REGIONAL CHARACTERISTICS OF STRESS FIELD AND ITS DYNAMICS IN AND AROUND THE NANKAI TROUGH, JAPAN

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1 CHINESE JOURNAL OF GEOPHYSICS Vol. 46, No. 4 July, , , , ,,, : -. (10 22 ), (60 85km) ; (40 ), (160km)..,. GPS,, -,,.,,, (2003) P , REGIONAL CHARACTERISTICS OF STRESS FIELD AND ITS DYNAMICS IN AND AROUND THE NANKAI TROUGH, JAPAN XU J IREN 1 ZHAO ZHIXIN 1 KONO YOSHITERU 2 KINOSHITA HAJ IMU 3 1 Institute of Geology, Chinese Academy of Geological Science, Beijing , China 2 Faculty of Science, Kanazawa University, Kanazawa , Japan 3 Japan Marine Science and Technology Center ( JAMSTEC), Yokosuka, , Japan Abstract The Nankai Trough of Japan is one of the most active seismic zones in the world. In order to understand the mechanisms of great interplate earthquakes and the tectonics, seismic activity and focal mechanism solutions are analyzed in detail in this research. Based on the geometry of slabs and state of stress within the slabs, the Nankai Trough can be divided into two parts, the eastern part ( Shikoku and the Kii peninsula) and western part ( Kyushu). In the east the seismic slab of the Philippine Sea plate subducts at shallow dip angels between and its leading edge reaches down only to about 60 85km depth beneath Shikoku and the Kii peninsula. While in the west the seismic slab with dip angle of 40 subducts and down to about 160km depth beneath Kyushu. There are two types of seismogenic stress field within the slabs of the ( , ).,,1970,1993,.. E2mail org. cn

2 4 : 489 Philippine Sea plate in the eastern and western parts of the Nankai Trough. The type of down2dip compression stress field exists in the region beneath the most of Shikoku and the Kii peninsula. On the other hand, the type of down2dip extension stress field within the slab is dominant beneath Kyushu. The analyses of gravity, heat flow, GPS and other geophysical data suggest that the tectonic features in the eastern part are different from that of the western part. Two types of characteristics of seismotectonics and intraslab stress field in the same trough might be related to the differences of the slab ages in the eastern and western parts. Key words Nankai Trough, Stress field within slab, Geophysical field, Subduction of high stress type, Young active slab. 1.,,..,, , 12 M7. 5, 6 M8., , , [1 ]. 1980, GPS,, , 1500 [2 ] ,,, 1 (a) M5 M9 (b) (a) EU PA PH ; (b),. [6,7 ]. Fig. 1 Plate tectonics (a) and horizontal projections of focal faults of earthquakes ( M5 M9) occurred in and around the Nankai Trough during (data from references [6 ] and [7 ]) (JMA) (JUNEC) 1) [3 ] [2 ] (1996),., Ishikawa SEIS2PC [4 ]. 8,,, [5 ] , 1) Y. Ishikawa,1999.

3 490 (Chinese J. Geophys. ) 46.,,., , M7. 5.,, M5 M6.,.,,., ( ) [3 ]. 2 ( 2a), ( 2b) ( 2c)., : -., 10 22, 60 80km( 2b,2c).. ( 2b), 10 11, 60km.,, 40, 160km( 2b).,,.,.. [8,9 ] ) ( 0 200km) (a) ; (b) ; (c). A B C. (1) (5). Fig. 2 Epicentral distribution of earthquakes during (depths : 0 200km)

4 4 : 491, M , M5 M6, M7, 8.,. 3, M3. 5 M8. 5.,. CMT [3 ] [10 ] P 3. Fig. 3 Horizontal projections of compressional axes (P2axes) of focal mechanism solutions and divided three areas based on their characteristics of the intraslab stress field in the Nankai Trough 4 P (a) T (b) ( , M3. 5 M8. 4) Fig. 4 Profile distributions of P2(a) and T2axes (b) along the NS section in the region from the western part of Shikoku to the southwestern coast of the Kii peninsula ( , M3. 5 M8. 4), respectively 5 P (a) T (b) ( , M3. 5 M8. 4) Fig. 5 Profile distributions of P2(a) and T2axes (b) in the Kyushu ( , M3. 5 M8. 4), respectively

