismicity. I also show that the dip angles of the westerly dipping fault planes of the thrust-type earthquakes off Miyagi prefecture are smaller by 6° in average than the dip of the slab surface in this region, except for the six years prior to the Tohoku-oki earthquake, i.e. prior to 2006. Furthermore, the slip vectors coincide with the relative motion between the overriding and subducting plates only during this period. I infer that the topmost earthquakes of the triple seismic zone off Miyagi prefecture prior to 2006 are thus likely to be within the crust of the subducting plate. The slow slips before and after the Tohoku-oki earthquake would have occurred not within the asperities but along the aseismic plate interface, and the short-period seismic waves would have been radiated due to fractures within the crust associated with the overshooting rupture at the time of the Tohoku-oki earthquake. Many of the so-called repeating earthquakes at the topmost surface of the subducting plate would be in fact intra-crustal events within the slab. M9 earthquakes would interact with the triple seismic zone, not only mechanically, but also through fluid migration, because earthquakes in the triple seismic zone involve dehydration reaction. The irregularity of the occurrence of M9 earthquakes might be due to the inhomogeneous distribution of hydrated minerals in the incoming plate. The subduction zones having M9 earthquakes or under Kanto have a collisional character. I propose to term subduction having both a collisional character and a triple seismic zone as “super-subduction”. The relative motion between the plates is accommodated by the deformations of the crust of the subducting slab as a “plate boundary zone”. The viewpoint of “super-subduction” is necessary to understand earthquakes in subduction zones with a collisional character and dehydration reactions in the slab.

著者

西田 究

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.61, no.Supplement, pp.115-121, 2009-07-31 (Released:2013-11-21)

参考文献数

55

---

It has long been believed that only large earthquakes excite free oscillations of the solid Earth. In 1998 a few Japanese groups reported existence of Earth’s background free oscillations even on seismically quiet days. The excited modes are almost exclusively fundamental spheroidal modes with amplitudes of about 0.5 nGal (10-11 m/s2), and they fluctuate persistently in little correlation with their neighboring modes. Cumulative effects of many small earthquakes are too small to explain the amplitudes. These features suggest that the background free oscillations are excited persistently by random disturbances globally distributed near the Earth’ surface. The intensities of these modes clearly show annual and semiannual variations with the largest peak in July and a secondary peak in January. The observed amplitudes of some modes are anomalously large relative to the adjacent modes. These are the modes that are theoretically expected to be coupled with the acoustic modes of the atmospheric free oscillations. All of these features suggest that atmospheric disturbance is one of the most likely excitation sources of this phenomenon. Assuming that atmospheric pressure disturbance acting on the Earth’s surface is a primary excitation source, some quantitative comparison has been made between the atmospheric pressure disturbance and Earth’s background free oscillations. Shortly after the discovery, some groups proposed oceanic excitation mechanism. This mechanism is based on two observations; (1) The typical frequency of Earth's background free oscillations of about 0.01 Hz coincides with that of ocean bottom pressure sensors. (2) The excitation sources are dominated in the northern Pacific in winter of the northern hemisphere and in the circum-Antarctic in winter of the southern hemisphere. The source distribution is consistent with oceanic wave height data. Based on these observations oceanic excitation mechanism by ocean infragravity waves through their nonlinear interaction was proposed. However, the proposed mechanisms work efficiently only in shore regions, and hence may not be consistent with the observed spatial extent of the excitation sources, and they cannot explain observed acoustic resonance. At present there is little consensus about the excitation mechanism that can explain all of the observed features. These observations suggest that excitation sources are superposition of oceanic and atmospheric phenomena. The phenomena of background free oscillations should be understood as those in a single system of the atmosphere, ocean and solid Earth.

