High-resolution seismic reflection survey across the Western Boundary Fault Zone of the Nagano Basin, Central Japan: Data acquisition and processing

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1 Bull. Earthq. Res. Inst. Univ. Tokyo Vol. 2+,**0 pp * : + * +, -. / , : -. / : 3 +* : +* High-resolution seismic reflection survey across the Western Boundary Fault Zone of the Nagano Basin, Central Japan: Data acquisition and processing Shinsuke Okada + *, Yasutaka Ikeda +, Tomoo Echigo,, Shigeru Toda -, Haruo Kimura., Hajime Kato /, Tatsuya Ishiyama 0, Kyoko Kagohara 1, Nobuto Morishita 1, Rina Okuda 2, Hiroki Watanabe 3 and Shin Oda +* + Graduate School of Science, The University of Tokyo,, Graduate School of Science, The University of Tokyo (Now at Geo-Research Institute), - Aichi University of Education,. Earthquake Research Institute, The University of Tokyo, / Faculty of Education and Human Sciences, Yamanashi University, 0 Active Fault Research Center, National Institute of Advanced Industrial Science and Tecnology, 1 Graduate School of Science, Tohoku University, 2 Aichi University of Education (Now at Iwakura Minami Elementary School), 3 Department of Earthsciences, Faculty of Science, The University of Tokyo, +* Graduate School of Science, The University of Tokyo (Now at Idemitsu Kosan Co., Ltd.) Abstract The Western Boundary Fault Zone of the Nagano Basin borders the eastern margin of Northern Fossa Magna, which has undergone strong horizontal shortening since Pliocene. The Western Boundary Fault Zone of the Nagano Basin is considered to be a back thrust that developed on the hanging wall side of the Itoigawa-Shizuoka Tectonic Line. To reveal the subsurface structure of the Western Boundary Fault Zone of the Nagano Basin, we carried out a high-resolution seismic reflection survey along the Saigawa River, southern Nagano City. The source used in this survey was a mini-vibrator (T-+/***). Source and receiver spacing was +* m. +2* channels of geophone arrays were used to record each shot. The seismic section obtained after careful data processing shows fairly flat Quaternary basin fillings in the eastern part of the seismic line. The Quaternary basin fillings are interpreted to be in west-dipping fault contact with west-dipping Neogene strata underlying the Saigawa Hills. Key words: high-resolution seismic reflection profiling, subsurface structure, the Western Boundary Fault Zone of the Nagano Basin, Northern Fossa Magna, back thrust * s_okada@eps.s.u-tokyo.ac.jp ++-**

+ NE-SW /, km Fig. + half graben 0 km tectonic inversion +333;,**,; Elouai et al.,,**. back thrust,**, Elouai et al.,**.,1 km, ++ km backthrust +330b,**/ 3 -* +* ++, ; +2.1 +.2 m,**,..1 km Fig.,,**. thrust front migration Ikeda, +32- NW Fig.

2 + NE-SW /, km Fig. + half graben 0 km tectonic inversion +333;,**,; Elouai et al.,,**. back thrust,**, Elouai et al.,**.,1 km, ++ km backthrust +330b,**/ 3 -* +* ++, ; m,**,..1 km Fig.,,**. thrust front migration Ikeda, +32- NW Fig. +. Index map of the Nagano basin. Black solid line; active fault (after Ikeda et al.,,**,), gray solid line; seismic line. -.+ km, +.0 km - IVI, T-+/***; +330a +* m +* Hz + m * GDAPS-., msec 0 sec NW -*/* 172

uv;<pwxyz#4{ LMj< kl Fig.,. Location map of the seismic line (Sai-gawa,**/).

-. Example of shot gather at VP. +-12. VP. +**+ῌ++*2 +2*!"#$%& ' ()*+, -.

B#C D E F GHI J K LMN / O# P"#QR $%S T U V W /0 JF VP. ++*3ῌ+,-* #4 RP.

