第 四 紀 研 究 (The Quaternary Research) 30 (2) p. 161-174 July 1991

Active Faults and Neotectonics in Japan1)

Atsumasa OKADA2) and Yasutaka IKEDA3)

Wereview in this paperthe recent trend ofactive fault studies in Japan. The JapaneseIslands, includingthe continentalshelves and slopessurrounding them, form an activetectonic belt. The overalldistribution and regional characteristics of Quaternary faults have recently been clarified, and are nowbeing supplemented in detail. Some topics, including the studies of active faults on shallow sea and lake bottoms, the geomorphicand structural evolution of and the surface defromationassociated with thrust faults, and the excavationof active faults with or without historic activity, are also reviewed.

brief review, see RESEARCH GROUP FOR ACTIVE I. Introduction FAULTS, 1980b). The earlier edition has The Japanese Islands, including the continen- provided basic data for earthquake prediction tal shelves and slopes surrounding them, form an research and earthquake-disaster-prevention active tectonic belt delineated on the southeast planning. This book was extensively revised in by oceanic subduction systems along the north- 1991, introducing the mass of new data obtained west rim of the Pacific Ocean. Being closely in the past decade. Active faults were mapped related to this physiographic setting, tectonic for the Japanese Islands and adjacent sea features such as Quaternary faults, folds, and bottoms, to a uniform standard, and compiled associated tectonic landforms develop in and into a set of sheet maps (originally on a scale around the islands. Studies on the neotectonics 1:200,000) with an inventory of faults for each or recent crustal movements have been sheet map. Two smaller-scale compiled maps conducted mainly by geodetic, seismological, of active faults (1:3,000,000 and 1:1,000,000) were geomorphological and geological methods. also published; the former is shown in Figure 1. Here, we review the recent trend of geomorpho- The overall distribution of major active faults logical and geological research of the last decade. in Japan was first revealed in these works, and Special focus is placed on the studies of active the fundamental characteristics of the active faults. For reviews of other subjects in faults have been clarified. Average rates of slip neotectonics or active fault studies in the 1970's on these faults during the late Quaternary range or earlier, see KAIZUKA (1987), MATSUDA (1981a), from several mm/y to less than 0.01mm/y. MATSUDA and KINUGASA (1988), OTA (1980, 1985), The active faults on land were classified into YONEKURA and OTA (1986), and YONEKURA (1989). classes A (1-10mm/y), B (0.1-1mm/y), or C (0.01-0.1mm/y), according to their average II. Comprehensive mapping of active slip rates (RESEARCH GROUP FOR ACTIVE FAULTS, faults over the Japanese Islands 1980a, 1991; MATSUDA, 1981b). The most obvi- and adjacent sea bottoms ous active faults previously known on land in The details of active faults in and around the Japan belong to class A. The distribution of Japanese Islands have recently been elucidated active faults is not uniform over the Japanese in "Active Faults in Japan" (RESEARCH GROUP FOR Islands, but is remarkably different from region ACTIVE FAULTS, 1980a, revised edition, 1991; for a to region (Fig. 1). Figure 2 shows a classifica-

1) Received 15 April 1991. Accepted 22 May 1991. 2) Aichi Prefectural University, Takada-cho 3-28, Mizuho-ku, Nagoya 467. 3) Universtiy of Tokyo, Hongo, Bunkyo-ku, Tokyo 113. 102 The Quaternary Research Vol. 30 No. 3 July 1991