著者

松村 正三

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.58, no.1, pp.55-65, 2005-06-10 (Released:2010-03-11)

参考文献数

21

被引用文献数

2 1

---

Peculiar seismic activities are occurring beneath Lake Hamana inside the subducted Philippine Sea slab at depths of about 30km. They consist of three spindle-shaped earthquake clusters with NW-SE axes, in a left-stepping alignment in the EW direction, with several kilometers between them. Illustrations of the focal mechanisms indicate that they act like an open crack under NW-SE compression and NE-SW tension and are caused by a right-lateral shear force acting on the Philippine Sea slab. A stress pattern simulation model suggests the following explanation of the situation. A localized locked zone is positioned on the plate boundary just north of the clusters. Separated from the main locked zone, it is considered to be one of the satellite asperities surrounding the main one. The main locked zone is located eastward from Lake Hamana and is expected to become the seismogenic zone of the forthcoming Tokai earthquake. These clusters have demonstrated a remarkable decrease in activity since the second half of the year 2000. An anomalous tectonic movement detected by GPS measurements occurred almost simultaneously. This indicates that a slow slip event progressed on the plate boundary beneath Lake Hamana; that is, the locking must be released there. Since the current change in seismic activity corresponds with this movement, it can be attributed to tectonic stress change due to the slow slip. We estimate that at least three similar periods of quiescence have occurred during the last quarter century. Tidal gauge findings at Maisaka and the crustal tilt at Mikkabi, both of which were observed near Lake Hamana, have indicated almost simultaneous occurrences of similar anomalies. As a result, three episodes of slow slip were identified: the first occurred before 1980, the second around 1990, and the last has been ongoing since late 2000. This implies that the slow slip repeats quasi-periodically with an interval of about one decade. We consider that the locked zone beneath Lake Hamana is a small asperity with a potential of slowly and intermittently slipping due to a weak coupling condition in an area sandwiched between two seismogenic zones of the Tokai and Tonankai megathrust earthquakes.

著者

寺島 敦 大竹 政和 小沢 邦雄

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.28, no.3, pp.239-267, 1975-10-10 (Released:2010-03-11)

参考文献数

10

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Recently, it is suggested that underground water plays an important part in generating the earthquakes. Studies are made in the present paper on changes in flow rate, temperature and heat capacity of hot springs distributed in southern Izu peninsula. These changes are considered to be related closely with the change of underground water caused by the Izu-Hanto-Oki (off the Izu peninsula, Japan) Earthquake (138°48′E, 34°34′N, h: 10km, M=6.9, May 9, 1974).Both of the flow rate and temperature of these hot springs were found to be increased just after the earthquake.Increases of flow rate and heat capacity discharged in the Shimogamo spa contiguous to the epicentral area began about 5.7 years before the earthquake. This abnormality reverted just before the earthquake. This period of about 5.7 years fit the period of precursory phenomena prior to the occurrence of the earthquake M=6.9 expected by “Dilatancy Model”. Moreover, the behavior of the variation bears a close resemblance to what is expected for the behavior of underground water as precursory phenomena prior to earthquakes by “Dilatancy Model”.

著者

大見 士朗

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.68, no.1, pp.1-15, 2015-05-15 (Released:2015-07-07)

参考文献数

19

被引用文献数

2

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Seismic activity near Mt. Hotaka in the Hida Mountains, central Japan was analyzed by using the Matched Filter Method (MFM). In this analysis, MFM was implemented as an automatic hypocenter relocation system. We selected about 30 template earthquakes in the target region that enables us to detect more than 3,000 events and locate about 800 earthquakes in the time period from April 2013 to October 2013. Comparison with manually inspected results indicates that location errors by MFM system is within a couple of kilometers. The seismic activity in the target region started in April 2013 and most intense activity occurred in October 2013. The largest event took place on October 8, 2013 at 19:28 (JST) whose magnitude was 3.9 (JMA). Epicentral area extends about 4km in EW direction with 1km in NS direction at the eastern frank of Mt. Hotaka. Although manually inspected catalogue data is essential to evaluate seismic activity, we suppose MFM is one of the powerful tools to automatically obtain preliminary results for the swarm activity that concentrated in a small area such like this study or volcanic regions.

