断層は，富山・岐阜県境に位置するNE—SW走向，長さ約64 kmの右横ずれ活断層である．断層の最新活動は1858（安政5）年の飛越地震とされているが，その際，断層東部が破壊したかどうかは見解が分かれていた．本研究では，断層東部にあたる岐阜県飛騨市神岡町佐古において，未報告の断層露頭を発見した．断層により切断されている砂礫層の放射性炭素同位体年代測定値は100±30 yrs BP, 140±30 yrs BPであった．このことから，少なくともこの地点では飛越地震の際に，断層変位が生じたといえる．加えて，既往のトレンチ調査等の結果と合わせて考えると，最新活動時に断層全体が破壊した可能性が高い．

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断層は，1858（安政5）年に発生した安政飛越地震の震源断層であり，断層トレース周辺の地下では，現在も顕著な微小地震活動が観測されている．このため，断層系は1970年代から内陸活断層の地球物理学・測地学分野のテストフィールドとなってきた．近年，GPS測地をはじめ観測データが総合的に集積してきたことから，本邦日本海側の歪集中帯における地殻地震の発生メカニズムの解明が進んでいる．
本見学旅行では，断層の中央部から東端部に至る区間について見学を行う．各見学地点において，断層破砕帯や活断層の露頭，段丘の横ずれ変位，河道閉塞，流系のオフセット等を観察し，同断層の最新活動である飛越地震の地震像や，断層の運動学・力学的な観点からの現地討論を実施する予定である．

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Crustal structure models from the Noto Peninsula to the Hida mountains area crossing the Atotsugawa fault were estimated based on both detailed gravity anomaly and explosion seismological data. The results are as follows;
(1) Minor changes of short wavelength Bouguer anomalies were observed across the Atotsugawa fault. This is due to small density differences of rocks around the fault. However, a locally high anomaly zone remained along the fault.
(2) Variation of long wavelength Bouguer anomalies across the fault was about 12 to 20 mgal per 10km.
(3) Estimated crustal structure models indicated that the deeper subsurface structure changes suddenly under the fault. It suggests that the Atotsugawa fault is situated at the border of large scale variation of crustal structure between the Hida mountains and the Noto peninsula regions.

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Microearthquake activity near the Atotsugawa fault, one of first-class active geological faults in the northern Chubu region, has been routinely monitored since 1971.
The results obtained to data show 1) a clear lineation of microearthquakes along the Atotsugawa fault, with an intermittent zone of low seismicity, 2) less clear lineation but considerably high activity near a latent smaller fault along the Yamada river, 3) high activity with clustered epicenters just south of Mt. Norikura, and 4) extremely low seismicity over a region southeast of the Atotsugawa fault.
The above pattern of seismicity does not greatly vary with time except in the region along the Yamada river, compared with the preliminary results reported in the first paper.
Focal depths of these microearthquakes along the Atotsugawa fault are largely confined to depths shallower than 15-20km, which appears to suggest the lower boundary of the fault plane.
Composite distribution of first motion of P wave from a number of earthquakes does not seem inconsistent with right-lateral movement of the fault, indicating that maximum compressional, tectonic stresses lie in a direction slighly deviating from the E-W direction.

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The Atotsugawa fault system, which is located in the northern region of Central Honshu, Japan, is composed of the Amidagawara, Atotsugawa, Mannami, Mozumi-Sukenobe, and Ushikubi faults. The Atotsugawa fault in the central part is creeping where seismicity is low. In contrast, the edges of the creeping zone are locked showing relatively high seismicity. In order to study the relationship between the distribution of earthquakes and the fault structure, we analyzed seismic survey data along and across the fault system, and relocated hypocenters using a 1-D velocity structure determined in this study. P-wave velocity structures around the Atotsugawa fault system are determined by comparing first arrival travel time data from explosion surveys with the travel times from forward calculations by a ray tracing method. On the basis of the P-wave velocity structures, we have found that the crust around the Atotsugawa fault system consists of three layers, an upper crust which includes a surface layer, a middle crust and a lower crust. Furthermore, two distinct reflectors are located at depths of about 11km and 20km below the Atotsugawa fault system. The depth of the shallower reflector is close to that of low resistivity layer. Comparing the relocated hypocenters with the depths of the reflectors, the shallower reflector is roughly coincident with the base of the seismogenic layer and the second reflector is several kilometers deeper. In addition, seismicity is concentrated in the upper crust (with velocities of 5.9-6.2km/s) and only a few earthquakes occur at the bottom of the middle crust. The difference in seismic structures between the creeping zone and the other regions is not clear. This is probably because the creeping zone does not have a significantly different velocity structure that is detectable from these data.

