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doi:10.2204/iodp.proc.343343T.204.2016

Introduction

Coseismic thrust displacement during the 2011 Tohoku-Oki Mw 9.0 earthquake reached ~40–80 m near the trench, the largest slip ever recorded for an earthquake (Fujiwara et al., 2011; Fujii et al., 2011; Ide et al., 2011; Lay et al., 2011; Pollitz et al., 2011; Yue and Lay, 2011). Integrated Ocean Drilling Program (IODP) Expedition 343/343T (the Japan Trench Fast Drilling Project [JFAST] project) sailed 1 y after the earthquake to investigate the conditions and mechanisms that facilitated large, shallow slip. The primary goals of the JFAST project included identifying and sampling the rocks that were deformed during the earthquake to establish the processes active on the fault during slip and to establish the stress state on the fault before, during, and after the earthquake (Mori et al., 2012). Structural data are critical for locating the plate boundary décollement, where both long-term and coseismic displacements are typically localized. The attitudes of fractures and faults around the fault also provide information about the stress field when they formed (see the “Expedition 343/343T summary” chapter [Expedition 343/343T Scientists, 2013a]).

Drilling was undertaken at Site C0019, ~7 km landward of the trench in the region of maximum slip during the earthquake (Fig. F1) (see the “Expedition 343/343T summary” chapter [Expedition 343/343T Scientists, 2013a]). Cores were recovered at targeted intervals from Hole C0019E, and onboard visual core description followed standard IODP procedures (see the “Methods” chapter [Expedition 343/343T Scientists, 2013b]). Tectonic structures including Mode I fractures, phyllosilicate bands, shear fractures, secondary faults, and breccia zones were logged, and care was taken to differentiate drilling-induced deformation in the core from tectonic features.

Distinguishing tectonic structures from those induced by drilling and recovery processes is a key goal of visual core description but is difficult because tectonic and human-induced deformation may appear alike. Drilling mud may also resemble natural features and sediments. Moreover, structures in weak sedimentary rock, like those recovered during the JFAST expedition, can show very little mineralization, therefore increasing the difficulty of distinguishing natural from induced damage. Of the published guidelines for identifying induced damage in core (e.g., Pendexter and Rohn, 1954; Kidd, 1978; Kulander et al., 1979, 1990; Dengo, 1982; Leggett, 1982; Lundberg and Moore, 1986), few address induced damage in mudstones.

Results from Expedition 343/343T onboard visual core description provided insufficient data for robust estimates of the spatial distribution or orientation of structures throughout the frontal prism and in particular surrounding the plate boundary fault (see the “Expedition 343/343T summary” chapter [Expedition 343/343T Scientists, 2013a]; Kirkpatrick et al., 2015). Here, we reexamine the cores from Hole C0019E with the aims of (1) expanding on previous criteria for distinguishing induced and tectonic deformation to (2) evaluate the spatial variability of tectonic structure density and (3) establish whether structure density can be used to locate fault horizons as observed in IODP cores elsewhere (e.g., Brown and Behrmann, 1990; Maltman et al., 1993; Ujiie and Kimura, 2014).

Cores from Hole C0019E used for analysis

Twenty-one cores were recovered from Hole C0019E from the following depth intervals: 176.5–185.2, 648–660, ~690–725, and ~770–835 meters below seafloor (mbsf), with an overall recovery rate of 43%. The first 16 coring runs recovered rocks composed of Pliocene and Pleistocene siliceous mudstones of the hanging wall frontal prism (see the “Expedition 343/343T summary” chapter [Expedition 343/343T Scientists, 2013a]; Rabinowitz et al., 2015). Section 17R-1 is composed of highly sheared scaly clay interpreted to be the plate boundary décollement (821.5–822.5 mbsf) (see the “Expedition 343/343T summary” chapter [Expedition 343/343T Scientists, 2013a]; Rabinowitz et al., 2015). Below the décollement, rocks include middle to late Miocene brown siliceous mudstone, early Miocene stratified pelagic clay, and Late Cretaceous laminar chert at the total drilling depth (see the “Expedition 343/343T summary” chapter [Expedition 343/343T Scientists, 2013a]; Rabinowitz et al., 2015). Degree of lithification increases moderately with depth, indicated by increasing P-wave velocity (meters per second) and electrical resistivity (ohm-meters) (see the “Expedition 343/343T summary” chapter [Expedition 343/343T Scientists, 2013a]; Nakamura et al., 2014). Unconfined compressive strength of recovered rocks also increases with depth, ranging from 1 to 10 MPa (see the “Expedition 343/343T summary” chapter [Expedition 343/343T Scientists, 2013a]). All examined cores contained intact rock; the most poorly lithified units were pelagic clay, whereas the most well lithified units were chert.

The data reported here focus on deformation hypothesized to define the damage zone surrounding the plate boundary décollement, rather than all 21 cores collected from Hole C0019E. A damage zone is generally defined as the network of subsidiary structures—faults, fractures, and veins—that bound a fault core and are related to the development of the fault (e.g., Chester and Logan, 1986; Caine et al., 1996). Our study focuses on subsidiary structures in Cores 10R–20R, which surround the décollement and were recovered from 770 to 833.5 mbsf. We examined a total of 6.73 m of intact core below the décollement and a total of 9.38 m of intact core above the décollement. The total depth interval examined in this study is 63.5 m.

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