著者

齊藤 竜彦

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.61, no.Supplement, pp.93-101, 2009-07-31 (Released:2013-11-21)

参考文献数

58

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This paper deals with the stochastic scattering theory for short-period seismic waves in random media. The stochastic approaches to simulate the seismic wave propagation in the heterogeneous lithosphere was employed by K. Aki in seismology in 1970s. His pioneering studies proved that stochastic approaches are very useful for understanding short-period seismic wavefield which is usually too complicated to simulate by deterministic approaches. Numerous researches have since employed stochastic methods to study short-period seismograms. H. Sato and his colleagues have greatly contributed to the development of the scattering theory. Analyses of seismograms based on the theory have revealed that the random inhomogeneity in the lithosphere is well characterized by a power-law spectrum, with the regional variation related to seismotectonic conditions. Recent observations and advanced data analyses, however, are requiring more development of the scattering theory to simulate seismic wave propagation not only in isotropic random media but also in anisotropic random media. This paper covers multiple-forward scattering theories for velocity shift and seismogram envelopes. The achievement of the scattering theory in isotropic random media and the recent progress in anisotropic random media are reported.

著者

岸上 冬彦 飯田 汲事

出版者

公益社団法人 日本地震学会

雑誌

地震 第1輯 (ISSN:00371114)

巻号頁・発行日

vol.11, no.8, pp.365-371, 1939-08-25 (Released:2010-03-09)

参考文献数

5

著者

小田切 聡子 島崎 邦彦

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.54, no.1, pp.47-61, 2001-07-15 (Released:2010-03-11)

参考文献数

79

被引用文献数

2

著者

樋渡 康子 佐藤 魂夫 今村 文彦

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.54, no.4, pp.431-440, 2002-04-25 (Released:2010-03-09)

参考文献数

15

被引用文献数

1

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Three earthquakes and accompanying tsunamis of 1704, 1793 and 1804 hit the coastal area of the Aomori-Akita districts in the northern part of Japan Sea, causing several hundreds of fatalities and serious damages to houses and ships. In this study, we simulate the generation and the propagation of these tsunamis by assuming several fault models, and determine a set of fault parameters for each earthquake that best fits the observed distributions of tsunami heights and crustal uplifts. The fault model previously proposed by Sato (1980) for the 1704 Iwadate earthquake generally explains the observed tsunami heights, but a similar model with the fault length 3km longer towards the north is more consistent with the reported crustal uplift at Henashi peninsula. For the 1793 Ajigasawa earthquake, a fault model lying off the coast of Tsugaru peninsula better explains the observed tsunami heights around Ajigasawa. The tsunami simulation also corroborates the observation that tsunami caused by the event might flush into the Jyusan Lake. Among several fault models tested for the 1804 Kisakata earthquake, a fault model located most offshore shows the best agreement with the observed distribution of tsunami heights. The tsunami simulations for the 1704 Iwadate and 1804 Kisakata earthquakes show that the tsunamis are largely amplified at Oga peninsula. The amplification is ascribed to an energy concentration by the reflection in the 1704 Iwadate tsunami and the edge wave trapped in the shallow sea in the 1804 Kisakata tsunami, respectively. Both are important mechanisms affecting the heights of tsunamis along the Japan Sea coast.

著者

行谷 佑一 矢田 俊文

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.66, no.4, pp.73-81, 2014-03-25 (Released:2014-05-20)