tion of active fault provinces proposed by the Western Hokkaido and the western half of RESEARCH GROUP FOR ACTIVE FAULTS (1980a, 1991).northern Honshu constitute a province of Each province is briefly described below. reverse faults trending nearly N-S (e.g., AWATA The densest occurring zones of active faulting and KAKIMI, 1985; WATANABE, 1989). Major are along the and the Nankai strike-slip faults do not develop in this province. trough, where a number of reverse faults form The northern to central part of central mega-thrust belts associated with the Eurasian- Japan, from the Chubu to Kinki District, is Pacific or Eurasian- characterized by numerous active faults of both boundaries. Each mega-thrust belt as a whole strike-slip and reverse types. The density of ranks as class AA, with a slip rate one order of active faults in this province is extremely high. magnitude higher than class A; the recurrence Strike-slip faults constitute a typical conjugate interval of major earthquakes (with a magni- set; faults trending NW are left-slip, and those tude possibly as high as 8) from each mega- trending NE are right-slip. The conjugate set of thrust belt is estimated to be about 100 to 200 faults delineate many fault blocks of a rhombic years. shape. There are also reverse faults trending The secondary dense zone is at the eastern N-S, some of which delineate the fronts of small- margin of the Japan Basin off Northeast Japan. scale mountain ranges 10 20km wide. There are N-S trending faults, which are The is the northern-most part of generally reverse, and have generated large the Izu-Bonin arc in the Philippine Sea plate. (M=7-8) earthquakes in historic times. Numerous strike-slip and normal faults of short NAKAMURA (1983) proposed that a nascent con- length are distributed in and around the vergent boundary between the North American peninsula; faults trending N-S are generally and Eurasian plates is developing here. left-slip, and those trending NW are generally 1991年7月 第 四 紀 研 究 第30巻 第3号 163

Fig. 1 Compiled map showing the active faults in and around the Japanese Islands (simplified from the original map on a scale of 1:3,000,000, of the RESEARCH GROUP FOR ACTIVE FAULTS, 1991) right-slip. The regional stress field of this area In the inner zone of western Japan, active is quite exceptional; the Japanese Islands are strike-slip and reverse faults develop at a generally subjected to E-W to WNW-ESE moderate density. compression, whereas the maximum principal Back-arc spreading is now taking place in the horizontal stress in this area is in the NNW- , both sides of which are SSE direction. This exceptional stress field has bordered by normal faults. The Quaternary been attributed to the collision of the Izu volcanic field of central Kyushu, where Peninsula with Honshu to the north (RESEARCH numerous normal faults of short length trending GROUP FOR ACTIVE FAULTS, 1980a, 1991; MATSUDA,E-W develop, is believed by some workers 1984; NAKAMURA et al., 1984; KAIZUKA, 1984; (KIMURA, 1983; TADA, 1984, 1985) to be the YAMAZAKI, 1984). landward extension of the Okinawa trough. 164 The Quaternary Research Vol. 30 No. 3 July 1991

Fig. 2 Active fault provinces of Japan See Table 1 for the nature of respective provinces. (after the RESEARCH GROUP FOR ACTIVE FAULTS, 1991)

However, there is an alternative explanation; the bathymetry, geologic structure, magnetic normal faults in central Kyushu could be anomalies, and gravity anomalies, with an secondary faults caused by the terminal effect of explanatory text for each quadrangle. The the Median Tectonic Line, a large right-slip fault, Geological Survey has published the Marine the western-most portion of which borders the Geology Map Series (more than 30 sheet maps) southern margin of this normal fault province with scales of 1:200,000 to 1:3,000,000, which (OKADA, 1980). Detailed mapping of active mainly cover continental slopes and deep sea faults on the continental shelves off western areas. Kyushu is required to solve this problem. Mapping active faults in shallow sea or lake Although active faults are not distributed in areas is important for planning disaster- the outer zone, the mountainous region on the prevention programmes. Recent studies of Pacific side of western Japan, crustal Holocene faults beneath Beppu Bay in central deformation with long-wave swells or undula- Kyushu, and the Iyo-nada Sea west of Shikoku tions has progressively occurred in the have demonstrated that acoustic profilers Quaternary. provide extremely high resolution images, to a depth about several ten meters below the sea III. Active faults on lake and sea bottoms bottom, in which normal and strike-slip faults Recently much data on offshore active faults are clearly recognized (SHIMAZAKI et al., 1986, have been acquired by using newly-developed 1990; TSUTSUMI et al., 1990). Moreover, a instruments such as narrow multi-beam number of continuous reflectors in the acoustic depthmeters, multichannel seismic profilers, sections make it possible to identify progressive- and submersibles. The distribution of active ly offset features of some faults in late Holocene faults beneath the sea has been mapped mainly time. Sediments were sampled on both sides of by the Maritime Safety Agency, the Geological such faults using a piston corerer, and were Survey of Japan, and the Geographical Survey analyzed by the sedimentological and micro- Institute. The Maritime Safety Agency has paleontological methods. Although details of published a series of quadrangle maps, called the these results have not yet been published, such Basic Maps of the Sea, for shallow sea areas (1:10,000.4 a method, combining acoustic profiling with 1:50,000) and continental shelf areas (1:200,000). piston coring, is quite effective for revealing These maps contain information about recurrence intervals of earthquakes generated 1991年7月 第 四 紀 研 究 第30巻 第3号 165

