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Site C00221

M. Strasser, B. Dugan, K. Kanagawa, G.F. Moore, S. Toczko, L. Maeda, Y. Kido, K.T. Moe, Y. Sanada, L. Esteban, O. Fabbri, J. Geersen, S. Hammerschmidt, H. Hayashi, K. Heirman, A. Hüpers, M.J. Jurado Rodriguez, K. Kameo, T. Kanamatsu, H. Kitajima, H. Masuda, K. Milliken, R. Mishra, I. Motoyama, K. Olcott, K. Oohashi, K.T. Pickering, S.G. Ramirez, H. Rashid, D. Sawyer, A. Schleicher, Y. Shan, R. Skarbek, I. Song, T. Takeshita, T. Toki, J. Tudge, S. Webb, D.J. Wilson, H.-Y. Wu, and A. Yamaguchi2

Background and objectives

During Integrated Ocean Drilling Program (IODP) Expedition 338, logging while drilling (LWD)/measurement while drilling (MWD) and coring were conducted at IODP Site C0022, which is located in the trench slope basin previously drilled at IODP Sites C0004 and C0008 (Fig. F1).

During IODP Expeditions 314 and 316, LWD and coring, respectively, were conducted at Site C0004 (Expedition 314 Scientists, 2009b; Expedition 316 Scientists, 2009a). Site C0004 targeted the uppermost 400 meters below seafloor (mbsf) near the tip of the megasplay fault zone where the seaward-most branch of this fault system approaches the surface (Figs. F1, F2) (Moore et al., 2007, 2009). This megasplay fault is thought to coincide with the rupture area of the 1944 Tonankai earthquake, and its slip was likely responsible for the devastating tsunami at the time of that earthquake (Park et al., 2002; Moore et al., 2007).

Logging data obtained during Expedition 314 suggested three logging units corresponding to the slope sediment, accretionary prism, and underthrust sediment, the latter two of which are bounded by the megasplay branch fault (Expedition 314 Scientists, 2009b). The sediment cored during Expedition 316 is divided into four lithostratigraphic units: Pleistocene upper slope apron (Unit I, 0–78.06 mbsf), Pliocene accretionary complex (Unit II, 78.06–258.01 mbsf), Pliocene fault-bounded unit (Unit III, 258.01–307.52 mbsf), and Pleistocene underthrust slope basin (Unit IV, 307.52–398.79 mbsf) (Expedition 316 Scientists, 2009a). Two biostratigraphic age reversals were found within Unit III, which represents the megasplay branch fault zone. Unit III is characterized by a fractured and brecciated zone, and the fault zone within Unit III was sampled at 271 mbsf, where it consists of microbreccia and a narrow dark layer ~1 cm thick that dips at ~50° to the core vertical axis (Expedition 316 Scientists, 2009a). Analyses of this fault sample revealed evidence for frictional heat, including an increase in vitrinite reflectance, enrichment in Al and K and depletion in Ca and Sr, and an increase in illite crystallinity, implying possible propagation of seismic slips to this shallow portion of the megasplay branch fault (Yamaguchi et al., 2011; Sakaguchi et al., 2011a). In addition, the top 1 m of core contains repeated occurrences of earthquake-induced mud breccia, the uppermost of which was found by 210Pb dating to have been deposited at the time of the 1944 Tonankai earthquake (Sakaguchi et al., 2011b). However, the seismic profile and reconstruction of splay fault activity through time indicate that the surface layers younger than ~1.24 Ma are not displaced by this megasplay branch fault (Fig. F2) (Expedition 316 Scientists, 2009a; Strasser et al., 2009), which implies that this fault has been inactive recently. Alternatively, Kimura et al. (2011) suggested ongoing splay fault activity may not only be inferred by stratal ages and architecture across the fault itself but also by broader and distributed deformation, manifest by deformation of the lower slope basin and erosion and redeposition of slope sediment by successive oversteepening and mass transport and accumulation of mass transport deposits overlying the fault.

Site C0008, located ~1 km seaward of Site C0004, was also drilled and cored during Expedition 316 (Expedition 316 Scientists, 2009b). Drilling objectives were to help assess the timing and relative age of past fault motions via identification of provenance and age of basin material. Seismic horizons identified in the basin sediment at Site C0004 continue laterally across the basin to Site C0008, and the lithologies are similar (Expedition 316 Scientists, 2009b).

As a contingency operation during Expedition 338, LWD and coring to 420 mbsf were conducted at Site C0022 (proposed Site NT2-13A), which is located ~350 m southeast of Site C0004 (Fig. F2). Objectives of LWD and coring at Site C0022 include collection of logging data and samples across the shallow end (~100 mbsf) of the megasplay branch fault in order to characterize fault materials and properties, search for signs of seismic rupture propagation, and constrain activity of this fault. Thus, LWD and coring at Site C0022 provide additional information on the activity of the megasplay branch fault and its bearing on the 1944 Tonankai earthquake and tsunami.

1 Strasser, M., Dugan, B., Kanagawa, K., Moore, G.F., Toczko, S., Maeda, L., Kido, Y., Moe, K.T., Sanada, Y., Esteban, L., Fabbri, O., Geersen, J., Hammerschmidt, S., Hayashi, H., Heirman, K., Hüpers, A., Jurado Rodriguez, M.J., Kameo, K., Kanamatsu, T., Kitajima, H., Masuda, H., Milliken, K., Mishra, R., Motoyama, I., Olcott, K., Oohashi, K., Pickering, K.T., Ramirez, S.G., Rashid, H., Sawyer, D., Schleicher, A., Shan, Y., Skarbek, R., Song, I., Takeshita, T., Toki, T., Tudge, J., Webb, S., Wilson, D.J., Wu, H.-Y., and Yamaguchi, A., 2014. Site C0022. In Strasser, M., Dugan, B., Kanagawa, K., Moore, G.F., Toczko, S., Maeda, L., and the Expedition 338 Scientists, Proc. IODP, 338: Yokohama (Integrated Ocean Drilling Program).

2Expedition 338 Scientists’ addresses.

Publication: 13 January 2014
MS 338-107