Seismic Reflection Profiling Across the Onshore Source Area of the,**1 Noto Hanto Earthquake, Central Japan

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1 Bull. Earthq. Res. Inst. Univ. Tokyo Vol. 2,,**1 pp.,0/,1-,**1 * +,, , - Seismic Reflection Profiling Across the Onshore Source Area of the,**1 Noto Hanto Earthquake, Central Japan +,, + + Hiroshi Sato *, Shinji Kawasaki, Susumu Abe, Naoko Kato, Iwasaki Takaya, Tanio Ito +, - - Earthquake Research Institute, the University of Tokyo JGI Inc. Graduate School of Science, Chiba University Abstract The seismogenic source fault of the,**1 Noto Hanto earthquake (M 0. 3JMA) runs beneath the northwestern part of the Noto peninsula and its o# shore trending ENE-WSW. To reveal the relationship between a seismogenic source fault and geologic structure, shallow, high-resolution seismic reflection and refraction profiling was undertaken across the source fault along the western coast of the Noto peninsula for +* km distance. The seismic source was a vibroseis and seismic data were recorded by fixed /1/ channels. The P-wave refraction analysis delineated the three layered velocity structure down to + km;,2. km/s, -,. km/s and.+. km/s layers. The -,. km/s layer, which corresponds to the lower Miocene fluvial sediments, shows northward thickening with steps. On the resultant depth converted seismic section, south and north-dipping, faults are interpreted based on the systematic change on dip-angle of the reflectors in the lower Miocene fluvial sediments. By comparing with the aftershock distribution, the deeper extension of the south-dipping fault corresponds to the source fault of the,**1 Noto Hanto earthquake. Key words:,**1 Noto Hanto earthquake, earthquake source fault, seismic reflection profiling, geologic structure, central Japan +,**1 -,/ 3.,,**+. M 0. 3 Sakai et al.,,**2 JMA ENE-WSW 0*,**/;,**1 ;,**1,**1; Ozawa et al.,,** * km Yamada et al.,,**2; Sakai et al.,,**2,**1 * satow@eri.u-tokyo.ac.jp ++- **-,

, +30/ +301 +33* - Fig. + -+ Kano et al.,,**,;,**/ +333 3. + km Fig. + Lopez and Ishiwatari,,**, ENE-WSW Table + ENE-WSW Industrial Vehicle International EnviroVibe +33*,* m 2 +** Hz,* Fig. +. Geological map showing the seismic line Monzen *2.

2 , +30/ * - Fig Kano et al.,,**,;,**/ km Fig. + Lopez and Ishiwatari,,**, ENE-WSW Table + ENE-WSW Industrial Vehicle International EnviroVibe +33*,* m 2 +** Hz,* Fig. +. Geological map showing the seismic line Monzen *2. Major geologic structure is after Editorial committee of civil engineering geologic map of Hokuriku ( +33* ). Pre-N: mainly pre-neogene rocks with minor distribution of lower Miocene andesite. 266

,**1 / Fig., A 1/ +** */. *2. ++. G-DAPS. A + RP+ -10 +* m,/ m VP.3* 1/ Fig.,.3, B RP,22 /0/ VP1 +**,.3 VP0*. +** RP.3* / km -, VP,01 /,/. km JGI Super X Minimum Phase Conversion Table +.

3 ,**1 / Fig., A 1/ +** */. * G-DAPS. A + RP * m,/ m VP.3* 1/ Fig.,.3, B RP,22 /0/ VP1 +**,.3 VP0*. +** RP.3* / km -, VP,01 /,/. km JGI Super X Minimum Phase Conversion Table +. Data acquisition parameters for the Monzen *2 seismic survey. Trace Edit CMP CMP Fig. + CMP : +0./ CMP : /.** m, : 2,,. km Gain Recovery +,** AGC Fig.,. Examples of shot gathers of the Monzen *2 seismic survey. A: a shot gather for reflection profiling at VP,01, B : a shot gather for refraction analysis at VP.3*. Location of shot points is shown in Fig

$* : */. :. : Time Valiant Refraction Statics Fig... Optimum stacking velocities determined by velocity analysis.$

4 Fig. -. Surface structure determined by time-term analysis. (a) time-terms, (b) velocities of second layer, (c) topography and geometry of surface low velocity layer. Deconvolution :,*** :,.* : */. :. : Time Valiant Refraction Statics Fig... Optimum stacking velocities determined by velocity analysis. mal Moveout NMO Fig. - V +,*. *2. kms V, Residual Statics +1.,1. kms *.* m NMO NMO Velocity Analysis Constant Velocity Stack CMP CMP Stack +** CMP /** m Fig. Final Filtered Stack., NMO NMO Correction Nor- Fig. / A 268

