Atera Fault Research Articles

Holocene faulting on the Yugamine fault of the Atera fault zone, central Japan

Holocene faulting on the Yugamine fault of the Atera fault zone, central Japan

Subsurface radar survey across the north-central part of the Atera Fault System

Subsurface radar survey across the north-central part of the Atera Fault System

岐阜県中津川市，小秀山北西斜面に分布する流紋岩質溶岩 (小秀山流紋岩) のK-Ar年代

Mt. Kohide (Nakatsugawa City, Gifu Prefecture) is located ∼ 7 km north of the Atera fault. A partial collapse of the northwestern flank of Mt. Kohide has exposed a 5-m-wide dyke-like conduit ∼ 15-30 m beneath the extrusive rhyolite lava that was fed by it. The rocks of the extrusive lava and the feeder are nearly aphyric (< 0.5 vol% phenocrysts), biotite-rhyolite (∼ 73 wt% whole-rock SiO2). The extrusive lava yields a K-Ar age of 23.01 ± 0.36 Ma. Within the measurement errors, this age coincides with the previously determined K-Ar ages of biotite-rhyolite dykes intruded along the Atera fault (23-22 Ma) and the rhyolitic welded tuff of the Hachiya formation (∼ 40 km southwest of Mt. Kohide; 22 Ma) as well as the K-Ar and fission track ages of the rift-type alkalic moonstone rhyolites from the Hokuriku area (25-22 Ma). This suggests that rhyolitic volcanism occurred contemporaneously in central Japan between the Late Oligocene and Early Miocene.

中部領家帯に分布する領家花崗岩類の巨視き裂構造と弾性波速度との関係

Based on geological field survey on granitoids distributed in the Chubu Ryoke Belt, orientation and occurrence of macroscopic (outcrop scale) fractures were analyzed to characterize the anisotropic fracture network system in the study area. Variation of P-wave velocity was measured to identify preferred orientation of weak planes developed in the granitic rocks. In northeast part of the Inagawa granite, parallelepiped block structure composed from fractures with three preferred orientation was observed. The structure is inferred to be derived from tensile stress field in the granitic body. On the contrary, in southwest part of the Inagawa granite and the old Ryoke granitic rocks, complex sheared fractures were dominant. Predominant orientations of those macroscopic fractures were close to the directions of major and representative active fault systems of the Chubu Ryoke area such as the Median Tectonic Line (NE -SW strike) and the Atera fault (NW-SE strike).P-wave velocity measurement revealed that anisotropy of the P-wave velocity was closely related to the distribution of parallelepiped fractures, however, shear fractures had no clear relation to variation of the P-wave velocity.P-wave velocity measurement has an advantage on prediction of major orientations of parallelepiped joint system developed in granitic rocks, however in the case of highly stressed rock mass, better approximation on preferred orientations of fractures is likely to be achieved with consideration for occurrence of major faults and tectonic stress regime distributed in the area.

Quaternary faulting and channel migration along the Gero fault of the Atera fault belt, central Japan

Quaternary faulting and channel migration along the Gero fault of the Atera fault belt, central Japan

Subsurface structure under a basaltic monogenetic volcano near the active Atera fault

In this study, we examined the subsurface structure in the vicinity of the active Atera fault beneath at least one monogenetic volcano that forms the Sakashita body of the Ueno basalts, central Japan. Microgravity and seismic reflection surveys over an area of about 3×3 km (horizontal)×1 km (vertical from surface) revealed that (1) a low-density body interpreted as the damaged zone of the Atera fault reaches a depth more than 200 m below sea level; and (2) a weak zone of crust that may exist about 1.5 km southwest of the Atera fault in a direction parallel to it. By combining the shape of the Sakashita body and (2), we propose that a dyke extending in a NW–SE direction with several vent alignments in the same direction may exist along this weak zone. This suggests that the studied area was governed in the NW–SE compressional and NE–SW extensional stress field when the Sakashita body formed.