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The Kamioka deposit is composed of skarn type lead-zinc orebodies formed by the replacement of limestone, a member of the Hida metamorphic rocks. The mine contains three main deposits, namely Mozumi, Maruyama and Tochibora distributed from the north to south.
The Atotsugawa district locates between the Mozumi and the Maruyama deposit and 2 km south of the Mozumi deposit. Although no ore exposure has been detected at the suface, a blind skarn type lead-zinc deposit has been expected to exist on the basis of some studying in the Mozumi deposit and surface geology. That is, the series of upper limits of lead-zinc mineralizations of Mozumi deposits are estimated to locate deeper from north toward the south direction. The Minami 8 you fault which is considered as one of the major ore fluids pass intersects at the district and wide distribution of the limestone and Inishi rocks which are main orebed at the Kamioka mining area.
A course of intense exploration has been carried out by means of surface and undergroud diamond drilling. That confirmed the blind lead-zinc mineralization at the levels around 180 m above sea level and elucidated the following characters.
(1) The orebodies are mainly distributed in the Inishi rocks and gneisses and classed as disseminated type ore (MACHIDA et al. 1987).
(2) The ore is mainly composed of sphalerite with epidote-chlorite alteration zone.
(3) The mineralization is considered to be closely related to the Minami 8 you fault and dykes.
Above feature indicate that other areas which are located at the intersection between faults (and fissures) and dykes, the passway of the ore-forming fluid, are ought to be aimed for new exploration targets.
On the basis of the indication, another exploration is now being performed in the Sakonishi district, 2 km east of the Mozum deposit, and some hopeful blind lead-zinc mineralizations have been discovered to date.

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The Atotsugawa fault extends 60km or more in the northern Hida Mountains of central Japan, linearly running from ENE to WSW, and forms a master active fault system with predominantly right-lateral component of displacement. A historical large earthquake (1858, M≅7.0) occurred in this region, and many small to micro-earthquakes are taking place along this fault system up the present.
A deep trench was excavated across the central part of the Atotsugawa fault where the 5m-high scarplet disturbed the lowest river terrace along the Miya (-gawa) River. The size approximately N-S trending trench was 13m deep in maximum and 22m long across the scarplet (Fig. 1B & 5), and an additional small excavation was made down to clarify the last event at the west side of the main trench, exactly at the southern foot of the scarplet. Several fault planes were exposed on the four trench walls, showing recent movements of the fault which separates the granitic rocks on the hanging-wall side (north) and the younger sediments on the foot wall side. The strike of the main fault is about N70° E and the angle dips 65° N at the surface and 75° N in the trench bottom, increasing the fault angle toward the deeper part.
The main results from this survey are summarized as follows :
1) The accumulated vertical offset forming the scarplet is so large at this site that we were not able to correlate the each layers, and sedimentary environment of each side has been quite different due to the displacement, except the basal terrace gravel across the fault. The gravel in the down-thrown side is 2m thicker than that in the up-thrown side. Intrapolating the average vertical slip, at least two events were presumed in the basal gravel.
2) We recognized the “structure D” within the deposits along the main fault disturbed zone, and interpreted that this structure was created by slumping of slope materials from the fault scarplet (see Fig. 13). This phenomenon indicates the sudden movement (earthquake event) of the fault. Dated four events are <820y. B. P., 5200± 200y B. P., 7500±800y. B. P., and 8600±00y. B. P. The latest event (No. 1) was considered to be the 1858 Hietsu Earthquake, as the uppermost humic soils (<820y. B. P.) is thrust up by granitic rocks of the hanging-wall. And, this is also confirmed by existence of a low scarplet at Hayashi (Locality a in Fig. 2), which suggests probably historical fault topography formed within the settlement.
3) Through detailed observation of the trench walls, we found more than 10 events after the formation of the basal gravel which approximate age was estimated to be ca. 12000-13000. However, events of sand deposits at the lower part are not so reliable because it is not easy to detect the key structure in the soft sediments.
4) The average recurrence interval of the main Atotsugawa fault for last 4 events is about 2800years. However, the recurrence interval scatters from 1100 to 5100 years. It is assumed that some events might be missed due to artificial modifications of the fault scarplet and the superficial deposits. In this case, the recurrence interval becomes shorter and the sporadicalness smoother.

