We compiled a totally new 1: 2,000,000 active faults map of Japan based on the digitized GIS data of active faults in the scale of 1: 25,000 by Active Faults Map Working Group (1999). The new map differs from the existing active faults maps of Japan in t wo points. One is the definition of active faults and the other the scale of base maps. We defined active faults as faults that have repeatedly moved during the late Quaternary with intervals of one thousand to ten thousand years forming distinctive fault-related tectonic features on the earth surface. The Research Group for Active Fault Studies (1980,1991) and other previous studies commonly defined active faults as faults that repeatedly moved during Quaternary and are the potential sources for future earthquakes. Therefore, our criterion is stricter for recognition of active faults than those used for the previous mapping. The other difference is accuracy in mapping of active faults; our map is based on the active faults map data in scale of 1: 25,000 while others mostly based on much smaller scale data. Through our mapping, some faults previously recognized as active because of existence of Quaternary faults were not treated as active if these faults are not associated with fault-related tectonic features on the ground. On the other hand, many faults are newly found as active by our detailed photo-interpretation. We assembled active faults about 140 fault groups based on the f ollowing criteria; 1. Faults more than 10km long in dimension,2. Isolated from the neighboring faults at least 5 km apart, and similar type of faulting, strike and sense of faulting. We compared the active faults map with geoloic map to know their characteristic occurrence of faults. We prepared maps for type, activity, and density of faulting to know the regional characteristic of active faulting in Japan. We also examined the possibility of occurrence of new active faults and expansion of faults based on the mode of cumulative slips distribution along their traces.
Following high demand on detailed information for active faults in Japan after the 1995 Hyogokennambu earthquake, we compiled completely new active faults map of Japan with a scale of 1: 25,000 based on precise airphoto interpretation. We also digitized location of active fault traces and their parameters such as displacement, fault reference etc. to GIS data set. This digital fault map differs much in accuracy from“Active fault in Japan: Sheet map (1: 200,000) and inventories”and may provide useful information to geoscientists and engineers for their purposes.
The horizontal strain rate caused by fault activity for the past several hundred thousand years was calculated using the latest active fault database. Features of fault activity at the scale of the Japanese Islands were estimated, and the results were compared with the strain rates that the pre-existing literature showed. The strain rate required using the latest active fault database resembles the strain rate requ ired using the pre-existing data on active faults in the Quaternary period (Kaizuka and Imaizumi,1984). The distribution of the strain rate required from active fault data resembles the distribution of the strain rate required from the GPS observations (Sagiyaet al.,1999) in many regions, with the exception of the Pacific coast region. However the value of strain rate required from active faults data (10-8/year) is smaller than the value of strain rate required from the GPS data (10-7/year). In the region along the Ou Mountains, Northeast Japan, the value of strain rate required from the latest active fault data was nearly equal to the strain rate required from the geological section (Sato,1989).
The Japanese on-land seismogenic faults are listed with historical shallow earthquakes of greater than M 5.0, based on‘Active Faults in Japan’published by the Research Group for Active Faults of Japan, in 1991. Total numbers of listed seismogenic faults (defined by Matsuda,1990) and earthquakes over the Japanese Islands are 263 and 449, respectively. The numbers of faultsper 10,000km2 in Japan on land are about 7 on an average. The numbers are the largest in the Chubu (21) and Kinki (17) districts and the smallest in Hokkaido (3). About a half of the seismogenic faults are shorter than 20km in length. Faults longer than 80km are only 9 in number (3%). Faults with strike-slip component in the displacement are very common in western Japan, while faults with no strikeslip component, mostly reverse faults, are dominant (more than 90% in the areas) in Hokkaido and Tohoku districts. The known historical shallow on-land earthquakes larger than M5.0 in the period from AD1600 to 1999, are 420 in number, which are more than 90% of the total known since AD600. In 1900's, the earthquake of M>7,7>M>6,6>M>5 are 13,83,134 in number, respectively. The number of M>5 earthquakes per 10,000km2is the largest in Chubu district (10/10,000km2), followed by Kyushu-Ryukyu and Kanto districts. The successive earthquakes of M>5 in one year and less than 15km apart from each other have occurred mostly on the back arc side of volcanic fronts on the Islands. The half of the successive earthquakes relates in distribution to volcanic areas rather than known seimogenic faults. There is a close relationship between distr i b utions of seismogenic faults and of historical earthquakes of M>5: the Chubu district is the highest in distribution density both of faults and earthquakes. The most (abou t 80%) of earthquakes of M>7 have been occurred in areas within 15km in distance from seismogenic faults.
Two paleoliquifactions were described at the archaeological sites in the southern part of Ishikari Lowland, Hokkaido; one from Eniwa City and the other from the campus of Hokkaido University in Sapporo. Timing of occurrence was examined on the basis of 14C dates and Tarumae-a tephra. Both the liquifactions might have occurred associated with the same earthquake probably in AD12-13 Century.
