Abstract
Abstract. The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is designed to investigate fault mechanics and seismogenesis along a subduction megathrust, with objectives that include characterizing fault slip, strain accumulation, fault and wall rock composition, fault architecture, and state variables throughout an active plate boundary system. Integrated Ocean Drilling Program (IODP) Expedition 338 was planned to extend and case riser Hole C0002F from 856 to 3600 meters below the seafloor (m b.s.f.). Riser operations extended the hole to 2005.5 m b.s.f., collecting logging-while-drilling (LWD) and measurement-while-drilling, mud gas, and cuttings data. Results reveal two lithologic units within the inner wedge of the accretionary prism that are separated by a prominent fault zone at ~ 1640 m b.s.f. Due to damage to the riser during unfavorable winds and strong currents, riser operations were suspended, and Hole C0002F left for re-entry during future riser drilling operations. Contingency riserless operations included coring at the forearc basin site (C0002) and at two slope basin sites (C0021 and C0022), and LWD at one input site (C0012) and at three slope basin sites (C0018, C0021 and C0022). Cores and logs from these sites comprehensively characterize the alteration stage of the oceanic basement input to the subduction zone, the early stage of Kumano Basin evolution, gas hydrates in the forearc basin, and recent activity of the shallow megasplay fault zone system and associated submarine landslides.
Highlights
Subduction zones generate Earth’s most destructive earthquakes, but much of what we thought we knew about great earthquakes, and the tsunamis they generate, was turned upside down by the 2004 Sumatra and 2011 Tohoku events
The Nankai Trough is formed by subduction of the Philippine Sea Plate to the northwest beneath the Eurasian Plate at a rate of ∼ 4.1–6.5 cm yr−1 (Fig. 1) (Seno et al, 1993; Miyazaki and Heki, 2001), and Shikoku Basin oceanic plate sediment is actively accreting at the deformation front
This fault system includes the region where a cluster of very low frequency (VLF) seismic events occurred in 2004–2005 (Ito and Obara, 2006) and the first tectonic tremor recorded in an accretionary prism setting was found (Obana and Kodaira, 2009)
Summary
Subduction zones generate Earth’s most destructive earthquakes, but much of what we thought we knew about great earthquakes, and the tsunamis they generate, was turned upside down by the 2004 Sumatra and 2011 Tohoku events. During stages 1 and 2 of NanTroSEIZE (IODP Expeditions 314, 315, 316, 319, 322, 332, and 333), eight riserless drilling sites and one riser drilling site targeted the incoming Philippine Sea Plate, the frontal thrust region, the midslope megasplay fault region, and the Kumano forearc basin Site C0002 is planned to access the plate interface fault system where it is believed to be capable of seismogenic locking and slip, and to have slipped coseismically in the Tonankai earthquake (Ichinose et al, 2003) This fault system includes the region where a cluster of very low frequency (VLF) seismic events occurred in 2004–2005 (Ito and Obara, 2006) and the first tectonic tremor recorded in an accretionary prism setting was found (Obana and Kodaira, 2009). We characterize the petrophysical properties and lithological/structural associations determined from our log, cuttings and core data
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