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Scientific objectives

Expedition 319 was noteworthy because it marked the first riser drilling in IODP history, as well as the first observatory installation for the Chikyu. Riser drilling provided the opportunity for several measurements new to IODP, including in situ stress magnitude and pore pressure using the Modular Formation Dynamics Tester (MDT) wireline tool, real-time mud gas analysis, and analysis of drill cuttings. In addition, the operations plan included a leak-off test (LOT) at a casing set point for engineering purposes, which provided an additional measurement of minimum stress magnitude. Our operational and scientific plan also included a long-offset (30 km) "walkaway" vertical seismic profile (VSP) at the riser Site C0009 and the first installation of long-term borehole monitoring instruments by the Chikyu using a simple instrument package (smart plug) connected to a retrievable casing packer at the riserless Site C0010 (Araki et al., 2009).

Our objectives at Site C0009 were to drill, sample, log, and case the rocks and sediments above the locked portion of the coseismically active plate boundary thrust (Fig. F4A; see also Figs. F1, F2, F3). These operations also provide preparation for later observatory placement, which will monitor deformation, seismicity, pore pressure, and temperature. The scientific objectives at Site C0009 included:

  1. Documenting the lithology, structural geology, physical properties, and fluid and rock composition of the upper ~1600 m of basin fill and possibly accreted sediments in the hanging wall of the plate boundary fault;

  2. Collecting core at or near the depth of potential observatory installation, primarily to obtain samples for shore-based geotechnical and mechanical analyses;

  3. Conducting downhole tests to measure in situ pore pressure and stress state; and

  4. Conducting a two-ship VSP experiment to constrain the seismic velocity structure of the deep interior of the accretionary wedge and image the plate boundary below the drill site.

At Site C0010, our objective was to drill with measurement while drilling (MWD) and a basic suite of LWD tools (geoVISION resistivity tool [GVR] and gamma ray) to penetrate the megasplay fault at ~410 meters below seafloor (mbsf) and into the footwall to a total depth (TD) of 560 mbsf (Fig. F4B). The major scientific objectives at Site C0010 included:

  1. Characterizing the lithology, structural geology, and physical properties of the hanging wall, megasplay fault zone, and footwall using logging data;

  2. Correlating between Site C0010 and previously drilled IODP Site C0004 (~3.5 km along strike) to characterize local variations in fault zone architecture and physical properties;

  3. Conducting a "dummy run" of a seismometer and strainmeter package designed for placement in a future permanent observatory at this site; and

  4. Installing a temporary monitoring package to monitor pore pressure and temperature within the fault zone accessed through a screened casing.