5 ,**1 Fig. /. Filtered stacked section and post stacked, migrated time section of the Monzen *2 seismic line. F-X :.3. kms : 1 : +*** : /** +322 *.* *./ :,/ -* // 0/ Hz, *./ +.* : +2,-./ // Hz, +.* /.* : +* +* -/./ Hz Time Migration : 2* -. Fig. / B Depth Conversion +1. kms. kms Fig. 1 Fig. 0A. m *2. kms,2. kms -,. kms --,2. - kms -/. kms,2. kms -1. kms, -,. kms kms. kms / km.-. kms 269

$F+ and F,: inferred fault, Red and black broken lines: boundary of velocity layers deduced from the refraction analysis. 1** m, /* m,**, +0** m Fig. 0 B CMP.** +,** -. CMP+ --* F + ; Fig. 0B +1.$

6 Fig. 0. Depth converted seismic section (A) and its geologic interpretation (B) of the Mozen seismic line. F+ and F,: inferred fault, Red and black broken lines: boundary of velocity layers deduced from the refraction analysis. 1** m, /* m,**, +0** m Fig. 0 B CMP.** +,** -. CMP+ --* F + ; Fig. 0B +1. km CMP -**.** -., kms 1/* m,2. kms -1. kms,.+. kms CMP+,** +0** Yamaji, CMP++** +*** -* +33* ; Sato and Amano, km Kano et al., F, 270

$,**1 Fig. 1. Velocity model of the Monzen *2 seismic line obtained by refraction analysis. (a) Comparison between theoretical travel time curves (solid lines) and first arrival data (solid circles).$

7 ,**1 Fig. 1. Velocity model of the Monzen *2 seismic line obtained by refraction analysis. (a) Comparison between theoretical travel time curves (solid lines) and first arrival data (solid circles). (b) Ray diagrams corresponding to the first arrivals from seven shot points. (c) Velocity model with inferred P-wave velocities (in km/s). F+ F+,**1 F+ -** m F,,** m /,**1 N,-E N0* E./, km, km, Fig. 2 F+ F + 271

8 Fig. 2. Comparison between geologic structure and aftershock distribution. Aftershock distribution is after Sakai et al. (,**2) and Yamada et al. (,**2). The aftershocks were projected to the plane, which is perpendicular to the strike of the source fault and passes through the center of the seismic line (Line +* in Figure + from Sato et al.,,**1), and rotated to the general strike of the CMP line of the seismic section. On this figure the dip angle of source fault shows apparent dip angle. +3,**1 : ****+,**1 :,**1,**1 2, -*+ -+,. +322,.+,0-,00.,**1,** /0, / * 133p /-,33-*2. Kano, K., T. Yoshikawa, Y. Yanagisawa, K. Ogasawara and T. Danhara,,**,, An unconformity in the early Miocene syn-rifting succession, northern Noto Peninsula, Japan : Evidence for short-term uplifting precedent to the rapid opening of the Japan Sea, Island Arc, ++, +1* +2..,**/ *. +30/ 1 / p.,**/ +++,20,33. Lopez, C. J. and Ishiwatari, A.,,**,, Petrogenesis of the tholeiitic basalt, calc-alkaline basaltic andsite and high magnesian andesite lava succession of the Oligo- Miocene Anamizu Formation in northeastern Noto Peninsula,central Japan, J. Min. Petrol. Sci., 31, 2/ ++-.,**1 no. 0+ CD Ozawa, S., H. Yarai, M. Tobita, H. Une and T. Nishimura,,**2, Crustal deformation associated with the Noto Hanto Earthquake in,**1 in Japan, Earth Planets Space, 0*, 3/ 32. Sakai, S., A. Kato, T. Iidaka, T. Iwasaki, E. Kurashimo, T. Igarashi, N. Hirata, T. Kanazawa and the group for the joint aftershock observation of the,**1 Noto Hanto Earthquake,,**2, Highly resolved distribution of aftershocks of the,**1 Noto Hanto Earthquake by a dense seismic observation, Earth Planets Space, 0*,

9 ,**1 +301,**1,* + 2, Sato, H. and K. Amano, Relationship between tecton- Yamada, T., K. Mochizuki, M. Shinohara, T. Kanazawa, S. ics, volcanism, sedimentation and basin development, Kuwano, K. Nakahigashi, R. Hino, K. Uehira, T. Yagi, N. Late Cenozoic, central part of Northern Honshu, Japan. Takeda and S. Hashimoto,,**2, Aftershock observation Sediment. Geology, 1., -,--.-. of the Noto Hanto earthquake in,**1using ocean bottom seismometers, Earth Planets Space (in press).,**+ Yamaji, A., +33*, Rapid intra-arc rifting in Miocene north- ++* east Japan, Tectonics, 3, -0/ -12. Received January,+,,**2,**1 Accepted March +1,,**2 273

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