Table 1 Characteristics of active fault provinces of Japan

(modified from RESEARCH OF ACTIVE FAULTS, 1991) from a fault on a lake or sea bottom (SHIMAZAKI et 1983), gave the ages of the reflectors identified in al., 1986, 1989; TSUTSUMI et al., 1990). the profiler records. It was revealed that the In Lake Biwa, the largest and oldest (5Ma) active fault/flexures are closely releated to the inland lake in Japan, active faults and flexures in evolution of the lake, and that the rates of lacustrine sediments were found by interpreting vertical slip on some of these faults have the records of a high-resolution acoustic profiler accelerated since 0.4Ma (UEMURA and TAISHI, and a multi-channel seismic profiler (UEMURA 1990). and TAISHI, 1990), Deep drilling, reaching to a depth of 1,400 meters (YOKOYAMA and TAKEMURA, 166 The Quaternary Research Vol. 30 No. 3 July 1991

"frontal faults/flexures") develop several km IV. Precise mapping of active faults in front of the boundary faults (IKEDA and From the view point of disaster prevention, YONEKURA, 1979; SAWA, 1981; OTA and SANGAWA, precise mapping of active faults, on scales of 1982; IKEDA,1983; SANGAWA et al.,1985; WATANABE, 1:50,000 to 1:10,000, is indispensable especially 1985; SUZUKI, 1988). On the upthrown side of a for planning land use in urban and industrial frontal fault/flexure, basin fills are strongly areas. Most of the large cities in Japan are deformed and eroded to form foothills. A thick located on alluvial plains and coastal plains, pile of basin fills on the downthrown side of a mainly of Holocene age, where thick young boundary fault indicates that the boundary fault deposits and extensive artificial modifications had moved progressively until sometime in the have obscured the topographic expression of recent geologic past, whereas the strong active faults. It is therefore difficult to detect deformation in the foothills indicates that the active faults in such highly populated regions thrust front has migrated basin-ward. only from aerial photographs. Thus, it is IKEDA and YONEKURA (1979) and IKEDA (1983) necessary to develop new techniques for modelled the migration of thrust fronts, using detecting active faults in such regions. examples from Japan and the Transverse Ranges However, aerial photographs were taken over of California. They found that the pattern of some of the large city areas just after the Second deformation in the vicinity of a migrated thrust World War, when artifical modification was not front is the result of fault plane bending (in such so extensive. Projects to publish large-scale a way that the shallower portion of the fault dips strip maps for selected active faults are now at a significantly lower angle than the deeper underway at university institutes and the portion). They also suggested that the Geological Survey of Japan. formation of the bend is closely related to the Active faults data for Kyushu were compiled migration of thrust fromt: 1) As the boundary by the RESEARCH GROUP FOR ACTIVE TECTONICSfault IN moves, basin fills accumulate on its KYUSHU (1989) into a set of 161 sheet maps, downthrown side. 2) Once the sedimentary originally on a scale 1:50,000, with related wedge fully develops, faulting in the basement information such as photo-geologic lineaments, rocks causes stress concentration in the wedge geomorphological classification, landslide and along the wedge-basement interface as well as volcanic landforms. An inventory of active along the updip extension of the basement fault faults is attached to each sheet map. This (the boundary fault), because of the marked research is based mainly on the interpretation of difference in rigidity between the wedge and the aerial photographs, of 1:20,000 and 1:40,000 basement. 3) Mechanical anisotropy of the scales. Some of the faults were checked in the wedge due to its stratified structure makes it field. easier for the fault to propagate updip along the flat-lying interface as a low-angle detachment V. Thrust faults and related problems fault than along the boundary fault. As a 1. Migration of thrust fronts result, the thrust front jumps basin-ward. Many ranges and intervening basins have 2. Subsurface structure of a migrated developed in western Hokkaido, Northeast thrust front Japan, and the Kinki District. These ranges are The ma Valley fault zone (IVFZ) in central bordered on one side, or both sides, by thrust Japan is a typical example of thrust-front faults (referred to as "boundary faults"), along migration. The boundary fault of the IVFZ which, rocks composing the range are thrust marks the physiographic boundary between the over syntectonic basin fills. The thickness of Kiso range on the west and the ma Valley on the the basin fills generally increases toward the east. The frontal fault is located 1-5km east boundary fault to form a sedimentary wedge. of the boundary fault. Since the boundary fault In many cases, boundary faults are not active; is still active, the migration of the thrust front in instead, active faults or flexures (referred to as the IVFZ is not yet finished. As shown later, 1991年7月 第 四 紀 研 究 第30巻 第3号 167