Activity of the Atera fault based on analysis of outcrops on the left bank of the Kiso River, Yamaguchi Village, Nagano Prefecture, central Japan

Activity of the Atera fault based on analysis of outcrops on the left bank of the Kiso River, Yamaguchi Village, Nagano Prefecture, central Japan

Miocene clockwise rotation of southwest Japan and formation of curvature of the Median Tectonic Line: Paleomagnetic implications

The origin of the curvature of the Median Tectonic Line (MTL) in southwest Japan is still controversial. In order to investigate its formation, we carried out a paleomagnetic study in the Nohi area, north of the curved part of the MTL in the eastern part of southwest Japan. More than 420 paleomagnetic samples were collected at 35 sites from Cretaceous to Paleogene welded tuffs in the Kasagatake Rhyolites, Oamamiyama Group, and Nohi Rhyolite. Characteristic directions with high unblocking temperature component above 560°C were isolated from 32 sites. Paleomagnetic directions of the Kasagatake Rhyolites and Oamamiyama Group (55–66 Ma) have a clockwise deflected declination (D = 51.7°, I = 52.6°, α95 = 8.6°) which is consistent with the characteristic Paleogene paleomagnetic direction of the central part of southwest Japan. A similarly deflected declination is also observed in the peripheral part of the Nohi Rhyolite region, far away from the Atera fault system. Paleomagnetic data indicate that the Nohi area experienced a clockwise rotation through more than 45° with respect to the Asian continent as a part of southwest Japan but that the amount of rotation of the Nohi area is 23° smaller than that of the central part of southwest Japan. A new model is developed to explain the tectonic rotation. At about 15 Ma during the latest stage of the clockwise rotation of southwest Japan, the eastern end moved southward by more than 60 km along a left‐lateral strike‐slip fault which comprised the present north‐south segment of the MTL and the Akaishi Tectonic Line. We conclude that the curvature of the MTL is a composite of the 15 Ma strike‐slip fault system and a small bend of the MTL associated with the small differential rotation of the Nohi area with respect to the central part of southwest Japan.

An Audiomagnetotelluric View of the Atera Fault.

We made audiomagnetotelluric sounding to image the subsurface structure across the Atera fault, which is a 66 km long active strike-slip fault in central Japan. A major feature of the resistivity model is existence of two dipping conductors at 0.5-2.0 km depth: one dipping southwest and the other northeast. These imply fractured zones, created either by strike slip movements or by vertical ones. An audiomagnetotelluric imaging is suited for mapping subsurface fractures zones in active fault regions.

阿寺断層系の活動と1586年天正地震: 小郷地区, 青野原地区, 伝田原地区トレンチ掘削調査

阿寺断層系の活動と1586年天正地震: 小郷地区, 青野原地区, 伝田原地区トレンチ掘削調査

阿寺断層の最新活動時期

阿寺断層の最新活動時期

Electric and magnetic anomalies at the Yanagase fault and the Atera fault system-comparison with the trenching excavation study

Electric and magnetic anomalies at the Yanagase fault and the Atera fault system-comparison with the trenching excavation study

Gravimetric Evidences for Active Faults around Mt. Ontake, Central Japan: Specifically for the Hidden Faulting of the 1984 Western Nagano Prefecture Earthquake.

A gravity survey was extensively carried out around Mt. Ontake as a part of the 1986 Joint Seismological Research in Western Nagano Prefecture, central Japan (the JSR'86). Together with the overall revision of the existing data sets compiled by the Nagoya University, we completed a Bouguer anomaly map around Mt. Ontake with a high resolution of about 1 mgal. It delineates detailed gravity features in the area surrounding the focal region of the 1984 Western Nagano Prefecture Earthquake. A triangular crustal block around Mt. Ontake and a linear anomaly belt along the Atera Fault mark specific tectonic features. A characteristically strong gradient belt of Bouguer anomaly is outstanding in the middle of the triangular crustal block. The seismic fault plane of the 1984 Western Nagano Prefecture Earthquake runs along this belt and dips steeply northward. The fault plane of the largest aftershock exactly coincides with the western end of the gradient belt of Bouguer anomaly.

Microstructure of deformed biotite defining foliation in cataclasite zones in granite, central Japan

Foliated cataclasite occurs along both left-lateral faults, striking N10°–40°W, and right-lateral faults, striking N10°–25°E, formed in the Late Cretaceous Naegi-Agematsu granite along and near the Atera fault in central Japan. Quartz and feldspars in the cataclasite are angular to subangular in shape. Biotites, which are granular or elongate, exhibit kink bands and cleavage separation, or cleavage steps and biotite fish. Arrays of deformed biotite define a P-foliation that formed in the fault zones. The angle (θ) between the P-foliation and the (001) biotite cleavage and the ratio ( W/ L) of apparent width ( W) to length ( L) of deformed biotites are related to the type of biotite microstructure developed. In the samples studied, there were no values of θ in the ranges 30–75° and 105–150°. The cleavage-step microstructure exhibits an angle, θ, of less than 30°, with W/ L less than 0.2. Kink band and/or cleavage separation microstructures have an angle, θ, of 75–105°, with W/ L between 0.5 and 1.5. Biotite fish have an angle, θ, of more than 150°, with W/ L less than 0.2. These data imply that the orientation of cleavage in biotite with respect to the plane of shear strongly controlled the response of biotite to deformation. The resulting microstructures provide good kinematic indicators in biotite-bearing shear zones and faults.

