The Nankai Trough (南海トラフ, Nankai Torafu; Southern Sea Trough) is a submarine trough located south of the Nankaidō region of Japan's island of Honshu, extending approximately 900 km (559 mi) offshore. The underlying fault, the Nankai megathrust, is the source of the devastating Nankai megathrust earthquakes, while the trough itself is potentially a major source of hydrocarbon fuel, in the form of methane clathrate. In plate tectonics, the Nankai Trough marks a subduction zone that is caused by subduction of the Philippine Sea plate beneath Japan, part of the Eurasian plate (Kanda et al., 2004). This plate boundary would be an oceanic trench except for a high flux of sediments that fills the trench. Within the Nankai Trough there is a large amount of deformed trench sediments (Ike, 2004), making one of Earth's best examples of accretionary wedge. Furthermore, seismic reflection studies have revealed the presence of basement highs that are interpreted as seamounts that are covered in sediments (Ike, 2004). The northern part of the trough is known as the Suruga Trough, while to the east is the Sagami Trough. The Nankai Trough runs roughly parallel to the Japan Median Tectonic Line.
Rates of tectonic motion
Conventional geologic estimates of plate movement velocities are difficult in the Nankai Trough because there are no spreading ridges that bound the tectonic plate. This area was not in the original NUVEL models (DeMets et al., 1990). However, a more recent study that included the Philippine Sea plate was based on data from the NUVEL-1A model (Zang et al., 2002). This study estimates that subduction in the Nankai Trough is about 43 mm/yr. REVEL-based calculations indicate that there is no accumulation of strain at the trench. The rates of movement have been calculated to be in a range of 3.0 ± 1.8 mm/yr to 11.1 ± 1.7 mm/yr (Sella et al., 2002). As mentioned previously, the NUVEL-1A plate motion model does not include the Philippine Sea plate. This is because the mathematics of this model only used twelve plates, and the Philippine Sea and Eurasian convergent margin were not included. However, using the Eurasia to North America plate motion, the estimated rate was 2–4 mm/yr (DeMets et al., 1990). This is not in agreement with the REVEL model, seemingly indicating that the NUVEL-1A model may need further revision.
Sedimentology The deposits in the trough are primarily trench-wedge turbidites (Spinelli et al., 2007). There are indications of an increase in the retention of porosity within the rock. Typically porosity decreases with increasing depth. However, there is an anomalous preservation of porosity at depth at drill site 1173. This has been attributed to post-depositional opal cementation that is preserving the porosity (Spinelli et al., 2007). The detrital clays, primarily smectite, display variation over time and location in the Nankai Trough as well as the Shikoku basin. At depth there is an increase in the smectite clay content in the sediments, inferring that there has been a change in the deposition source rock (Steurer et al., 2003). Furthermore, there is a geothermal alteration of the smectite, converting it to illite clay (Steurer et al., 2003).
Tectonic structure The Nankai Trough is actively deforming and marks a region of seismic activity. Deformation is concentrated in the outermost imbricate zone, with a significant amount of "out of sequence" thrusting occurring landward. Based on the work of Operto et al., 2006, several areas of intense tectonic activity in the Nankai Trough were identified using full waveform tomography. The upper portion of the upper accretionary prism and the underlying backstop are currently undergoing a great deal of compressional pressure. Several thrust faults were identified by Operto et al., 2006, of which the thrust faults closest to the subduction zone are active. Furthermore, Pisani et al., 2006, identified protothrusts and decollement surfaces along the Nankai Trough. Recently there has been an increase in interest in the release of water from illite clays in subducting sediments. The conversion of smectite to illite (illitization) in subduction zones is likely driven by the higher temperature found in the subduction zone as opposed to non-subducting sediments (Saffer et al., 2005). IODP Expedition 370 will seek to find the temperature limit of the deepest life on Earth by drilling in the Nankai Trough, where heat flow is particularly high near its boundary with the subducting young, hot Philippine Sea tectonic plate. At the targeted site, the geothermal gradient is about four times steeper than elsewhere in the Pacific Ocean. Reaching temperatures of approximately 130 °C in other areas would require collecting cores from approximately 4 kilometers below the seafloor, rather than 1.2 kilometers as planned by Expedition 370. Eventually, IODP Expedition 370 reached a temperature of ~120 °C at 1.2 kilometers below the seafloor with mineral evidence showing that there are localized depths with significantly higher temperatures due to hot fluids.
Seismicity
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