講演題目：伊予灘中央構造線活断層系上灘セグメントの完新世活動度評価?下灘沖ボーリングコア解析の成果?

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1 No. 2, p , 2002 Regional imaging and segmentation of the MTL active fault system in the Iyo-nada Sea - off Saganoseki area Futoshi Nanayama 1, Kazuhiro Otsuka 2, Kenichiro Miura 3, Michiharu Ikeda 4, Seiichi Kanayama 5, Shuji Kobayashi 6, Masashi Hasegawa 7, Yuichi Sugiyama 8 and Eikichi Tsukuda 9 1, 8, 9 Active Fault Research, [email protected], Center, GSJ/AIST [email protected], [email protected] 2, 5, 6 Shikoku [email protected], Research Institute [email protected], Inc., [email protected] 3 NEDO Former NEDO fellow ult at Research Active Fa Center, GSJ/AIST 4 NEDO Active Fault Research Center, GSJ/AIST, present affiliation: Kisojiban Consultants Co., Ltd., [email protected] 7 Sogo Geophysical., Ltd., [email protected] Exploration Co Abstract: Single-channel seismic profiling in the Iyo-nada Sea - off Saganoseki area has revealed two right-overstepping fault jogs about 13 km long off Gushi and off Misaki. These extensional jogs divide the Median Tectonic Line (MTL) active fault system in this area into three geometrical segments: 21-km-long Iyo segment, 42-km-long Iyo-nada segment, and 23-km-long Hoyo Strait segment from east to west. The profiling also indicates that the Hoyo Strait segment meets off Saganoseki with the ESE-trending Beppu-wan (Beppu Bay) active fault system. The Holocene activity of the Hoyo Strait segment is inferred to be lower than those of Iyo and Iyo-nada segments of MTL and of the Beppu-wan active fault system.,,,,, Key words: Iyo-nada, Saganoseki, Median Tectonic Line (MTL) active fault system, Beppu-wan active fault system, (fault) segment, extensional (fault) jog 1213 MTL MTL 300 km MTL 1992 MTL W. G. C. E. P., 1995 Schwartz and Coppersmith (1984) characteristic earthquake MTL

2 MTL T km T2 T1 T m 1975 = 40m Ba Bk 11 7 Be T1, T MTL NE-SW 1

3 MTL NNE-SSW MTL 1992 R/S WNW-ESE NE-SW MTL MTL 1997 MTL MTL MTL MTL Ohno et al., 1998 MTL MTL 1990, MTL 23km 35km 8 13km MTL N60 E 9km km N60 E 42km N55 E 23km

4 ENE-WSW NE-SW MTL 1992 WNW-ESE no. 40, MTL 13km /25,000 MTL , no. 68, Ohno, Y., Kobayashi, S. and Hasegawa, S. (1998) Determination of design basis earthquakes from active faults. Proceeding of 8th IAEG Congress, Balkema, II, no. 1, 215 Schwartz, D. A. and Coopersmith, K. J. (1984) Fault behavior and chracteristic earthquakes: examples Emre, O.Duman, from the Wasatch and San Andreas fault zones. Jour. T.Y.Dogan, A Geoph. Res., 89, B7, Izmit 1984 no. 1, no. 28, no. 72, no. 35, p a 1: 25000, 100, b 1: : , no. 31, I 5 1 no. 1, , 65p Working Group on California Earthquake Probabilities (1995) Seismic hazards in southern California: probable earthquakes, 1994 to Bull. Seism. no. 17, Soc. Am., 85, , 1-35.

5 伊予灘 ~ 佐賀関沖 MTL 活断層系の広域イメージングとセグメント区分第 1 図. 四国 ~ 伊予灘における中央構造線活断層系の分布概要 ( 上 )と既存資料による伊予灘 ~ 別府湾域の活断層の分布 ( 下 ). 今回の調査海域と探査測線の位置を下図に示す. Fig. 1. Index map for the MTL active fault system in Shikoku and Iyo-nada Sea (top), and distribution of active faults in the Iyo-nada Sea ~ Beppu Bay area after the published research data (bottom). The area of our 2001 survey and location of acoustic profiling lines are shown in the bottom figure.

6 七山太池田倫治大塚一広三浦健一郎金山清一小林修二長谷川正杉山雄一佃栄吉第 2 図. 豊予海峡周辺海域におけるジオパルス探査の測線配置と探査によって得られた断層撓曲の分布. Fig. 2. Survey lines for acoustic profiling with geopulse and obtained distribution of faults and flexures in and around the Hoyo Strait.

7 伊予灘 ~ 佐賀関沖 MTL 活断層系の広域イメージングとセグメント区分第 3 図.ソノプローブとジオパルスによる音波探査記録の比較. 測線位置は第 2 図参照. Fig. 3. Comparison of acoustic profiles obtained by sonoprobe and geopulse. See Fig. 2 for profiling lines.

8 七山太池田倫治大塚一広三浦健一郎金山清一小林修二長谷川正杉山雄一佃栄吉第 4 図. 音響ユニット区分の例. 測線位置は第 2 図参照. Fig. 4. Examples of acoustic unit classification. See Fig. 2 for profiling lines.

9 伊予灘 ~ 佐賀関沖 MTL 活断層系の広域イメージングとセグメント区分第 5 図. 佐田岬半島沖に分布する活断層の探査記録例 (1). 測線位置は第 2 図参照. Fig. 5. Acoustic profiles of active faults off the Satamisaki Peninsula (1). See Fig. 2 for profiling lines.

10 七山太池田倫治大塚一広三浦健一郎金山清一小林修二長谷川正杉山雄一佃栄吉第 6 図. 佐田岬半島沖に分布する活断層の探査記録例 (2). 測線位置は第 2 図参照. Fig. 6. Acoustic profiles of active faults off the Satamisaki Peninsula (2). See Fig. 2 for profiling lines.

11 伊予灘 ~ 佐賀関沖 MTL 活断層系の広域イメージングとセグメント区分第 7 図. 佐賀関沖の別府湾活断層系の探査記録例と断層両側の各反射面の標高差. 測線位置は第 2 図参照. Fig. 7. Acoustic profiles of the Beppu-wan active fault system off Saganoseki, and altitude difference of each reflector between the both sides of the fault. See Fig. 2 for profiling lines.

12 七山太池田倫治大塚一広三浦健一郎金山清一小林修二長谷川正杉山雄一佃栄吉第 8 図. 伊予灘 ~ 別府湾域における活断層の分布とセグメント区分. 海底地形データは株式会社四国総合研究所による. 本図の作成には, 今回の探査結果に加えて, 四国電力株式会社 (1984), 国土地理院 (1992a, 1992b, 2001), 松岡岡村 (2000), 島崎ほか (2000) 等のデータを用いた. 陸域の米湊断層および伊予断層は岡田ほか (1998), 本郡断層は長谷川ほか (1999), 佐賀関断層は吉岡ほか (1997) による. Fig. 8. Active fault distribution and segmentation in the Iyo-nada Sea - Beppu Bay area.

Fig. 1. Active faults in the Kanto district (after Coordinating Committee for Earthquake Prediction, 1980). A-A' PROFILE DOUGUER ANOMALY RESIDUAL ANOM

Fig. 1. Active faults in the Kanto district (after Coordinating Committee for Earthquake Prediction, 1980). A-A' PROFILE DOUGUER ANOMALY RESIDUAL ANOM Structure of the Basement and the Gravity Anomaly in the Kanto Plain -A Geophysical Study of Active Fault- Takashi TADA Crustal Dynamics Department, Geographical Survey Institute (Received Apirl 7, 1983)