Shallow Subseafloor Structure in the Inshore Area o# the Southwestern Part of the Boso Peninsula Imaged by Marine Seismic Reflection Profiling

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1 Bull. Earthq. Res. Inst. Univ. Tokyo Vol. 2.,**3 pp. -*1-,3 + +,, * +, -... Shallow Subseafloor Structure in the Inshore Area o# the Southwestern Part of the Boso Peninsula Imaged by Marine Seismic Reflection Profiling + +,, + Hiroshi Furuya *, Tanio Ito, Hiroshi Sato, Naoshi Hirata, Nozomi Komada, Noriko Tsumura, Kazumi Asao, Ryoyu Arai +, -. and Yasuhiro Hanba Graduate School of Science, Chiba University Earthquake Research Institute, The University of Tokyo Fire,Earthquake and Disaster Prevention Division, Chiba Prefecture Kawasaki Geological Engineering Co., Ltd Abstract The southern part of the Boso Peninsula and its surroundings constitute one of the best regions for researching crustal movements causing disastrous earthquakes in the Tokyo metropolitan area, because records of crustal movements associated with these earthquakes are well preserved in Pleistocene trench slope sediments overlying Mio-Pliocene accretionary complexes. To reconstruct the last one million years of vertical crustal movements we performed a multi-channel seismic reflection survey called Boso,**2 in the inshore area o# the southwestern part of the Boso Peninsula in September,**2. A,*-km-long seismic line was deployed from o# Kanaya to Tateyama Bay in the N-S direction,and a 1-km-long seismic line was deployed in Tateyama bay in the E-W direction. Seismic data sets were obtained using an air-gun of.0* cu. inch and a -**-m-long streamer cable with,. channel hydrophones. Source and receiver spacings were +,./ m. The seismic data were processed by the common mid-point method and the seismic profile obtained clearly demonstrates the detailed geologic structure of the trench slope sediments of the Toyofusa Group. The two units, which correspond to the lower ( +*. *1. Ma) and upper (*1. *-. Ma) Toyofusa Group, are identified on the seismic profile. Key words: marine seismic reflection survey, Boso Peninsula, inshore area, trench slope sediments ; Matsuda et al., ** * hiroshi_ f@graduate.chiba-u.jp,0-2/,,

2 +**,**,; Miyauchi et al.,,**0 +** -/*,**2 +33/ -* +** +33/ +33, +33, -** m +** 1* 1* -* Fig., +33/ Boso,**2,**2 Line, 3 WSW - Line + +33/ Line, Fig /,, P,**,,**-,**2 MS,**. P P,,+,1** ms -/** ms,/** ms -/** ms,*** ms,/** ms Line + :,*** ms : +2** ms +33, Fig. + Line + - Line, Fig. +,**2 Boso,**2,,**, Miyauchi et al.,**0 +33,

3 Fig. +. Simplified geological map of the southern part of the Boso Peninsula showing the location of seismic lines of the Boso,**2 seismic survey. Geological map is based on Kotake ( +322 ), Saito ( +33, ) and Kotake et al. ( +33/ ) Boso,**2 Table + Boso,**2 - km - km -,,* km Line + 1 km Line,, Super X-C.0* cbi Line + Line CDP, +,./ m Line + Line, +,./ m Fig. - Line + Line, CDP Sort 309

4 Fig.,. Ages of the trench slope sediments after Kotake et al. ( +33/ ) Fig. -. Flow chart of data-processing. Table +. Data acquisition parameters for the Boso,**2 seismic survey. CDP 0,/. m First Break Mute Gain Recovery n t t n +2. Deconvolution : +,** msec : +0* msec : + msec sliding : +/** msec NMO NMO Correction,Velocity Analysis CDP 310

5 NMO aperture : 0** m :./ ivas RMS : 2/ Time-Variant Band Pass Filter RMS -*** m Line + 00 Line,,+ :,.* msec DMO DMO DMO Correc- : +** msec tion, DMO Velocity Analysis phase : minimum phase filter NMO cut-off : * /** msec: -/./ +,* +.* Hz /** +*** msec: -*.* +,* +.* Hz NMO +***,*** msec:,* -* +** +,* Hz DMO. RMS +/** msec Line + Line,./ Fig.. Trim Statics Line + Fig. / + Fig. /, Line, Fig. 0 + Fig. 0, Trim Line + Line, Fig. / + Fig. 0 Statics Line + + +*. +/. : +* msec Line + Fig. / : +0* msec, a-a cut-off : -2.*0* h-h Fig. 1 + f-f CDP CDP Stack Line + Line, Fig. 0, NMO DMO Fig. 1, CDP g-g F-X Prediction Filter a-a c-c d-d e-e f-f : - : -/ :,/* msec Line + : +,/ msec Kirchho# Time CDP + 3** : Migration CDP 3** +/** : CDP +/**,,** : 311