5 492 (Chinese J. Geophys. ) 46 P, ( A) ( B ) ( C) 3,. 4 5 ( 3 A C ) P (a) T (b). 4, P, T.,,. P T ( 5), P, T,. 5 4, -.,,.,., ( 3). 8, ( 1). 4,,, GPS,, ( I),,., 5km. 6,, mGal,, - 120mGal [10 ].,.,.,. 4. 2,,.,., mwπm 2, 200mWΠm mwπm 2, [11 ]., -,.,., GPS GPS. GPS,. ( GSI) GPS [12 ],. 7. 7a,, ;,. ( 7b),,.,. GPS,, ;,. GPS -

6 4 : 493,,,,. 7 GPS (a) (b),, Fig. 7 Horizontal (a) and vertical (b) crustal movement in and around the Nankai Trough deduced from GPS measurements ,,., [13 ]. 8,.,, -,,.,, 8,. 1 Table 1 Regional characteristics in the eastern and the western parts of the subduction zone in the Nankai Trough ( ) ( - ) ( ) M6 M7, M8 GPS,,

7 494 (Chinese J. Geophys. ) ,, [14 ].,, -,., [15 17 ],,, -,.,,.,,,, 8.,. Ruff [18 ],., GPS., 30 15Ma [19 ].,,,.,,,. 21 ( OD21),.,,. [ 1 ] Ando M. An outline : for the future Nankai earthquake, what must be done in Japan? Chikyu Monthly, 1999, 24 : 6 13 [ 2 ].. :, 1996 Utsu T. Encyclopedia of Earthquake (in Japanese). Tokyo : Asakura Shoten Publisher, 1996 [ 3 ] Nakamura M, Watanabe H, Konami T, et al. Characteristic activities of subcrustal earthquakes along the outer zone of southwestern Japan. Annuals of Disas, Prev, Res, Inst., Kyoto Univ., 1997, (40 B21) : 1 20 [ 4 ] Ishikawa Y. Introduction to the revision of SEIS2PC. Geological Information, 1986, 11 : [ 5 ] Ishibashi K. Specification of a soon2to2occur seismic faulting in the Tokai district, central Japan, based upon seismotectonics. Ewing Ser. 4, 1981, Maurice [ 6 ]. ± g g g - g g g g. :,1995 Sata R. Parameter Handbook of Earthquake Faults, Japan ( in Japanese). Tokyo : Kajima Press, 1995 [ 7 ] Kikuchi M. EIC newsletter. http :ΠΠwww. eri. u2tokyo. ac. jpπjhome. html, 2000 [ 8 ] Kodaira S, Takahashi N, Park J, et al. Western Nankai Trough seismologenic zone : Result from wide2angle Ocean Bottom Seismic survey. J. Geophys. Res., 2000, 105 : [ 9 ] Kodaira S, Takahashi N, Park J, et al. Subducted seamount imaged in the rupture zone of the 1946 Nankaido earthquake. Science, 2001, 289 : [10 ] Xu J, Kono Y. Seismogenic Stress Field and Its Tectonics in and around the Nankai Trough, Japan. Abstract of AGU Fall Meeting, 1999, [11 ] Kinoshita M, Yamano M. Heat flow distribution in the Nankai Trough region, Japan2Russia2China Monograph. Tokyo : TERRAPUB, [12 ] Geograhical Survey Institute. Crustal Deformation of Japan detected by GEONET. http :ΠΠmekira. gsi. go. jpπhendou new. html, 1999 [13 ] Kanamori H. Seismic and aseismic slip along subduction zones and their tectonic implications. Maurice Ewing Ser. 1977, 1 : [14 ] Isacks B L, Molnar P. Distribution of stresses in the descending lithosphere from a global survey of focal mechanism solutions of mantle earthquakes. 174 Rev. Geophys. Space Phys., 1971, 9 : 103 [15 ] Kimura S, Okano K. Characteristics of focal depth distribution of mantel earthquakes in the central and western part of Shikoku. (in Japanese), 1994, 47 (2) : Zisin [16 ] Shiono K. Seismicity of the SW Japan Arc2Subduction of the Young Shikoku Basin. Modern Geology, 1988, 12 : [17 ] Seno T. Regional Stress Fields in Kyushu : Implications to Deep Processes. Program and abstracts of 1998 Fall Meeting. the Seismological Society of Japan, 1998,C37 [18 ] Ruff L J, Kanamori H. Seismicity and the subduction process. Phys. Earth Planet. Inter., 1980, 23 : [19 ] Kagami H, Shiono K, Taira A. Subducting of plate and formation of accretionary prism in Nankai trough, Formation of Japan Islands. Tokyo : Iwanami Shorten Publisher,

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