Seismic Expedition in the Hinagu Fault Area, Kyushu Island, Japan

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1 Bull. Earthq. Res. Inst. Univ. Tokyo Vol. 2-,**2 pp. +*- +-* Seismic Expedition in the Hinagu Fault Area, Kyushu Island, Japan +,** Kodaira et al.,,**,;,**,; Kurashimo et al.,,**-; Sato et kmo +* km al.,,**.;,**0,**-, +333,***,**, a, b; Iwasaki et Fig. +,**- al.,,**.,**,,**.;,**. +333,*** M.-../. /-. Sato et al.,,**.;,**, * iwasaki@eri.u-tokyo.ac.jp ++- **-, ,**- The Research Group for the,**- Hinagu Fault Seismic Expedition Abstract The Hinagu fault system, Kyushu, Japan, is located in the westernmost part of the Beppu- Shimabara graben. This fault system is characterized by a quite high seismic activity and estimated to have a higher seismic risk among active faults in Kyushu Island. In,**-, we conducted an extensive seismic expedition in and around the Hinagu fault area. This expedition involves seismic refraction/wide-angle reflection experiment using dynamite shots and seismic array observation both for the active and passive seismic sources. For the refraction/wide-angle reflection study, two profile lines of /0.. and -,. + km lengths were set in EW and NNE-SSW directions, respectively, on which 1 dynamite shots of +**,** kg charge and -/3 recorders were deployed. The array observations, which were designed for high-resolution imaging of crustal scatterors and reflectors, were undertaken at / sites in the fault region. This paper presents the outline of this seismic expedition and fundamental data obtained. Key words: crust, structure, Hinagu, fault, Kyushu, Beppu-Shimabara graben

$The refraction/wide-angle reflection experiment was carried out for NNE-SSW line and EW line on which -/3 receivers (small solid circles) were deployed.$

2 ,**- Fig. +. Location map of the,**- seismic expedition. The refraction/wide-angle reflection experiment was carried out for NNE-SSW line and EW line on which -/3 receivers (small solid circles) were deployed. Seven shots (S+- S 1 ) of +**,** -kg charges are shown by stars. Locations of array observations (A+-A/) are shown by open circle. -* Fig. + S+ S1 1,**,;,**/;,**/,**- Table + -/./ /+. + m,**.,**/,**. +,**. +, +2 +3, /* m. S-,**- +/ 2 +, Fig., /0.. km, S-+ S-1 -,. + km,,1.,,. -*. -..,+. -+.,**-,**...* kms / +,3,**. Fig. +,-*,/* m Table, Mark Products L-,, D,., Hz,,,. k W *1. 104

3 Table +. Shot time, location and charge size of,**- seismic expedition. Fig.,. Travel-time diagrams near shot points. These observations were performed to determine seismic velocities at the shallowest part of the crust. (a) S+ -S-. (b) S. -S 1. LS,**. 2*** +330 / ms - Fig. + A+ -A/ / Fig. - Table - +, S / -./,**. ; 105

4 ,**- Table,. Receivers of,**- seismic expedition. 106

5 Table,. (Continued) 107

6 ,**- Table,. (Continued) 108

7 Table,. (Continued) Fig. -. Configurations of array observations (A + / ). Locations of these arrays are shown in Fig. +. Stars and circles respectively denote shot and receiver locations. Surface faults are indicated by broken lines. 109

8 ,**- Table -. Specification of array observation. GPS 0 kms, -km *,. *-. sec unms / dulation S- Fig.. 1 Fig.. / inline shot Fig. 0 1 offline shot Table, reduction velocity 0 kms /,/ km,*,/ km S, A, S- S/ Fig. +, 23 Table. Table / sec A+ -. / S- Fig sec **+. s A **-. s B C SN,* L km Fig. 23. inline shot,**- Fig. +* ++ offline shot,**.,**/ / km *,. *-. sec undulation 0kms intercept time *.. *2. sec 110

9 Fig... Record sections for inline shots on the NNE-SSW line. Horizontal axis is an o# set in km from the individual shot point. Travel time is reduced for a velocity of 0 km/s. (a) S 0. (b) S -. (c) S

10 ,**- Fig. /. Record sections for inline shots on the EW line. Horizontal axis is an o# set in km from the individual shot point. Travel time is reduced for a velocity of 0 km/s. (a) S +. (b) S,. (c) S -. (d) S.. (e) S /. 112