6 312

7 Fig. /. Seismic sections along Line +. ( + ) Migrated time section. (,) Depth converted section. V : H + :

8 Fig. /. (Contimued) A-AD-D -* Fig CDP3** CDP,,**,3** : -*,* A-A CDP,3** : /** m Fig. 1 -,,/ /** m f-f CDP+.** +*** m 315

9 Fig. /. (Contimued) 316

10 Fig. 0. Seismic sections along Line,. ( + ) Migrated time section. (,) Depth converted section.v : H + :

11 B-B A-A B-B Fig CDP,+** 0** m C-C CDP,3** CDP IIICDP+/**,,**,2/* +*** m+,/* m D-D Fig. + / c-c Fig. 1 +, Fig c-c,** ms c-c Fig. 2 Line + Line, c-c /** m,1** ms c-c CDP+3**,+** Line + ICDP + 3** CDP+,** a-a CDP /** 2** +3-1 ; +32+ CDP2** 3** Fig A-A A +33, A-A A-A b-b.** ms b-b, IICDP3**,,*** ms +/** b-b III IV CDP,,** B-B B CDP+/** CDP,,** C-C Fig. 1 CDP C-C, +/**,* IV&V CDP,,** Miyauchi et al.,,**0 B-B e-e CDP,/**.* B-B B-B d-d Fig. 2,,*** ms. km,**, MS d-d+2** ms 318

12 Fig. 1. Predominant reflectors and boundaries between reflective domains. For details refer to the text. Line +. a-ah-h : Predominant reflectors (Thin red lines indicates reflectors oblique to their overlying reflectors. Broken lines are for obscure parts.). A-A to D-D : Boundaries between characteristic reflective domains (Broken lines are for obscure parts.) V : H + : +. [ + ] [ /] : See (-) and Fig. 2 ( + ). (,) Line, g-g and h-h : Predominant reflectors (Broken lines are for obscure parts.). (-) Detailed profiles of [ + ] to [/] in ( + ). -+3-,*

13 Fig. 1. (Contimued) 321

14 Fig. 1. (Contimued) 322

15 323

16 e-e,* f-f,* D-D D-D Fig. + 0 Line + Line, Fig. 3 Fig. 3 Fig. 2. (Contimued) +**,***,/** ms,, e-ed-dfig. 2 +.,.** ms CDP-,** f-f +33/ ;,**2 f-f f-f Fig. 2 + /,-** ms f-f g-g,*** ms Line, f-f f-f h-h,+** ms,/** ms h-h 324

17 Fig. 3. Geologic interpretation of the Boso,**2 seismic lines. Blue heavy lines indicate the base of trench slope sediments. -,/ -,0

18 Fig. 3. (Contimued) 327

Fig. 3. (Contimued),**. 0/ 31 ++2. +322 3. - +21,*0. +33/ +*+ 1 /+/ /-+. Matsuda, T., Y. Ota, M. Ando and N.

19 Fig. 3. (Contimued),**. 0/ ,*0. +33/ +*+ 1 /+/ /-+. Matsuda, T., Y. Ota, M. Ando and N. Yonekura, +312, Fault mechanism and recurrence time of major earthquakes in southern Kanto district, Japan,as deduced from coastal terrance data, Geol. Soc. Amer. Bull.,,-, +0+* +0+2.,**, +*3,2..,**, MS, Miyauchi, T., T. Minawa, T. Ito, H. Kato, T. Kawamura, T. Ikawa, and K. Asao,,**0, Structurally controlled geomorphology on the southern Boso Peninsula, Central Japan: Investigation using seismic reflection profiling, Quatern. Res.,./,, / + 328

20 +*1 p.,**, ,,3-, ,**2 +-,*+,*1. /1* p. +3.3,**2MS, ** +33, 3, (Received February +/,,*+*) (Accepted March,0,,*+*),**-, 329

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