At the middle portion of the Itoigawa-Shizuoka Tectonic Line, two lines of bulges(pressure ridges) are formed, trending parallel to the faults. The internal structure and sedimentary facies of the western tectonic bulge were examined to reveal the geologic processes of forming the bulge in Wakamiya, Fujimi town, Nagano Prefecture. The tectonic bulge forms anticline and its western flank is displaced by some faults with low angle striations. Together with the these features, this bulges shows a positive flower structure, formed by left-lateral faulting. The clast composition and sedimentary facies demonstrates that the formation of western tectonic bulge is later than the eastern one. Judged by the deformation of volcanic ash(Pm-I), the Wakamiya tectonic bulge was formed since 100,000yr. BP.
The 70-km-long Atera fault system is composed of several fault segments with mainly left-lateral strikeslip displacement in the eastern part of southwest Japan. The Atera fault is the longest segment in this fault system. We investigated geomorphology and geology around a fault outcrop on the left bank of Kiso River, Yamaguchi Village, Nagano Prefecture. Vertical displacements of Kiso-gawa volcanic mudflow deposit, M2surface, and the deposit of L 1 surface are 15.4m,13.0m., and 8.5m, respectively. Vertical slip rate of the Atera fault is 0.3-0.4mm/yr in the study area.
The Ouchigata Plain in western Hokuriku area is interpreted as a tectonic depression bounded on the north and south by northeast-trending reverse faults. The Sekidosan fault on the south displaces geomorphic surfaces of late Pleistocene to Holocene age along its entire length. However, the slip rate and history of Holocene faulting on the fault was not well constrained by previous studies. We have obtained subsurface geologic information by geoslicer and a core sampler across monoclinal scarps 4.6 m high on a Holocene alluvial surface in Hakui City, Ishikawa Prefecture. The samples obtained at 6 sites are 3.5 to 5.3 m long. Correlation of strata and radiocarbon dating show that a peat layer dated at about 6000 years BP is warped into a monocline. The vertical displacement of the top of the peat layer is estimated at 5.2 - 6.1 m, which gives a vertical slip rate of 0.8 - 1.0 mm/yr for the Sekidosan fault. This rate is more than twice greater than previous estimates, which requires a reevaluation of seismic hazards from this fault.
The Median Tectonic Line active fault zone (MTL), with slip rates as high as 5-10 mm/yr, is one of the most active inland faults in Japan. However, the seismic hazard evaluation on the MTL is hampered by insufficient paleoseismological data, mainly due to sparse distribution of trenching sites along the fault zone. We have conducted a first major paleoseismological study of the Kawakami fault, a segment of the MTL in northwestern Shikoku. The study includes aerial photograph interpretation and paleoseismic trenching. Two trenches were excavated across the Kawakami fault northeast of downtown Komatsu, Ehime Prefecture in 1997. The sediments exposed on the walls in the eastern trench are middle to late Holocene fluvial deposits ranging in texture from silt to gravel. These sediments are offset along an almost vertical,3-m-wide fault zone. The sense of apparent displacement across the fault zone is down to the north, consistent with Holocene scarps around the trench site. The sedimen t s in the trench contain evidence for three episodes of surface-rupturing earthquake in the past 4000 years B. P. The most recent surface-rupturing earthquake, event A, on the Kawakami fault occurred sometime during the 7th and 19th century AD. Historical documents report extensive damage around the trench site during an earthquake in 1596, and this earthquake may be correlated to the latest faulting on the Kawakami fault. The penultimate event, event B, occurred sometime during the 1st and 10th century AD. Another earthquake, event C, occurred after the deposition of layer VII, which was radiometrically dated at about 4000years B. P.
The Hitoyoshi Basin is located at the southern part of Kumamoto Prefecture, southwest Japan. The basin has been considered to be a fault angle depression bounded by fault at the southern margin of the basin, named here the southern marginal fault of the Hitoyoshi Basin. This fault extends NE-SW direction for about 22 km, and has a sense of right lateral strike-slip with a northwest downthrown vertical component of displacement.
The Nishiyama fault zone in North Kyushu runs from Tsuyazaki to Iizuka. We have carried out three trench excavations across this fault at Anzunosato, Nuyama and Myoujyouji, where we had detected terrace deposits displaced vertically in aerial photographs. Trench excavations revealed the following. The faults, observed in these trenching studies, strike from NW to SE. They ap p eared as high angle faults. The south-western side of the fault apparently uplifted relatively. The latest event of the fault occurred after 11,000 y. B. P. at Anzunosato. At Nuyama, the latest event occurred between 1 1,000 y. B. P. and the 5th cent ury. At Myoujyouji, the latest event is confirmed to occur before 2,100 y. B. P. Therefore the latest event of the Nishiyma fault zone probably occurred between 11,000 y. B. P. and 2,100 y. B. P. The apparent vertical displacement of the latest event is 25cm at Nuyama trench. The exploratory drilling at Myoujyouji revealed that the apparent vertical fault displacement amounts to 38m after middle Pleistocene.