阿寺断層周辺の苗木‐上松花こう岩に認められる塑性変形した黒雲母の発見とその意味

阿寺断層周辺の苗木‐上松花こう岩に認められる塑性変形した黒雲母の発見とその意味

Mechanical interaction between neighboring active faults—an application to the Atera fault, central Japan

Static stress changes across the Atera fault, central Japan, due to the neighboring large earthquakes are examined. Several large earthquakes in the past 100 yrs which might have dominantly affected the fault are selected in order to calculate coseismic changes in the fracture stress. The result suggests that the stress increased at both the northern and southern ends of the fault, which amounted to as much as 6 bar in total. Among these earthquakes, the Nagano-seibu earthquake ( M = 6.8) of 14 September, 1984, seems to have most seriously affected the Atera fault in such a sense as to increase the fracture stress.

An analysis of history and recurrence interval for large earthquakes on the Hagiwara fault, Atera fault system, by the trench excavation survey

An analysis of history and recurrence interval for large earthquakes on the Hagiwara fault, Atera fault system, by the trench excavation survey

Gravity survey in the Central Ranges, Honshu, Japan.

An extensive gravity survey was carried out from 1978 through 1980 in the area of the Central Ranges (Japan Alps), Honshu, which consist of the three largest mountain ranges in Japan (Hida, Akaishi, and Kiso). The Central Ranges area of 40, 000km2 has provided about 3, 700 gravity data including 2, 214 newly measured data, from which a Bouguer anomaly map was drawn. Bouguer and terrain corrections were made with a topographic data file of 230×280m2 grid at a distance of up to 80km from any measuring point, taking the earth's sphericity into account. A value of 2.64g/cm3 was used for the average density of surface features, and this was derived from the observed gravity data using a newly proposed least-squares method. The possible errors involved in the analysis were critically evaluated. The newly drawn anomaly map has an overall accuracy of 5mgal (=5×10-5m·sec-2). The Central Ranges area is known as a broad zone of strong negative anomaly, which has now been resolved into two parts, one along the axis of the Hida mountain range and the other along the northern Fossa Magna. Bouguer anomaly and topography are correlated negatively in the Hida range, marginally in the Akaishi range and positively in the Kiso range, suggesting that the correlation becomes progressively negative with increasing massiveness of mountain structure. The Central Ranges are bounded by the Itoigawa-Shizuoka Tectonic Line to the east and by the Atera fault to the southwest across which the regional Bouguer anomaly pattern changes sharply. The Atera fault is a firstclass active fault along which a narrow but very clear Bouguer low is observed that may be attributable to fault gouge material.

PREHISTORIC LARGE EARTHQUAKES DEDUCED FROM FAULT ACTIVITIES ALONG THE ATERA FAULT, CENTRAL JAPAN

Prehistoric fault activities along the Atera fault, one of the most prominent active strike slip faults in central Japan, have been revealed from studies on faulted humic soil layers and river terraces. Many organic samples for radiocarbon datings were collected from the outcrop on the Atera fault at Rgo, Kashimo Village, Gifu Prefecture, in order to determine the periods of prehistoric earthquakes. Displacements of river terraces around Sakashita Town on the right bank of the Kiso river, have been measured in order to deduce the amounts of coseisimic displacements. Prehistoric earthquakes deduced from fault activities have been repeatedly taken place along the fault at the periods of before 10, 300 y. B. P., 7, 900 y. B. P., 5, 500 y. B. P, and 1, 850 y. B. P., Recurrence intervals between succeeding two earthquakes are not equal but differ from each other judging from the amounts of displacements associated with each earthquake. The average vertical slip rate has been calculated as 0.71 m/103 yrs. from the amounts of coseismic displacements obtained from faulted river terraces and the receurrence intervals of earthquakes. The average lateral slip rate is calculated as 3.55 m/103 yrs., since it is known that the left lateral displacement is about 5 times larger than the vertical one (Sugimura and Matsuda, 1965). Along the Atera fault a great earthquake is presumed to occur within a thousand years in future, based on “Time-predictable recurrence model for large earthquakes” (Shimazaki and Nakata 1980).

阿寺断層における精密水準測量による傾斜観測(1976-1981)

阿寺断層における精密水準測量による傾斜観測(1976-1981)