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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.

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Routine observation of microearthquakes in the vicinity of the Atotsugawa fault in the Chubu District has been carried on by the Kamitakara Crustal Movement Observatory, Kyoto University.
Microseismicity in this area seems to consists of three groups; namely, 1) a linear arrangement of microearthquakes along the Atotsugawa fault, 2) a large scale block of microearthquake occurrence extending from Mt. Norikura to Takayama, and 3) a distribution of N-S direction along the Central Mountains.
The linear distribution of microearthquakes along the Atotsugawa fault is particularly remarkable, which is the same properties as those along the strike-slip faults in northwestern Kinki District. Superposed push-pull distributions of the initial motion of P waves of these earthquakes seem to indicate that the general tendency of the tectonic compression acting in this area is nearly in horizontal and E25°S-W25°N direction.

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断層は,安政飛越地震(1858年)の震源断層とされ,地震に伴って同断層の東端付近で地震断層が出現したとする考えがある.筆者らは断層の東端付近の真川流域において,「真川露頭」の斜面上方で,新たに断層露頭を見い出した.断層面の傾斜は,斜面上方へほぼ鉛直から北西側に向かって緩傾斜となる.露頭と周辺地形を観察した結果,(1)この傾斜した断層面の一部を利用して真川側へ滑動する地すべりが存在すること,(2)この地すべりの頭部には断続的な地すべり運動に伴って形成された開口亀裂を埋積する堆積層が複数認められ,いずれも断層に切られていないことが判明した.これらの堆積層のうち,もっとも新しい腐植土層の¹⁴C年代値は1,040±70yrsBP(AMS)である.これは,少なくとも安政飛越地震(1858年)の際に,断層東端付近において地震断層が出現しなかったことを示す.

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The Kamioka mine is a representative lead-zinc skarn deposit in Japan which was formed mainly by the replacement of limestone, a member of the Hida metamorphic rocks. The mine is composed of three main deposits, Mozumi, Maruyama and Tochibora from the north to south.
The Sakonishi district is located at 2km south-east of the Mozumi deposit. The Kita 20 you fault which is cosidered as one major conduit of ore fluids responsible for the formation of the Mozumi deposit intersects the district. Although little ore exposure has been detected at the surface, a promising zinc deposit was discovered at vertical depth of 250m below the surface. The downhole length of the ore is 44.4m with an average grade of 13.4% Zn, 0.02% Pb and 7g/t Ag. The deposit develops with NE-SW strikes and steep dips.
The ores in the Sakonishi district are classified on the basis of the mode of occurrence into three categories, namely "disseminated type", "Mokuji type" and "Network (small vein) type". The hopeful ore is mainly composed of the disseminated type ore with sphalerite and minor amounts of galena, chalcopyrite and pyrite. Epidote, chlorite and hematite are as gangue minerals distributed in the Inishi rocks. The mineralization is considered to have been closely related to the activity of the Kita 20 you fault, the Atostugawa 1 you fault and porphyrite dyke.
The homogenization temperature of fluid inclusions in the sphalerite of the disseminated type ore is low ranging from 90°C to 180°C. The sphalerite varies in colour dependent on the iron content and shows "chalcopyrite disease". The limestone core samples which were taken near the orebody display more depletion of oxygen isotope ratios than the unaltered limestones. This indicates that the hydrothermal fluid responsible for the mineralization was of meteoric water origin similarly to other deposits in the Kamioka mine.
It is assumed from these observations that the mineralization in the Sakonishi district took place at two stages, (1) the formation of the Mokuji ore from the pyrometasomatic fluid along the Kita 20 you fault, and (2) that of the disseminated and network ore under hydrothermal conditions. Further exploration will provide additional informations on the rigorous model responsible for the high-grade zinc mineralization.

