Expedition 380 summary

The multiexpedition Integrated Ocean Drilling Program/International Ocean Discovery Program (IODP) Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) project was designed to investigate fault mechanics and seismogenesis along subduction megathrusts through direct sampling, in situ measurements, and long-term monitoring in conjunction with allied laboratory and numerical modeling studies. Overall NanTroSEIZE scientific objectives include characterizing the nature of fault slip and strain accumulation, fault and wall rock composition, fault architecture, and state variables throughout the active plate boundary system. Expedition 380 was the twelfth NanTroSEIZE expedition since 2007. Refer to Kopf et al. (2017) for a comprehensive summary of objectives, operations, and results during the first 11 expeditions. Expedition 380 focused on one primary objective: riserless deployment of a long-term borehole monitoring system (LTBMS) in Hole C0006G in the overriding plate at the toe of the Nankai accretionary prism. The LTBMS installed in Hole C0006G incorporates multilevel pore-pressure sensing and a volumetric strainmeter, tiltmeter, geophone, broadband seismometer, accelerometer, and thermistor string. Similar previous LTBMS installations were completed farther upslope at IODP Sites C0002 and C0010. The ~35 km trench– normal transect of three LTBMS installations will provide monitoring within and above regions of contrasting behavior in the megasplay fault and the plate boundary as a whole, including a site above the updip edge of the locked zone (Site C0002), a shallow site in the megasplay fault zone and its footwall (Site C0010), and a site at the tip of the accretionary prism (the Expedition 380 installation at Site C0006). In combination, this suite of observatories has the potential to capture stress and deformation spanning a wide range of timescales (e.g., seismic and microseismic activity, slow slip, and interseismic strain accumulation) across the transect from neartrench to the seismogenic zone. Expedition 380 achieved its primary scientific and operational goal with successful installation of the LTBMS to a total depth of 457 m below seafloor in Hole C0006G. The installation was conducted in considerably less time than budgeted, partly because the Kuroshio Current had shifted away from the NanTroSEIZE area after 10 y of seriously affecting D/V Chikyu NanTroSEIZE operations. After Expedition 380, the LTBMS was successfully connected to the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) in March 2018 using the remotely operated vehicle Hyper-Dolphin from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) R/V Shinsei Maru. Introduction and geological setting Subduction zones account for ~90% of the global seismic moment release and can generate damaging earthquakes and tsunamis with potentially disastrous effects on heavily populated coastal areas (e.g., Lay et al., 2005; Moreno et al., 2010; Simons et al., 2011). Understanding the processes that govern the strength, nature, and distribution of slip along these plate boundary fault systems is crucial in evaluating earthquake and tsunami hazards. Characterizing fault slip behavior and mechanical state at convergent plate boundaries through direct sampling, near-field geophysical observations, measurement of in situ conditions, and shore-based laboratory experiments is a fundamental and societally relevant goal of current earth science and the overarching goal of the Integrated Ocean Drilling Program/International Ocean Discovery Program (IODP) Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) project. Expedition 380 was the twelfth Integrated Ocean Drilling Program/IODP expedition since 2007 to address the NanTroSEIZE project’s objectives. The fundamental scientific objectives of NanTroSEIZE include characterizing the nature of fault slip and strain accumulation, fault and wall rock composition, fault architecture, and state variables throughout the active plate boundary system K. Becker et al. Expedition 380 summary through direct sampling, in situ measurements, downhole experiments such as walk-away vertical seismic profiles, and long-term monitoring in conjunction with allied laboratory and numerical modeling studies (Tobin and Kinoshita, 2006). See Kopf et al. (2017) for the most recent comprehensive summary of the rationale and history of the NanTroSEIZE project and regional survey data. To date, NanTroSEIZE has involved coring, logging, and installation of long-term borehole observatories at 14 sites in a trench–normal transect across the Nankai Trough subduction zone offshore the Kii Peninsula of southwest Honshu Island. This transect runs from the Kumano Basin in the fore-arc region ~60 km landward of the trench axis to the Kashinosaki Knoll on the downgoing plate ~40 km seaward (Figures F1, F2). The Nankai Trough is formed by subduction of the Philippine Sea plate to the northwest beneath the Eurasian plate at a rate of ~65 mm/y (Miyazaki and Heki, 2001; DeMets et al., 2010). The convergence direction is slightly oblique to the trench, and trench wedge turbidites and underlying Shikoku Basin hemipelagic sediments accrete at the deformation front. The Nankai Trough region has 1300 y of historical and archeological records of recurring great earthquakes that are typically tsunamigenic, including the 1944 Tonankai Mw 8.2 and 1946 Nankaido Mw 8.3 earthquakes (Ando, 1975; Hori et al., 2004). The rupture area and zone of tsunami generation for the 1944 event (in which the Expedition 380 site is located) are now reasonably well understood (Tanioka and Satake, 2001; Ichinose et al., 2003; Kikuchi et al., 2003; Baba and Cummins, 2005; Baba et al., 2005, 2006). Land-based offshore geodetic studies suggest that the plate boundary thrust is currently strongly locked (Miyazaki and Heki, 2001; Yokota et al., 2016). Similarly, lower microseismicity levels near the updip edge of the rupture area of the 1940s earthquakes (Obana et al., 2001) imply significant interseismic strain accumulation on the megathrust. Recent geological studies at the Nankai margin (Sakaguchi et al., 2011; Yamaguchi et al., 2011), direct observational evidence from the 2011 Tohoku earthquake (e.g., Fujiwara et al., 2011; Ito et al., 2013), and new results from both a seafloor cabled network and Figure F1. Site C0006 location (solid red circle). Previous LTBMS Sites C0002 and C0010 are shown. Inset shows the region in relation to Japan. Black arrow = motion of Kii Peninsula determined from GPS measurements (Heki, 2007). Yellow arrows = computed far-field convergence vectors between the Philippine Sea plate and Japan (Seno, 1993; Miyazaki and Heki, 2001). 136°20'E 136°30' 136°40' 136°50' 137°00' 32°50' 33°00' 33°10' 33°20' 33°30' N