-・-・-Observed ------Assumed trend

-・--・---・-Residual ○○○Computed

Reduction density=2.67g/cm3

Fig. 3 Bouguer gravity anomaly (top) across the northern part of the Ina Valley fault zone, central Japan, and a cross section of the density structure (bottom) deduced from the gravity anomaly (IKEDA, 1988)

MBF: the boundary fault, FAF: the frontal fault, dotted pattern: Mesozoic-Paleozoic rooks horizontal shortening at an extremely high rate photographs, we can observe only vertical and/ has occurred in the IVFZ during the late or strike separations across a fault; we have Quaternary. almost no chance to directly observe net slip. If Recently, high spatial-resolution gravity the dip angle of the fault is known, net slip could measurements were carried out along two routes be determined from observed separations. across the Ina Valley, in order to determine the However, it is in general very rare for us to subsurface structure of the IVFZ (IKEDA, 1988). encounter outcrops of the specific fault plane, Figure 3 shows the geologic cross section of the particularly in the case of thrust faults. northern ma Valley deduced from gravity data. Moreover, complex deformation of surface The basin fills underlying the ma Valley form a sediments would prevent us from evaluating to sedimentary wedge, and are thrust under the what extent a fault plane in a shallow outcrop basement rocks of the Kiso range for about 4km. constrains the slip direction. Since there is no significant offset at the base of IKEDA and YONEKURA (1986) developed a new the sedimentary wedge, the frontal fault is likely geomorphological method for determining the to be a detachment fault developing along the rate and direction of net slip on a low-angle wedge-basement interface. These features thrust. A low-angle, dip-slip fault has a large are in accordance with the model presented by component of horizontal slip. The amount of IKEDA and YONEKURA (1979) and IKEDA (1983). offset (normal horizontal separation) of a linear 3. Determination of net-slip rates fault reference (such as erosional scarps and on low angle thrust faults streams) is a simple function of three The rate of slip on a fault is one of the best parameters: the amount and the direction of criteria in evaluating its activity. However, it is horizontal slip, and the azimuth of the fault generally difficult to determine the rate of net reference. Thus, if we have two or more sets slip on a dip-slip fault. In the field or on aerial of observed azimuth-offset values for fault 168 The Quaternary Research Vol. 30 No. 3 July 1991 references of known ages, we can determine both IMAIZUMI et al., 1989). At the main site on a the direction and rate of horizontal slip using a narrow valley floor, five trenches 3-4m deep least squares method. and 5-20m long were dug across the 2-4m This new method was applied to the ma Valley high fault scarplets that were formed at the time fault zone. It was found that the rate of of the 1896 earthquake. At the other site, 2km horizontal slip on each fault constituting the north of the main site, a trench 3m deep and 12m fault zone is as high as 3-6mm/y, and that the long was made across the surface break. Two direction of horizontal slip is approximately subparallel fault planes were exposed in the perpendicular to the general strike of the fault trenches. The dip angles of the faults are as low zone (IKEDA and YONEKURA, 1986; ABE and IKEDA, as 15-20°near the trench floors, and decrease 1987). Because the vertical slip rate on each updip; near the surface, paddy field soil is in fault is much smaller (0.1-0.7mm/y), the subhorizontal fault contact with the overlying horizontal slip rate is virtually equivalent to the gravel bed. The spraying nature of the fault net-slip rate. This indicates also that the fault near the surface was also confirmed by shallow plane dips nearly horizontal at shallow depths. drilling at the trench sites (IMAIZUMI et al., 1989). The rate of net slip on the ma Valley fault zone as Close examination of trench logs has revealed a whole is as high as 9±3mm/y (IKEDA and three faulting events on the Senya fault, YONEKURA, 1986; ABE and IKEDA,1987). including the 1896 earthquake. Radiocarbon In Japan, the activity of dip-slip faults has been dating of sediments from the trenches indicates evaluated only in terms of vertical slip rates. As that the penultimate event occurred about 3,500 described before, there are many thrust faults, years ago. The interval between the latest mainly in Northeast Japan and the Kinki District, two events is consistent with the previously most of which are believed to be of class B. estimated average recurrence interval, which However, the above example has clearly was based on the average slip rate of the fault in indicated that the activity of these thrust faults the late Quaternary and on the assumption that is likely to have been underestimated and hence the amount of slip in one late Quaternary should be revaluated. faulting event is equal to that observed in 1896. 2. Atera fault VI. Excavation of active faults The Atera fault, a major active left-slip fault in Excavation surveys of active faults in Japan central Japan, extends NW-SE for 80km, with have been carried out since 1978, and have been a relatively small dip-slip component with the continuously planned by universities, govern- NE side upthrown. The total amounts of ment and private institutions. Approximately horizontal and vertical displacement are about 50 trenches in about 30 faults have been 7km and 1km, respectively, most of which has excavated up to 1990 (Fig. 4 and Table 2). occurred in the Quaternary (e.g., YAMADA, 1978a, Among these faults, 20 have historical records of 1978b; OKADA, 1981). The average rate of left- surface faulting associated with a large slip is estimated to be 3-5mm/y. Excavation earthquake. Here we briefly review typical was made at 5 sites along the main trace of this results from selected exploratory trenches. fault (YAMAZAKI and TSUKUDA, 1982; TSUKUDA and 1. Senya fault YAMAZAKI, 1984; AWATA et al., 1986). Ages of The Senya fault, one of the major reverse faulting events determined at these sites faults in Northeast Japan, borders the Yokote correspond fairly well to each other, and the basin on the west and the Mahiru range on the average recurrence interval is estimated to be east, and extends N-S for about 50km. Surface ca. 1,700 years during the past 15,000 years. breaks appeared along the Senya fault in This value corresponds well with the average association with the Rikuu earthquake (M=7.2) recurrence interval obtained from previous field of 1896 (e.g., MATSUDA et al., 1980). Excavation studies. Although it was fairly difficult to date was done at two sites in the central part of this the latest event because of artificial modification fault (RESEARCH GROUP FOR THE SENYA FAULT, 1986;of surface materials, results of recent excavation 1991年7月 第 四 紀 研 究 第30巻 第3号 169

