Km Km - Yuasa, Sakamoto et al,.. m WGS YK- JAM- STEC, YK- JAMSTEC, YK- JAMSTEC, YK- YK- Jarvis Jarvis YK- - Km. Km Km m m m m m /m Yuasa et al. NW Km

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1 Topographical and geological characteristics of Sofugan Tectonic Line (STL) area, Izu-Ogasawara Arc. Izumi SAKAMOTO Toshiya FUJIWARA and Osamu ISHIZUKA Sofugan Tectonic Line STL locates on the middle part of Izu-Ogasawara Arc IOA, and divides it into Northernand Southern part. Four major topographical characters such as seamounts, knolls, and trough and fault are observed in this area. And three major fault tectonic direction are confirmed such as NE-SW, NW-SE, and N-S trending. The investigated area is classified into four gedogical units such as fore volcanic front zone, volcanic front zone, back-arc knolls zone, and back-arc seamounts on the basis of their geological phenomenon. These topographical and geological characters indicate each stages for developing activity of Oceanic Island Arc. Keywords : Izu-Ogasawara Arc, Sofugan Tectonic Line, Seamount, Knoll, Fault OD OD Program Office, JAMSTEC Deep Sea Research Dept., JAMSTEC Geochemical Dept., GSJ

2 Km Km - Yuasa, Sakamoto et al,.. m WGS YK- JAM- STEC, YK- JAMSTEC, YK- JAMSTEC, YK- YK- Jarvis Jarvis YK- - Km. Km Km m m m m m /m Yuasa et al. NW Km m Km m Km m m S- m KmKm m -

3 Fig. Location and indexmap of around Sofugan Tectonic Line.

4 YK- YK- Fig. Bathymetric map of around study area using the YK- to YK- cruise data. Contour interval is m.

5 m Kmm Km m Km m Km m m Km m / m Km m Km m m m m m Km m km mm m m m GOKaS Fig. Bathymetric map of around the northern part of Shichiyo seamount chain. G Getsuyo seamount, O Ohomachi seamount, Ka Kayo seamount, S Suiyo seamount. Contour interval is m.

6 S-S- S- Km K m - Km NW - Km Km Km N ENE /m m / m m m S-S- Km Kmm m m /ms- NE Fig MKiD Bathymetric map of around the southern part of Shichiyo seamount chain. M Mokuyo seamount, Ki Kinyo seamount, D Doyo seamount. Contour interval is m.

7 S-S- Km KmKm m / m NS NE S-Km Km S-S- S- S-S- Km S- Km m S-S- S- S-S- S- Km Km m Fig. Bathymetric map of around Sofu Block so-called. Contour interval is m.

8 S- Km KmKmm NE Km KmKm m m /m /m Km KmKmm / m m Mn Km KmKmm m m - Mn Km KmKm m /m Km KmKm m /m S-Km Km Km KmKm KmKm m /m /m S- -, - - Km m -South -North S- Km KmS- S-m S-

9 Fig. Bathymetric map of around the NW-SE trending small knolls and Sofu depression. Contour interval is m. KmKm Km m m Fig. Rose diagram for long axis direction of small knolls where locates around the Sofugan Tectonic Line m Km Km Km m S- m/m m /m

10 Fig. Distribution map of depression and fault around the survey area m m NW m Km Km Km m Km Kmm m S- S- m m NW S-

11 S-S- KmKm m m S-S- Km Km S- S- m S- Km Km m S Ma SiO.% Yuasa et al., Honza and Tamaki, Yuasa and Nohara, Fig. Geological columnar section from around Sofugan Tectonic Line after Sakamoto et al.,

12 Yuasa and Nohara, m m S- CNb-b. -. Ma CNb. Ma HFSNb. Ma K-Ar in-situ S- + + S-S- - Ma S- Mn Mn S- Mn S- NW S- CNb.-.Ma S- Ma Ar-Ar CNb.Ma S- m HFS Nb Ma Ar-Ar S- m S-S- Km m -

13 Fig. Geological tectonic map around Sofugan Tectonic Line - - S- S- - -

14 Ma Ma Tsunakawa, Umino, Honza and Tamaki, S-S- Ma.. Ma Ma Ma Mn Ma + + Yuasa et al. - - S-S- -. Ma S-. Ma- -, -. Honza E., and Tamaki K.: The Bonin Arc. in The Ocean Basin and Margins. eds. by Martin et al. : A, -.., -. Ishizuka, O., Uto, K., Yuasa,M., and Hochestaedter,A.G. : K-Arages from seamount chains in the back-arc region of the Izu-Ogasawara arc. The Island Arc,, -. No., -. JAMSTEC: Onboard report of YK- Cruise p. JAMSTEC: Onboard report of YK- Cruise, p. JAMSTEC: Onboard report of YK- Cruise, p. YK-,. YK-,. YK-, Hochestaedter, S. No

15 -. JAMSTEC J.Deep Sea Res.,, P.A.Jarvis YK- JAMSTEC -. Sakamoto I., Fujiwara T., Ishiduka O., and Yuasa M.: Geology and Tectonics of the Sofugan Tectonic Line, Central part of Izu-Ogasawara Arc. Western Pacific Geophysical Meeting, Tokyo, WP. p-. vol. -. Taylor, B.: Rifting and the volcanic-tectonic evolution of the Izu-Bonin-Mariana arc. in Proc. ODP, Sci.Results, B. Taylor and K. Fujioka, eds., College Station, -. Tsunakawa,H.: K-Ar dating of volcanic rocks in the Bonin Islands and its tectonic implication. Tectonophys.,, -. Umino, S.: Volcanicgeology of Chichijima, the Bonin Islands Ogasawara islands. J. Geol. Soc. Japan, -. Yuasa M.,Nishimura A., and Makimoto H.: Sediments, rocks and Ferromanganese oxide. Geol. Surv. Japan Cruise Report No., -. Yuasa, M.: Sofugan Tectonic Line, a new tectonic boundary separating northern and southern parts of the Ogasawara Bonin Arc, northwest Pacific. In N. Nasu et al., eds., Formation of Active Ocean Margins, Terra Pub. Tokyo, -. Vol., -., -. Yuasa, M., Murakami F., Saito, E., and Watanabe, K.: Submarine topography of seamounts on the volcanic front of the Izu-Ogasawara Bonin Arc. Bull.Geol. Survey of Japan,, -. Yuasa, M. and Nohara, M.: Petrographic and geochemical along-arc variations of volcanic rocks on the volcanic front of the Izu-Ogasawara Bonin Arc. Bull.Geol. Surv. Japan, vol., -. No., -.

７－３　2004年新潟県中越地震

７－３　2004年新潟県中越地震 04 Mid Niigata earthquake Earthquake Research Institute, University of Tokyo. 04 23 17 6 M6.8 7 18 12 M6.018 34 M6. 6 27 40 M6.1 11 8 11 1 M.9 30km M6 1 14 1) 2 RMS P 0.17 0.074 S 0.476 0.166 900 P 3 S