参考文献数

56

被引用文献数

11 14

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To find tsunamis that affected the Pacific coast of eastern Japan during period between A.D. 869 and the 1600s, we searched a historical earthquake and tsunami database for tsunami descriptions. We examined reliability of the descriptions considering the written dates and independence of documents. As a result, at least two events were found. One is the Oei tsunami which occurred on September 1st, 1420 on the Julian calendar (the 7th month 23rd day, 27th year of Oei era on the Japanese calendar). The tsunami affected Kawarago and Aiga villages in Hitachi city, Ibaraki prefecture. Sea receded 9 times during 4 hours, exposing the coastal seafloor, and many fish were carried to the land. No description of earthquake ground shaking was found. The other is the Kyotoku earthquake tsunami which occurred on December 12th, 1454 (11th month 23rd day, 3rd year of Kyotoku era). An earthquake occurred in night and the tsunami widely inundated the coastal area of “Oshu” (the old name of province from Aomori to Fukushima prefectures). Many people were killed in the tsunami. Another entry in the database on December 31st, 1455 (11th month 23rd day, first year of Kosho era) is likely to be a fake event.

著者

遠田 晋次 宮腰 勝義 井上 大栄 楠 建一郎 鈴木 浩一

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.1, pp.57-70, 1995-05-25 (Released:2010-03-11)

参考文献数

15

被引用文献数

1 2

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The Yamasaki fault system is located from the eastern Okayama to Hyogo Prefectures, southwest Japan, trending in NW-SE direction with a length of 87 kilometers. Earthquake risk evaluation of this fault system is not complete because the past seismic events have not been determined throughout the fault system. This paper reports a comprehensive survey of the Ohara fault, located at the northwestern end of the fault system. High resolution electrical exploration and five drillings at Ohara Town clearly identified the location of the fault underneath the sediment cover. Trench survey was then carried out to determine the past seismic events along the Ohara fault. The following conclusions were derived from these studies. (1) The Ohara fault shows up as a sharp resistivity contrast in the high resolution electrical exploration, reflecting mainly the difference in resistivity between acid tuff and black slate that constitute the northern and southern sides of the fault, respectively. (2) The trench observation in the log and radiocarbon dating of sediments revealed that the latest fault movement along the Ohara fault occurred between 150 and 1200 years B. P. The Harima Earthquake of 868 years AD is most likely to correspond to this fault movement. The timing of the event roughly coincides with the latest event of the Yasutomi fault (Okada et al., 1987) comprising the central part of the Yamasaki fault system. This strongly suggests that the Ohara and Yasutomi faults ruptured simultaneously or as a sequence of events during the Harima Earthquake. (3) The penultimate movement of the Ohara fault was estimated between 1500 and 3000 years B. P. If the latest event corresponds to the Harima Earthquake, then the interval between the last two events is estimated to be 400 to 1900 years. (4) The present trench survey revealed possibly four events along the Ohara fault during the Holocene. Thus the recurrence interval may be about 2500 years. Comparing this result with the interval between the last two events, movement of this fault system is likely to be aperiodic.

著者

長尾 年恭 鴨川 仁 服部 克巳

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.1, pp.69-85, 2006-08-31 (Released:2013-08-05)

参考文献数

80

被引用文献数

3 3

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Despite its extreme importance and decades of efforts, practical short-term earthquake prediction still remains to be achieved in future. However, the electromagnetic research has been demonstrating some promises. This paper briefly reviews the recent progress of what we call “seismo-electromagnetics”, mainly referring to Japanese studies by the observational point of view. We demonstrate some results of observations, in varied frequency ranges, on the anomalous telluric current, ULF geomagnetic transient change, VLF-HF natural emissions and anomalous transmission of VLF and VHF band radio waves. We also summarize proposed physical mechanisms of these phenomena, including the notion called “Lithosphere-Atmosphere-Ionosphere (LAI) Coupling”. We believe that electromagnetic studies will play an important role in not only earthquake prediction but also in understanding physical processes of earthquake generation.