1. Shikano fault 2. Yasutomi fault (Yamasaki F. S.) 3. (Kita-Izu F. S.) 4. Himenoyu fault (Kits-Izu F. S.) 5. Umehara fault (Nobi F. S.) 6. (Nobi F. S.) 7. Atera fault 8. Atotsugawa fault 9. Senya fault 10. Osawa fault (ISTL F. S.) 11. Okaya fault (ISTL F.S.) 12. Okamura fault (MTL F. S.) 13. Urata fault (Morioka F. Z.) 14. Sanage-Sakaigawa fault 15. Gomura fault (Kita-Tango F. S.) 16. Chuzenji fault (Kits-Tango F. S.) 17. Yamada fault (Kita-Tango F. S.) 18. Tachikawa fault 19. Kannonji fault 20. Takeyama fault 21. Gofukuji fault (ISTL F. Z.) 22. Arafune fault (Nagano Basin W. Boundary F. Z.) 23. Fukozu fault 24. Aizu Basin W. Boundary F. Z. 25. Kawafune fault 26. Kitakami Lowland W. Boundary F. Z. 27. Hinagu fault 28. Mihoro fault 29. Chichio fault (MTL F. S.)

Fig. 4 Sites of exploratory trenches dug during 1978-4990 studies have suggested that this last event may with the Kita-Izu earthquake (M=7.3) of 1930, be correlated to the historical earthquake (M= surface breaks appeared along the Tanna fault as 7.8) of 1586 (AWATA et al., 1986). well as a right-slip subsidiary fault (the 3. Tanna fault Himenoyu fault); 2-3m of offset occurred The Tanna fault is located in the Izu Peninsula along the main trace during the earthquake. on the Pacific side of central Japan, and extends A test trench was excavated in 1980 across the in a N-S direction for about 20km. The sense of central portion of the Tanna fault in the Tanna slip is predominantly left-lateral. Numerous basin, yielding evidence of the last two faulting NW-SE trending right-slip faults of short length events. In 1982, a deep trench (7m deep and are associated with the Tanna fault, forming a 30m long) was excavated at the same site, in conjugate fault set called the Kita-Izu fault order to obtain evidence of older events (TANNA system (e.g., MATSUDA, 1972). Several valleys FAULT TRENCHING RESEARCH GROUP, 1983). dissecting a stratovolcano of Middle Pleistocene Excavation was done again in 1985 at a site age (ca. 0.5Ma) are offset left-laterally by 1km, 400m south of the 1980/1982 site in the Tanna indicating that the average rate of slip on the basin, in order to clarify the three-dimensional Tanna fault is about 2mm/y. In association structure of the fault. At this site, three 170 The Quaternary Research Vol. 30 No. 3 July 1991

Table 2 Major results of trenching of active faults in Japan

trenches (2.5-5m deep, 20m long) were dug special attention in order to identify these across the fault, parallel to each other, and faulting events. The third youngest event is connected with each other at both ends by two probably correlated to the historical earthquake additional trenches (THIRD TANNA FAULT of 841 A.D., which caused severe damage in this TRENCHING RESEARCH GROUP, 1988). province, although the second youngest event In the 1982 trench, nine faulting events revealed by the excavation is not registered in the including the 1930 