11 Fig. /. (Continued) : : : 113

12 ,**- Fig. 0. Record sections for o% ine shots on the NNE-SSW line. Horizontal axis is a trace number (see Table,). Travel time is reduced for a velocity of 0 km/s. (a) S +. (b) S,. (c) S.. (d) S /. 114

13 Fig. 1. Record sections for o% ine shots on the EW line. Horizontal axis is a trace number (see Table,). Travel time is reduced for a velocity of 0km/s. (a) S 0. (b) S 1. : : : Glenda Monroyo Besana : Subesh Ghimire Nambukara Gamage Sunil Shanta Ismail Husain Fathi Arash Jafer Gandomi Yannis Panayotopoulos 115

14 ,**- 116

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16 ,**- 118

17 119

18 ,**- 120

19 121

20 ,**- 122

21 123

22 ,**- 124

23 125

24 ,**- Fig. 2. Travel-time plot for inline shots on the NNE-SSW line. 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) S 0. (b) S -. (c) S

25 Fig. 3. Travel-time plot for inline shots on the EW line. 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) S +. (b) S,. (c) S -. (d) S.. (e) S /. 127

26 ,**- Fig. +*. Travel-time plot for o% ine shots on the NNE-SSW line. The reduction velocity is taken to be 0*. km/s. The horizontal axis is a trace number (Table,). The quality of travel time data is shown with di# erent symbols (see text for explanation). (a) S +. (b) S,. (c) S.. (d) S /. 128

27 Fig. ++. Travel-time plot for o% ine shots on the EW line. The reduction velocity is taken to be 0*. km/s. The horizontal axis is a trace number (Table,). The quality of travel time data is shown with di# erent symbols (see text for explanation). (a) S 0. (b) S 1. Fig. +,. Examples of record section observed at array A, (see Fig. + ). (a) S -. (b) S /. 129

Fig. +-. Examples of record section for S- observed at arrays A +, -,., and /. (a) A +. (b) A -. (c) A.. (d) A. /,**, a, 11 +-3 +1,.,**, b, 11 +1- +32.,**-,**.,**. B */-.,**-,**/,**/ P *21.

28 Fig. +-. Examples of record section for S- observed at arrays A +, -,., and /. (a) A +. (b) A -. (c) A.. (d) A. /,**, a, ,.,**, b, ,**-,**.,**. B */-.,**-,**/,**/ P *21. Iwasaki, T., K. Adachi, T. Moriya, H. Miyamachi, T. Matsushima, K. Miyashita, T. Takeda, T., Taira, T., Yamada, and K. Ohtake, K.,,**., Upper and middle crustal deformation of an arc-arc collision across Hokkaido, Japan, inferred from seismic refraction/wide-angle reflection experiments, Tectonophysics, -22, /31-. Kodaira S., E. Kurashimo, J.-O. Park, N. Takahashi, A.,**- Nakanishi, S. Miura, T. Iwasaki, N. Hirata, K. Ito and Y. Kaneda,,**,, Structural factors controlling the rupture process of as megathrust earthquake at the Nankai trough seismogenic zone, Geophys. J. Int., +.3, 2+/ 2-/.,**-,**. Double-Difference Tomography -,**. P +,,.,**, /..23 /*/. Kurashimo, E., N., Hirata and T. Iwasaki,,**-, Physical properties of the top of the subducting Philippine Sea plate beneath the SW Japan arc by AVO analysis, Abstr. th +* Int. Symp. Deep Seismic Profiling of the Continents and Their Margins, p 2-.,**1 /3 +*1 ++0.,**/ +0,,**-,**. P,**. B* , Sato, T., S. Miura, G. Fujie., K. Obana, A. Ito, D.H. Kang, S. Kodaira, K. Suyehiro, Y. Kaneda and T. Iwasaki,,**.. Deep seismic structure in the margin of the southwestern Yamato Basin, Japan Sea by ocean bottom seismographic experiment, Abstr. ++ Int. Symp. Deep Seis- th mic Profiling of the Continents and Their Margins, p 31. Steven Harder Kate Miller Galen Kaip,**0 :,**, 2* / GPS /* ++3 +,..,**, -2 +,2 +--.,**/,**/ S*/, -P*++.,**-,**.,**-,**. S */- **-. Received December 1,,**1 Accepted March +2,,**2 130

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