著者

宇津 徳治

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.31, no.4, pp.367-382, 1978-12-25 (Released:2010-03-11)

参考文献数

7

被引用文献数

5 18

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In many cases, the log-frequency vs magnitude curves for earthquakes show considerable curvature, though the well-known G-R (Gutenberg-Richter) formula predicts a straight line. To represent such data, two modifications of the G-R formula have been proposed.log n(M)=a-bM M≤cn(M)=0 M>c} (2)and log n(M)=a-bM+log(c-M) M<cn(M)=0 M≥c} (3)These are called here the truncated G-R formula and the modified G-R formula, respectively. These equations can be written in the form of probability density function:f(x)=B/1-e-BCe-Bx C≥x≥0 (5)f(x)=B2/e-BC+BC-1e-Bx(C-x) C>x≥0 (6)where x=M-MS, B=b ln 10, C=c-MS, and MS is the lower limit of magnitude above which the data is complete.The estimation of B and C in equation (5) by the method of moments was discussed by Okada (1970) and Cosentino et al. (1977). The equations proposed here areexpC(C-2x)/Cx-x2=C2-2Cx-x2/2x2-x2 (16)B=(2x-C)/(x2-Cx) (15)or x2/x2=2-BC(BC+2)/(eBC-1)/1-BC/(eBC-1) (18)Bx=1-BC/(eBC-1) (19)The maximum likelihood method for equation (5) yields only one equation (equation (19), Page (1968), Okada (1970)). If we adopt C=Max(xi) as the second equation, the C value is considerably biased. To correct the bias, a correction ΔC which is a function of B and C is proposed. For this correction we must use some estimated values for B and C.To estimate B and C in equation (6) by the method of moments the following equations are used.x2/x2(e-BC+BC-1){e-BC(B2C2+4BC+6)+2BC-6}/{e-BC(BC+2)+BC-2}2 (25)Bx=e-BC(BC+2)+BC-2/e-BC+BC-1 (23)The maximum likelihood estimates of B and C in equation (6) can be obtained by the equation:2-Bx=C/S ∑Si=1 1/C-xi=BC(1-e-BC)/e-BC+BC-1 (27), (29)The accuracy of B and C values determined by the above methods is estimated by Monte Carlo technique for the cases of S=50, 100, 200, 400, and 800 and several values of B and C. If we adopt the truncated G-R formula, the second method (which uses C=Max(xi)+ΔC) gives more accurate C values, whereas the accuracy of B values is almost the same as that obtained by the method of moments. If we adopt the modified G-R formula, the maximum likelihood method gives more accurate B and C values than the method of moments. The η value (η=x2/x2) is a useful index for the deviation of the distribution of data from the G-R formula (for the G-R formula, theoretical value for η is 2). An application of the present methods shows regional variations in b, c, and η values of shallow earthquakes in Japan.

著者

海野 徳仁 長谷川 昭 小原 一成 松沢 暢 清水 洋 高木 章雄 田中 和夫 小菅 正裕

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.3, pp.399-410, 1985-09-25 (Released:2010-03-11)

参考文献数

13

被引用文献数

1 6

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Hypocenter determination for aftershocks of the 1983 Japan Sea Earthquake (M7.7) is made by using the seismic networks of Tohoku University and of Hirosaki University. The obtained aftershock area is 160km long in north-south direction with a width of 40km, distributing itself along the eastern margin of the Japan Basin. Most of the aftershocks are located within the area bordered by the 2000m and 3000m isobaths, northern and southern ends being surrounded by the Sado Ridge and the Oshima Plateau, respectively. Precise hypocenter distribution deliniates an eastward dipping fault plane with a shallow dip angle. Almost all the aftershocks are located in the crust, which is consistent with the fact that the PMP phase is clearly observed from most of the aftershocks.A remarkable later phase is observed at many stations 4-7 sec after the P arrival. This later phase is interpreted as the reflected wave both at the sea surface and at the Moho discontinuity (pwPMP). Focal depth distribution estimated from arrival time differences between PMP and pwPMP phases also shows the eastward dipping fault plane with a shallow dip angle.Foreshock activity started 12 days before the occurrence of the main shock within a concentrated area in the vicinity of the main shock hypocenter. All the foreshocks are classified into two groups: one with high peak-frequency and the other with low peak-frequency, each having very similar wave forms. Hypocenters of low peak-frequency events are located at shallower depths than those of the main shock and high peak-frequency events.