3 uv;<pwxyz#4{ LMj< kl Fig.,. Location map of the seismic line (Sai-gawa,**/). Table +. Data acquisition parameters for the Sai-gawa,**/ seismic line. Fig. -. Example of shot gather at VP VP. +**+ῌ++*2 +2*!"#$%& ' ()*+, -./0 12!3#4 567 =>? B#C D E F GHI J K LMN / O# P"#QR $%S T U V W /0 JF VP. ++*3ῌ+,-* #4 RP. ++++ῌ+,3* #ds0 I #ef:g1hi Table + #ds0 XY& $% P" +,*!" 0* : Z#.ῌ ῒῐΐ ῑ $% [\&/0 /]^_`ab c Fig. 173 LMj< kl*)mno pq rplm st

Super X-C Fig.. Table, stacking line Fig.

4 Super X-C Fig.. Table, stacking line Fig., fold Fig. / CMP CMP / m 3* P +., 0** msec AGC Table,. Processing parameters for CMP Sai-gawa,**/ seismic data. Fig... Flow chart of data-processing. Fig. /. Distribution of number of folds along a CMP stacking line 174

5 Fig. 0. Surface velocity structure determined by time-term analysis. (a) Time-terms. (b) Velocities of second layer. (c) Topography and geometry of surface low velocity layer.,.* msec,,*** msec, msec iras P Fig. 0 NMO CMP CMP NMO NMO Fig. 1 CMP NMO NMO CMP /*. msec NMO CMP CMP +*+/2*3* Hz 175

3** El#mn&Qo ) 5647#8%) 9 : ;<=>?@ p&c9 R SJKTU AB= CD!E5647 -FGHAB 9!! Fig.

6 ~ Fig. 1. Velocity structure along the Seismic line (Sai-gawa,**/). Fig. 2. Time section after migration. F-X!"#$%& SJKTU #% C9 R SJKTU VW % Càbc@ ῌ decfàgh_ idcàb '() *+,-./0123 4# cjk CMP /* /** 1**,-. 3** El#mn&Qo ) 5647#8%) 9 : ;<=>?@ p&c9 R SJKTU AB= CD!E5647 -FGHAB 9!! Fig. 2 E 5647EI2JK7L# - vawx JK1 5647EMK L# -* O6 POELA# /** msec "&Q VW#q&CN rst# JK1 N ghàbc Fig. 1 #y &Cz à#% Q vawx#dec9 vawx{evw }# Fig #8%&C9 u)9 R SJKTU VW XYZE '&Q[\ "AB ]^_R 176

7 Fig. 3. Depth converted seismic section. 177

8 Fig. +*. Geologic interpretation of the depth converted seismic section. Unit (+a) ; Lower Quaternary basin ﬁlls, Unit (+b) ; Quaternary basin ﬁlls, Unit (,a) ; Ogawa Formation Ronji Mudstone Member, Unit (,b) ; Ogawa Formation Susobana Tu# Upper member, Unit (-) ; Ogawa Formation Susobana Tu# Lower member. ῑ ῐ ῌ 178 ῌ

9 / Fig. 1 Fig. 2 Fig. 3 Fig. 3 Fig. +* Fig. 3 - CMP 0**3-* CMP,*,0* * m+** m,** m+** m * m+** m +** m +a +b +b * m +** m CMP /+*,** m+** m CMP /0* +a /** m1** m CMP 0**1.* CMP /31 CMP /31 CMP /+* +** m, + CMP ++*,a + CMP ++*,0*./,b +320,a,b CMP.2*0**.** m,** m ** m +/+1 B +/-**-** : GMT Wessel and Smith, +331 Elouai, D., H. Sato, N. Hirata, S. Kawasaki, T. Takeshita, N. Kato and T. Takeda,,**., Deep seismic reflection profiling across the Northern Fossa Magna: The ERI +331 and the JNOC +330 seismic lines, active faults and geological structures, Earth Planets Space, /0, Ikeda, Y., +32-, Thrust-front migration and its mechanism evolution of intraplate thrust fault systems, Bull. Dept. Geogr., Univ. Tokyo, +/, +,/+/3. 179

10 ,**,,/.p / + +,-p.,**., a +/ +**+*/. +330b +/ +,2+-,. +333,+ /03/1/.,**, +2.1,,.//.. Wessel, P. and W.H.F. Smith, +332, New, improved version of the Generic Mapping Tools released, EOS Trans. Amer. Geophys. U., 13, /13. Received November +/,,**0 Accepted January,-,,**1 180

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