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doi:10.2204/iodp.proc.314315316.204.2012

Introduction

Earthquakes and tsunamis are among the most unpredictable and destructive natural disasters, sometimes destroying life as well as buildings on a massive scale. Particularly destructive are those earthquakes occurring in subduction zones. Recent examples include the Sumatra earthquake and tsunami in 2004 and the earthquake and tsunami off the coast of Sendai (Tohoku) Japan in 2011.

To better understand how and why earthquakes and tsunamis occur in subduction zones, it is useful to measure the mechanical and hydrological properties of sediments and sedimentary rocks retrieved by scientific ocean drilling. Among those many properties, permeability has an important influence on sediment consolidation and strength through its affect on pore fluid pressure (Moore and Vrolijk, 1992; Saffer and Bekins, 2006). By characterizing hydrological properties both within and adjacent to fault zones at various depths, we can examine how geologic structures and permeability might influence one another over a range of effective stress values, thereby improving the understanding of fault dynamics.

In this report, we present the results of constant-flow permeability tests that were completed at the University of Missouri on whole-round core specimens retrieved from the Nankai Trough accretionary prism offshore Japan (Fig. F1). Three holes were cored at Integrated Ocean Drilling Program (IODP) Site C0001 during Expedition 315 (“Expedition 315 Site C0001” chapter [Expedition 315 Scientists, 2009]). All of the sampling at Site C0001 was limited to the hanging wall of a megasplay fault (see Moore et al., 2009). Six whole-round samples were collected for this study, with subbottom depths ranging from 25.21 to 400.33 m core depth below seafloor (CSF) (Fig. F1). The upper three samples come from lithologic Unit I (slope apron facies), whereas the lower three are from Unit II (upper accretionary prism). Bedding dips at the sampling depths are also shown on Figure F1. Unfortunately, the deepest sample could not be tested successfully because of severe fracturing.

Previous laboratory tests of natural clay-rich sediment and shale tend to show large ranges in permeability values because of differences in the material’s mineral composition and texture (Bennett et al., 1989; Neuzil, 1994; Dewhurst et al., 1999; Yang and Aplin, 2007; Gamage et al., 2011). Of particular interest to our study is the anisotropy of permeability (e.g., Clennell et al., 1999, Bolton et al., 2000) because that property is likely to change in response to tectonic loading and fault-related deformation. Typically, comparisons are made between horizontal (cross-core) permeability (k_h) and vertical (along-core) permeability (k_v) at the same sampling depth. A sediment or sedimentary rock is considered isotropic if the hydraulic conductivity (or intrinsic permeability) is the same in every direction (k_h = k_v) and anisotropic if hydraulic conductivity or permeability is unequal in different directions. In most cases, preferred alignment of platy mineral grains results in k_h > k_v with the k_h/k_v ratio for soils ranging from <1 to >10 (Mitchell, 1993).

Permeability anisotropy usually varies with the thickness of sedimentary layering (varves, laminae, etc.), depth of burial, and magnitude of applied effective stress. In most sedimentary basins, long-term burial loading and chemical diagenesis impart changes in the volume and orientation of platy clay minerals. The alignment of grains becomes almost perpendicular to the maximum principal effective stress (Sintubin, 1994; Anandarajah and Kuganenthira, 1995; Kim et al., 1999; Aplin et al., 2006), and fluids physically seek the more conductive flow path along rather than across their direction of alignment (Golin et al., 1992). In subduction systems, the orientation of maximum principal effective stress and grain fabric may change because of tectonic loading and/or shearing. The hydrological properties of sediment, moreover, depend on many inherited factors, including grain size and shape, sorting, the type of geometric arrangement, and the magnitude of the interparticle forces (Moon and Hurst, 1984; Bennett et al., 1989, 1991). Different scales of fabric also need to be taken into account (Mitchell, 1993). Microfabric refers to the aggregation of particles and very small pores, whereas minifabric contains larger pores, cracks, fissures, or laminations between aggregate assemblages, which can measure up to several tens of micrometers. Fluid flow tends to be enhanced through the larger interaggregate pores as opposed to the tiny intra-aggregate pores (Olsen, 1960; Delage and Lefebvre, 1984). When sediments are compressed at increasing effective stress levels, the collapse of structure is progressive. In general, the largest interaggregate pores are affected first, and as consolidation proceeds, smaller pores are affected. A structural anisotropy therefore develops with increasing compression and lithification (Delage and Lefebvre, 1984; Moon and Hurst, 1984).

With these complications in mind, the primary purpose of our study is to quantify the degree of permeability anisotropy at shallow levels of the Nankai Trough subduction system. A second objective is to relate depth-dependent changes, if any, to variations in the development of grain fabric within the two lithologic units (slope apron and upper accretionary prism).

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