Bull. Earthq. Res. Inst. Univ. Tokyo Vol. 2-,**2 pp. 11 +*+ Deep Seismic Refraction/Wide-angle Reflection Profiling in Central Japan The Research Group for Seismic Expedition in Central Japan Abstract An extensive onshore-o# shore seismic refraction/wide-angle reflection experiment was undertaken in,**+. The profile line was extended from the eastern part of the Nankai trough to Central Japan. The land profile line of,0+-km length was laid from the Tokai district to the coastal region of the Sea of Japan, crossing an expected fault area of the forthcoming Tokai earthquake, an aseismic slip region north of the fault area, the Median Tectonic Line and the Niigata-Kobe Tectonic line. On this profile, -3+ receivers were deployed to observe 0 dynamite shots with +** /** kg charge. Data obtained are of good quality, from which clear first arrivals are traced beyond +,* +0* km o# sets. The most important feature in these data is strong reflection from the subducted Philippine Sea plate observed in the southernmost part of the profile. This paper presents the outline of this seismic expedition and fundamental data obtained. Key words: crust, plate structure, subduction, Philippine Sea plate, Central Japan +,**+ Iidaka et al.,**-,**.,**+ 2 Fig. + Kodaira et al.,,**/ aseismic slip,**+ Kodaira et al.,,**.,**+ NNW-SSE 1*, aseismic slip Ozawa et al.,,**, Sagiya et al.,***,0+ km NNW-SSE J-J + / * e-mail: iwasaki@eri.u-tokyo.ac.jp ++- **-, + + + 77
/ Fig. +,**+ /** kg J. -J/ Texas +** kg T0 J-T - 0 Table + +*/.- mm, J+ -J/ 1/., 1/. 2 m, T 0 -*+. m + 2,/,0 /* m. 0 Fig., J+ -J/ T0 +0.,.. //..3..,. -0. kms, Table, J+ - J/ -,2 Table, + -,2 LS2*** +330 Mark Products L-,, D Fig. +. Location map of the,**+ seismic expedition.,,. Hz,,,. kw Six shots (J+-J/ and T 0 ) of +** /** -kg charges on *1. +* ms the onshore line are shown by stars. Our onshoreo# shore profile is running across the expected fault DAT area of the forthcoming Tokai earthquake indicated +331 by a pink area. The onshore line is also crossing Median Tectonic Line (MTL) and the Niigata-Kobe Tectonic line. The aseismic slip area in the Tokai 0* km district (Ozawa (,**,)) is located west of the + km 0- Table,.*+ southern part of our profile (south of J.)). Receiver.0- +* ms points of the onshore line are indicated by small circles. L,2 B../ Hz Table +. Shot time, location and charge size of,**+ seismic expedition. 78
Fig.,. Travel-time diagrams near shot points. These observations were performed to determine seismic velocities at the shallowest part of the crust. J/ 0* km /** m, C 2** m J/ SN - L S Iidaka et al.,**-,**. /./ J-J +, */. *2. sec GPS J, -J/ 0 +* kms ms 0 kms / kms SEGY format Barry et al., +31/ Fig. - -. kms reduction velocity 0 kms J+ -J. Fig. - +,* +0* km +/ -* km Fig.. Table - J/ Fig. - e **+. s J/,/ km A **-. s B 79
Table,. Receivers of,**+ seismic expedition. 80
Table,. (Continued) 81
Table,. (Continued) 82
Table,. (Continued) 83
(a) (b) Fig. -. Record sections. Horizontal axis is an o# set in km from the individual shot point. Travel time is reduced for a velocity of 0 km/s. Remarkable later phases are indicated by arrows. (a) J +. (b) J,. (c) J -. (d) J.. (e) J /. (f) T 0. 84
(c) (d) Fig. -. (Continued) 85
(e) (f) Fig. -. (Continued) 86
(a) (b) (c) (d) Fig... Travel-time plots. The reduction velocity is taken to be 0*. km/s. The horizontal axis is an o# set in km. The quality of travel time data is shown with di# erent symbols (see text for explanation). (a) J +. (b) J,. (c) J -. (d) J.. (e) J /. (f) T 0. 87
(e) (f) Fig... (Continued) 88
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.,**+ 2 aseismic slip Iidaka et al.,,**-,**. : : : : : : Ismail Husain Fathi 100 Barry, E.M., Cavers, D.A. and Kneale, C.W., +31/, Recommended standards for digital tape format, Geophysics,.*, -.. -/,. Iidaka, T., T. Iwasaki, T. Takeda, T. Moriya, I. Kumakawa, E. Kurashimo, T. Kawamura, F. Yamazaki, K. Koike and G. Aoki,,**-, Configuration of subducting Philippine Sea plate and crustal structure in the central Japan, Geophys. Res. Letter, -*, doi: +*,3 /,**, GL *+0/+1.. Iidaka, T., T. Takeda, E. Kurashimo, T. Kawamura, Y. Kaneda and T. Iwasaki,,**., Configuration of subducting Philippine Sea plate and crustal structure in the central Japan, Tectonophysics, -22, 1,*. Kodaira, S., T. Iidaka, A. Kato, J-O Park, T. Iwasaki and Y. Kaneda,,**., High pore pressure may cause silent slip in the Nankai Trough, Science, -*., +,3/ +,32. Kodaira S., T. Iidaka, A. Nakanishi, J.-O. Park, T. Iwasaki and Y. Kaneda,,**/, Onshore-o# shore seismic transect from the eastern Nankai Trough to central Japan crossing a zone of the Tokai slow slip event, Earth Planets Space, /1, 3.-3/3. +330.+ +,1 +-3. Ozawa, S., M. Murakami, M. Kaidu, T. Tada, T. Sagiya, Y. Hatanaka, H. Yarai and T. Nishimura,,**,, Detection and monitoring of ingoing aseismic slip in the Tokai region, central Japan, Science,,32, +**3 +*+,. Sagiya T., S. Miyazaki and T. Tada,,***, Continuous GPS array and present-day crustal deformation of Japan, Pure appl. geophys., +/1,,-*-,-,,. Kate Miller
Galen Kaip Amy,**+ Kwiatkowski,**+,**+ C -1.,**+ C -2. Received December 1,,**1 +331 GPS Accepted March +2,,**2 /* ++3 +,.. 101