D. Saffer , A. Kopf , S. Toczko , E. Araki , S. Carr , T. Kimura , C. Kinoshita , R. Kobayashi , Y. Machida , A. Rösner , L.M. Wallace , S. Chiyonobu , K. Kanagawa , T. Kanamatsu , G. Kimura , M.B. Underwood
05 August 2017
Site C0010 was originally drilled with logging while drilling (LWD) and measurement while drilling (MWD) tools and cased during Integrated Ocean Drilling Program Expedition 319. Operations included drilling across the megasplay fault to a total depth (TD) of 555 meters below seafloor (mbsf), casing the borehole (with casing screens spanning the fault), conducting an observatory “dummy run” to test strainmeter and seismometer deployment procedures, and installing a simple pore pressure and temperature monitoring system (SmartPlug) attached to a retrievable bridge plug (see Figures F1 and F3 in the Expedition 365 summary chapter. The hole was revisited during Integrated Ocean Drilling Program Expedition 332, when the SmartPlug was recovered and replaced with a similar temporary instrument package that also included geochemical and microbiological sampling coils and an in situ microbiological colonization experiment; this temporary instrument package is termed the “GeniusPlug” (see Figures F4 and F5 in the Expedition 365 summary chapter. Drilling during Expedition 319 identified three distinct lithologic packages at Site C0010, defined on the basis of LWD data and further guided by comparison with coring and LWD results at nearby Integrated Ocean Drilling Program Site C0004 (located 3.5 km to the northeast along strike). From top to bottom, these lithologic packages are hemipelagic slope deposits composed primarily of mud with minor distal turbidite interbeds (Unit I, 0–182.5 m LWD depth below seafloor [LSF]), a thrust wedge composed of overconsolidated and fractured clay- and mudstones (Unit II, 182.5–407 m LSF), and overridden slope deposits (Unit III, 407 m LSF to TD). At Site C0010, Unit I is divided into two subunits: Subunit IA (0–161.5 m LSF), characterized by gamma ray and resistivity patterns similar to those observed in Unit I at Site C0004 (Expedition 314 Scientists, 2009), and Subunit IB (161.5–182.8 m LSF), interpreted as slope sediments composed of material reworked from the underlying thrust wedge. For International Ocean Discovery Program (IODP) Expedition 365, the observatory system at Site C0010 consisted of an array of sensors (see LTBMS in the Expedition 365 methods chapter [Saffer et al., 2017]) designed to monitor crustal deformation and hydrologic processes in the offshore portion of the subduction system over a wide range of timescales, including seismicity and microseismicity, slow slip events and very low frequency (VLF) earthquakes, hydrologic transients, ambient pore pressure, and temperature. To ensure the long-term and continuous monitoring necessary to capture such events, the borehole observatory was designed for connection to the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) submarine cabled network (http://www.jamstec.go.jp/donet/e) following the drilling expedition; this connection was completed on 19 June 2016. For the long-term borehole monitoring systems (LTBMS) installed at Integrated Ocean Drilling Program Sites C0002 and C0010, both connected to DONET, the formation pressure data can be viewed and downloaded at an open-access observatory data portal (http://offshore.geosc.psu.edu).