earthquake were registered in historical records. The recurrence interval of the sediments, younger than the Akahoya ash faulting averaged over the past 6,000 years is horizon (6,300 y.B.P), which was derived from a estimated to be about 700-1,000 years (Fig. 5). large submarine caldera volcano off southern This value coincides well with the previously Kyushu. Observations of prism-shaped beds estimated recurrence interval; the average slip burying preexisting fault/flexure scarps, rate of 2mm/y deduced from the offset valleys deposits in open cracks along the fault zone, dissecting a middle Pleistocene stratovolcano, unconformites between the deformed lower beds together with the offset of two meters observed and the less deformed upper beds were accorded at the site for the 1930 earthquake, yields an 1991年7月 第 四 紀 研 究 第30巻 第3号 171

Table 2 (Continued)

For location, see Fig. 4. * (1) Number of trenches at each site, (2) maximum length and (3) maximum depth of trenches in meters. ** References: a: OKADA et al. (1981), b: OKADA et al. (1987), c: TANNA FAULT TRENCHING RESEARCH G ROUP (1983), d: TSUKUDA and YAMAZAKI (1984), e: DISASTER PREVENTION I NSTITUTE, KYOTO (1983), f: MIYAKOSHI et al. (1988), g: YAMAZAKI and TSUKUDA (1982), h: T SUKUDA and YAMAZAKI (1984), I: RESEARCH GROUP FOR ATOTSUGAWA FAULT (1989), j: RESEARCH GROUP FOR SENYA FAULT (1986), k: RESEARCH GROUP FOR THE ITOSHIZU TECTONIC LINE ACTIVE FAULT (1988), l: TOGOet al. (1989), m: OKADA (1989), n: TSUTSUMI et al. (1991).

average recurrence interval of 1,000 years, based no historical record of large earthquakes and no on the assumption that the 1930 earthquake was significant arrangement of microearthquakes "characteristic" (SCHWARZ and COPPERSMITH, 1984) along the fault (OKADA, 1980). to the fault. Several trenches have been excavated across 4. The median tectonic line the main active trace of the MTL at Saijo, The Median Tectonic Line (MTL) active fault western Shikoku (OKADA, 1989; TSUTSUMI et al., system extends about 350km along the median 1991). From these surveys the following results zone of southwest Japan and runs parallel to the were obtained: 1) The latest event occurred general trend of tonal structures of the island during the early 8 th century A.D., and pe- arc. The predominant movement along the nultimate event occurred about 3,000 y.B.P. 2) MTL is right-slip (e.g., OKADA, 1980), the driving Since 6m of horizontal displacement was force of which is believed to be produced by the estimated to have taken place during the last oblique subduction of the Philippine Sea plate event, an earthquake as large as 8 in magnitude beneath the at the may have been associated with this faulting (FITCH, 1972). Although the average rate of slip event. 3) The average recurrence interval is on the MTL is as high as several mm/y, there is about 1,000 years or slightly longer. 4) Young 172 The Quaternary Research Vol. 30 No. 3 July 1991

