Active Faults and Neotectonics in Japan1) Atsumasa OKADA2) And
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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 Japan trench 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-Philippine Sea plate 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 Izu Peninsula 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- Okinawa trough, 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).