46 4 2003 7 CHINESE JOURNAL OF GEOPHYSICS Vol. 46, No. 4 July, 2003 1 1 2 3 1, 100037 2, 920-1192 3, 237-0061,,, : -. (10 22 ), (60 85km) ; (40 ), (160km)..,. GPS,, -,,.,,,.. 0001-5733(2003) 04-0488 - 07 P541 2002-03 - 17,2003-01 - 07 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 100037, China 2 Faculty of Science, Kanazawa University, Kanazawa 920-1192, Japan 3 Japan Marine Science and Technology Center ( JAMSTEC), Yokosuka, 237-0061, 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 10 22 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 (2000445,2000444).,,1970,1993,.. E2mail :xujiren @ccsd. org. cn
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.,,..,,. 715 1200, 12 M7. 5, 6 M8., 8 100 200. 1944 7. 9 1946 8 50,. 1946 8, [1 ]. 1980, GPS,,. 2. 416, 1500 [2 ]. 684 8. 1926. 20,,, 1 (a) 1498 1998 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 1498 1998 (data from references [6 ] and [7 ]) (JMA) (JUNEC) 1) [3 ] [2 ] (1996),., Ishikawa SEIS2PC [4 ]. 8,,, [5 ]. 1 1498 500, 1) Y. Ishikawa,1999.
490 (Chinese J. Geophys. ) 46.,,.,1946 8 50, M7. 5.,, M5 M6.,.,,., (1985 1992 ) [3 ]. 2 ( 2a), ( 2b) ( 2c)., : -., 10 22, 60 80km( 2b,2c).. ( 2b), 10 11, 60km.,, 40, 160km( 2b).,,.,.. [8,9 ]. 2 1985 1992 1) ( 0 200km) (a) ; (b) ; (c). A B C. (1) (5). Fig. 2 Epicentral distribution of earthquakes during 1985 1992 (depths : 0 200km)
4 : 491, M7. 6 -., M5 M6, M7, 8.,. 3, 1936 647 M3. 5 M8. 5.,. CMT [3 ] [10 ]. 3 647 3 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) (1936 1998, 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 (1936 1998, M3. 5 M8. 4), respectively 5 P (a) T (b) (1936 1998, M3. 5 M8. 4) Fig. 5 Profile distributions of P2(a) and T2axes (b) in the Kyushu (1936 1998, M3. 5 M8. 4), respectively
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,,. 4. 1 6 ( I),,., 5km. 6,,. - 40 60mGal,, - 120mGal [10 ].,.,.,. 4. 2,,.,., 120 200 mwπm 2, 200mWΠm 2. 40 80 mwπm 2, [11 ]., -,.,.,. 4. 3 GPS GPS. GPS,. ( GSI) GPS [12 ],. 7. 7a,, ;,. ( 7b),,.,. GPS,, ;,. GPS -
4 : 493,,,,. 7 GPS (a) (b),, 1999 7 1998 7. Fig. 7 Horizontal (a) and vertical (b) crustal movement in and around the Nankai Trough deduced from GPS measurements 5 8 1.,,., [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,,
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