AWATA, Y, and KAKIMI, T. (1985) Quaternary tectonics and damaging earthquakes in Northeast Honshu, Japan. Earthq. Predict. Res., 3, p. 319-344. AWATA, Y., MIZUNO, K., TSUKUDA, E, and YAMAZAKI, H. (1986) The recurrence interval of faulting on the Atera fault and the age of its last activity. Program and Abstracts, Japan Assoc. Quat. Res., no. 16, p.132- 133. (J) DISASTER PREVENTION RESEARCH INSTITUTE, KYOTO UNIVERSITY (1983) Trenches across the trace of 1891 Nobi earthquake fault. Rep. Coord. Comm. Earthq. Prediction, 29, p. 360-367. (J) FITCH, T. J. (1972) Plate convergence, transcurrent Fig. 5 Diagram showing inferred ages of events faults, and internal deformation adjacent to and the recurence interval of faulting southeast Asia and the western Pacific. Join. along the Tanna fault, revealed by the Geophys. Res., 77, p. 4432-4460. excavation at Myoga, Izu Peninsula, IKEDA, Y. (1983) Thrust-front migration and its central Japan mechanism-evolution of intraplate thrust fault Solid bar indicates the time range of an event systems. Bull. Dept. Geogr., Univ. Tokyo, 15, p.125- based on the ages of 4 tephra layers; open bar, that 159. based on the same tephra layers plus selected IKEDA, Y. (1988) Subsurface structure of the ma Valley radiocarbon dates of peat and wood samples from fault zone. Program and Abstracts, Assoc. Japan. the trench. Geographers, no. 33, p.10-11. (J) (after the TANNA FAULT TREICHING RESEARCH GROUP FOR IKEDA, Y. and YONEKURA, N. (1979) Dislocation fault 1983). models of the San Fernando, California, earthquake of 1971-bending of fault plane and its tectonic significance. Zishin (Tour. Seismol. Soc. Japan), 32, sediments (Holocene to latest Pleistocene in age) p. 477-488. (JE) with quite different facies are in fault contact at IKEDA, Y. and YONEKURA, N. (1986) Determination of the nearly vertical shear zone, indicating lateral late Quaternary rates of net slip on two major fault zones in central Japan. Bull. Dept. Geogr., Univ. emplacements. Tokyo, 18, p. 49-63. 5. Summary IMAIZUMI, T., HIRANO, S. and MATSUDA, T. (1989) The timing of individual faulting events on an Subsurface features of the Senya fault, Akita active fault in the recent geologic past is a key to Prefecture, detected by drilling-Attitude of fault understanding the future behavior of the fault. plane to explain the sinuous fault trace. Active Fault Although our knowledge of recurrence intervals Res., 7, p. 32-42. (J) of earthquakes produced by active faults in Japan KAIZUKA, S. (1984) Landforms in and around the South is still limited, data available at present allow us Fossa Magna and their tectonic processes of growth. to draw tentative conclusions. Most of the The Quat. Res. (Daiyonki-Kenkyu), 23, p. 55-70. (JE) major (class A) active faults of land have moved KAIZUKA, S. (1987) Quaternary morphogenesis and repeatedly, at intervals on the order of one tectogenesis of Japan. Zeit. Geomorph., N. F., Suppl., thousand years, and secondary large faults 63, p. 61-73. KAIZUKA, S. and IMAIZUMI, I. (1984) Horizontal strain (classes B and C) have recurrence intervals of rates of the Japanese Islands estimated from several thousand to tens of thousands of years. Quaternary fault data. Geogr. Rep. Tokyo Metropol. These intervals are significantly longer than Univ., 19, p. 43-65. those for interplate faults. KIMURA, M. (1983) Formation of Okinawa trough References grabens. Mem. Geol. Soc. Japan, 22, p.141-157. (JE) MATSUDA, T. (1972) Surface faults associated with ABE, H, and IKEDA, Y. (1987) Late Quaternary rates of Kita-Izu earthquake of 1930 in Izu Peninsula. In M. net slip on active faults in the nothern ma Basin. HOSHINO and H. AOKI (eds.): Izu Peninsula. Tokai Geogr. Rev. Japan, 60A, p. 667-681. (JE) Univ. Press, p. 73-93. (JE) 1991年7月 第 四 紀 研 究 第30巻 第3号 173

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