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A new outcrop of the Atotsugawa fault, which is one of the major active faults in central Japan, was observed along the Atotsu River, tributary of the Takahara River. In this outcrop, the total of about 50m wide fractured fault zone, ENE-WSW-striking slicken planes were observed. The slickenlines indicate dextral wrenching of the Atotsugawa fault but the vertical displacement slightly exceeds, with uplifting the southern side of the fault. This result almost same as the previous studies on the other outcrops, showing the continuous distribution of the fault trace with local branching or stepping.

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As a case of the seismic landslide, Kaerigumo collapse occurred in the Atotsu-gawa shattered zone due to the Tensho great earthquake (1586). The debris formed by the landslide dammed up the main course of the R. Sho, taking shape of a natural dam for a time. Owing to the extraordinary geographical phenomenon, villages wero buried under debris and a number of people fell dead. The quantity of the debris produced by the landslide is estimated at about 9×10⁶m³.

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The Tateyama volcano is located in the central part of the Japanese Islands and situated on the northern part of Norikura volcanic zone.
Volcanic activities of the Tateyama volcano in the late Quaternary, were classified into four stages. Andestic lava and pyroclastic materials were erupted during those activities. Eruption of the second stage produced so many pyroclastic materials as forming caldera. Afterwards this caldera was eroded and grew larger. A part of caldera wall was broken at Shiraiwa about twenty thousand years ago, and as the result, a lot of debris flowed over the caldera into Joganji River. The Tateyama Sabo Work is one of the largest Sabo work in Japan. Its most important work is the control of the Tombidoro which was produced by the deformation of the Tombiyama (Mt. Tombi) in the earthquake induced by the Atotsugawa fault in 1858. Deformation of the caldera wall by the earthquake may have occurred at any place in the caldera. But why did Tombi collapse occur at Tombiyama? From investigating topography of Kanayamadani and the Tombi collapse, I may conclude the cause of Tombi collapse is that Tombiyama was the terminal place of the right lateral Atotsugawa fault, and force of tension stress and depression concentrated here.

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Intrafault materials derived from two localities within the Atotsugawa fault of Central Japan have been investigated to elucidate the characteristics of the fault. In each fractured zone within the fault, two different zones are clearly recognized; the one zone is composed mainly of very fine fragments and developing limitedly both side of the fractured zone in contact with parent rocks, and the other is composed mainly of coarse fragments and constituting the main part of fractured zone.
Textures of the intrafault materials are in random arrangement of fragments showing rounded-angular shape, or in complete arrangement of fragments showing lenticular-anastomising shape. The grain-size frequency curves indicate no peakes or the remarkable peak of d=10-3 μm. Quartz grains within the intrafault materials under the scanning electron microscope have smooth surfaces keeping breakage or fracture surface, or small undulated, a little corroded surface.
The mineralogy of 2μ<fractions from the intrafault materials is principally including illite, chlorite and montmorillonite, and occasionally with quartz and feldspar. Geochemically, within the zone composed mainly of very fine fragments, loss of SiO₂, Na₂O and K₂O, addition of H₂O⁺, H₂O^- and Total Fe, addition or loss of CaO have been relatively found, comparing with composition of parent rocks.

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