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Logging while drilling (LWD) data and core observations reveal abrupt structural and lithologic changes at approximately 820 mbsf, where the presence of a <5-m-thick scaly-clay layer marks the plate boundary decollement (Chester et al. Inspection of these cores yielded seven lithologic units (Figure 2), and the characteristics of each unit have now been described (Expedition 343/343T Scientists 2013c). This process resulted in the collection of 21 cores with a total length of approximately 53 m from borehole C0019E.
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Four separate depth intervals at over 137 m between 177 and 845 meters below the seafloor (mbsf) were cored. However, due to time constraints the coring operations were limited. These boreholes thus enabled geophysical logging, core sampling, and long-term temperature monitoring (Expedition 343/343T Scientists 2013a Lin et al. At this location, the three boreholes drilled over a small area (the largest distance between them is approximately 30 m) successfully penetrated the interface between the subducting Pacific Plate and the overriding North American Plate. Site C0019 is located directly above a horst block, approximately 1.2 km west of a normal-fault boundary between the horst and adjacent graben centered under the trench. The JFAST Drilling Site C0019 is located within an area of large shallow slip, approximately 93 km seaward of the 2011 Mw 9.0 Tohoku-Oki mainshock epicenter, and approximately 6 km landward of the trench axis (Figure 1). Relationships between thermal conductivity and thermal diffusivity are also found, and these allow extrapolation of the thermal properties with depth from the more extensively measured thermal conductivity values. Values of thermal conductivity measured using this technique are then compared to the values previously measured. In this study, we report both thermal conductivity and thermal diffusivity measurements performed on four whole-round core samples retrieved from the JFAST borehole, using a transient plane heat source method (also known as a hot-disk method). These techniques, however, are not suited to measuring thermal diffusivity, a key parameter for understanding the thermal decay associated with frictional heating or other transient disturbances. Details of the divided bar thermal conductivity measurements are given in Sass et al. Determination of thermal conductivity in these studies consisted of making 45 line source measurements on split half cores (Expedition 343/343T Scientists 2013b) and 38 measurements made using a divided bar technique on rock chips (Fulton et al. The previous works measuring the physical properties of rocks across the Tohoku-Oki fault zone have focused on thermal conductivity (which is a measure of a material’s ability to transfer heat and has a primary control on the background geothermal gradient) (Expedition 343/343T Scientists 2013b Fulton et al. Obtaining measurements of any two of these parameters allows the third to be calculated. The relationship between thermal diffusivity, α, and thermal conductivity, λ, can be expressed as follows: Volumetric heat capacity controls how heat is manifested in terms of temperature, and thermal diffusivity controls how a heat signal diffuses over space and time. To quantitatively interpret the observed 0.31☌ temperature anomaly along the plate boundary fault and to calculate the coseismic shear stress on the ruptured fault from this thermal signal, knowledge of the thermal conductivity, thermal diffusivity, and volumetric heat capacity is necessary. 2012), and one of the principal objectives of JFAST was to detect the residual temperature anomaly induced by the coseismic frictional heat during the earthquake (Fulton et al. This work was undertaken by the drilling vessel ( D/V) Chikyu, approximately 1 year after the earthquake at Site C0019 (Mori et al. To understand the reasons for the large displacement of the coseismic slip, the Japan Trench Fast Drilling Project (JFAST, Integrated Ocean Drilling Program (IODP) Expedition 343 and 343T) drilled through the plate boundary fault zone that ruptured during the Tohoku-Oki earthquake. The 2011 Mw 9.0 Tohoku-Oki, Japan, earthquake produced a maximum coseismic slip of >50 m near the Japan Trench in the Miyagi-Oki region, triggering a huge tsunami (Fujiwara et al.