著者

渡辺 邦彦

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.23, no.1, pp.32-40, 1970-04-28 (Released:2010-03-11)

参考文献数

6

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On February 21, 1968, a large earthquake occurred at Ebino plateau, south-west part of Miyazaki Pref. in Kyushu. Since then, Ebino earthquake swarm had been active for about one year. Abuyama Seis. Obs., Sakurajima Volcano. Obs. and Disast. Prey. Res. Inst. of Kyoto Univ. made the temporary observation at 5 stations. The main results on the nature of the Ebino earthquake swarm are as follows:1) The epicentral region characteristically changed its shape with time.2) The aspects of the push-pull distribution of the initial P motions appeared to be nearly the same both in large shocks and microearthquakes, showing the same focal mechanism in both cases. This phenomenon does not seem to vary throughout the whole period of this study.3) According to the wave-period analysis, we recognized the interesting relation between the location of the epicenter and the _frequency of those shocks. It may be considered that this was caused by the difference of the properties of the crustal rocks in the hypocentral region from those outside the focal area.

著者

宇津 徳治

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.28, no.4, pp.435-448, 1975-12-10 (Released:2010-03-11)

参考文献数

9

被引用文献数

1 2

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About 400 shallow earthquakes in central Japan occurring during 1967-1974 have been relocated using data supplied by the Japan Meteorological Agency and several university seismic stations. About 4200 P-residuals are obtained in the relocation. The residual is approximately normally distributed with a mean of 0.0sec and a standard deviation of 0.59sec. Therefore, the probability that a residual exceeds 0.4sec is 0.25. If the actual travel-time for paths crossing the focal region of an impending earthquake is increased by 0.4sec, the probability that an observed residual for one of these paths exceeds 0.4sec will be 0.50. Let R denotes the ratio of the number of paths with residuals larger than 0.4sec to the total number of the paths crossing a certain region. The R-values for the focal regions of the central Gifu earthquake of 1969 (M=6.6), the Izu-hanto-oki earthquake of 1974 (M=6.9), and other 24 earthquakes of smaller magnitudes during some time-intervals before the occurrence of them have been determined to be about 0.5 or more. These values suggest the decrease in P-velocity before the earthquakes. A map has been made showing the distribution of R-values in 204 areas of 0.2°×0.2° in central Japan. Significantly high R-values are found in the areas containing the focal regions of the above-mentioned two earthquakes in the maps covering certain periods before the earthquakes. However, there are many other areas of high R-values, which are not connected with the occurrence of large earthquakes until now. Most of these areas may correspond to inherent low-velocity regions in the crust.

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.34, no.2, pp.197-211, 1981-08-25 (Released:2010-03-11)

参考文献数

18

被引用文献数

1 1

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A three-dimensional finite element method is used to simulate the observed stress state in the Southwestern Japan, which has been inferred by SHIONO (1977). Two different types of model are presented here, one is a locked model and the other is a decoupled model, where the former preserves a tight contact between the subducting Philippine Sea plate and the overriding continental plate, while the latter has a weak zone between them. In these models three different types of force are applied;(1) a negative buoyancy due to the density contrast between the subducting plate and surrounding mantle, (2) a northwestward compressional force generated by the movement of the Philippine Sea plate, (3) a westward compressional force due to the subducting Pacific plate.It is found that the observed extensional stress parallel to the leading edge of the subducting Philippine Sea plate may be caused mainly by a negative buoyancy, suggesting that the plate seems to sink down into the mantle by its own weight. The observed E-W compressional stress field prevailing in the Inner zone appears to be closely associated with the subduction of the Pacific plate. The northwestward horizontal compressional force may be smaller than the westward compressional force, and seems to be less than 500bars. The present calculations suggest that a low velocity layer between the oceanic and continental plates gives appreciable effects on low-angled thrust faulting off the Kii peninsula and the Shikoku island.