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42-458: The area south of the “ Central Tectonic Line ” is called Nanki ( 南紀 ) , and is home to reef-like coral communities which are amongst the northernmost in the world (apart from cold-water corals ) due to the presence of the warm Kuroshio Current , though these are threatened by global warming and human interference. Because of the Kuroshio’s strong influence, the climate of Nankii is the wettest in

84-432: A 90 to 150-year cycle (Mitsui, et al., 2004; Tanioka et al., 2004), similar slip occurrences along the fault segments, the order of subdivision faulting, and finally, different failure features. Hydrologic observatories were placed in boreholes drilled in 2000 ( IODP sites 808 and 1173) in an attempt to quantify changes in pore-fluid pressure that are a result of the oncoming Philippine Sea plate (Davis et al., 2006). Site 808

126-468: 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 prism . 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

168-505: A large amount of gas hydrates and is one of the best studied sites of gas hydrate formations (Collett, 2002; Saito et al., 2007). The information concerning the gas hydrates in the Nankai Trough was initially published in 2000 by the Japan National Oil Corporations. The data in the news release came from a series of boreholes what were started in the late 1990s. In this area, the main sedimentological controls for

210-523: 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. The Nankai Trough is the near-surface extension of a zone of active seismicity that dips beneath SW Japan. The rupture zone has been subdivided into five areas with respect to seismic modelling (Mitsui et al., 2004). These five subdivisions show interesting differences in earthquake behavior: frequency of earthquakes varying on

252-736: Is Japan's longest fault system. The MTL begins near Ibaraki Prefecture , where it connects with the Itoigawa-Shizuoka Tectonic Line (ISTL) and the Fossa Magna . It runs parallel to Japan's volcanic arc, passing through central Honshū to near Nagoya, through Mikawa Bay , then through the Inland Sea from the Kii Channel and Naruto Strait to Shikoku along the Sadamisaki Peninsula and

294-410: Is affected by the dual-crises of severe population decline and widespread poverty. Wakayama Prefecture occupies much of the area, including the entire southern part. To the northwest of Wakayama Prefecture is Osaka Prefecture , whose southern part is on the peninsula. East of Osaka Prefecture is landlocked Nara Prefecture ; farther east is Mie Prefecture . The Seto Inland Sea lies to the west of

336-406: Is an area with high heat flow. Its thermal history is complex due to multiple heating events or property changes. IODP Expeditions drilled the accretionary complex of the Nankai Trough and reveal the thermal history with drill core analyses. The area was originally a basin (Shikoku Basin) with active hydrothermal activity during its formation. As basin formation stopped and sedimentation took place,

378-420: Is located in the front section of the main thrust fault, while site 1173 is located approximately 11 km from the frontal thrust zone (Hitoshi et al., 2006). Other interesting results of the pressure measurements were the pressure changes that resulted from sediment deformation near boreholes and the effect of very low earthquake swarms at the time of pressure changes (Davis et al., 2006). The working hypothesis

420-521: 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

462-459: 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

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504-404: Is that pressure changes indicate a change in the elastic strain within the formation (Davis et al., 2006). A seaward change in the pressure as measured by the borehole instruments likely indicates a relaxation of the sediments from the previous major thrust earthquake. Furthermore, the short period seismicity appears to have some degree of dependency on bathymetric highs such as seamounts. This

546-489: Is the MTL. The most important part of the MTL is called the MTL fault zone ( 中央構造線断層帯 , Chūō Kōzōsen Dansōtai ) which is an active fault and considered hazardous. It is separated into six regions defined by the most recent seismic activities. Although the MTL is a less seismically active region than others, earthquakes with magnitudes of 6 to 7 have occurred. The oldest known activity occurred about 3000 years ago, and

588-738: The Akihasan shrine in Shizuoka Prefecture follows the Japan Median Tectonic Line. The Japan Median Tectonic Line Museum in Ōshika is dedicated to the history of and research on the tectonic line. Nankai Trough 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 ,

630-649: The Bungo Channel and Hōyo Strait to Kyūshū . The sense of motion on the MTL is right-lateral strike-slip, at a rate of about 5–10 mm/yr. This sense of motion is consistent with the direction of oblique convergence at the Nankai Trough . The rate of motion on the MTL is much less than the rate of convergence at the plate boundary , making it difficult to distinguish the motion on the MTL from interseismic elastic straining in GPS data. The upper part of

672-628: 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

714-624: The Suruga Trough , while to the east is the Sagami Trough . The Nankai Trough runs roughly parallel to the Japan Median Tectonic Line . 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

756-528: The Earth’s subtropics with rainfall in the southern mountains believed to reach 5 metres (200 in) per year and averaging 3.85 metres (151.6 in) in the southeastern town of Owase , comparable to Ketchikan , Alaska or Tortel in southern Chile. When typhoons hit Japan, the Kii Peninsula is typically the worst affected area and daily rainfalls as high as 940 millimetres (37 in) are known so

798-638: The Japanese islands was created at the edge of the Eurasian plate 180 million years ago. Then, 50 million years later, the lower part was created at the southern part of the Yangtze continent . The lower part then rode on the Izanagi plate and moved to the upper part. These islands were joined into one and formed the Japanese islands 70 million years ago. The boundary line between the upper part and lower part

840-586: The Kii Peninsula is often referred to as the Typhoon Ginza (after Ginza in Tokyo ). The peninsula was severely affected by the 2011 typhoon season , which caused severe damage and many deaths. A significant amount of damage remains today from the 2011 landslides caused by the typhoons. The natural landscape of the Kii Peninsula is dense temperate rainforest , but the vast majority of forests are monoculture pine plantations which were planted to rebuild after

882-701: The Kii Peninsula. To the south and east is the Pacific Ocean and to the north is the valley of the Kiso Three Rivers and Ise Bay . Notable places in the Kii Peninsula include: The Kii Peninsula is the location of a UNESCO World Heritage Site : Sacred Sites and Pilgrimage Routes in the Kii Mountain Range . In 2004, UNESCO designated three other locations on the Kii Peninsula as World Heritage Sites. They are: Japan Median Tectonic Line Japan Median Tectonic Line ( 中央構造線 , Chūō Kōzō Sen ) , also Median Tectonic Line ( MTL ),

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924-734: The MTL lies 3000 meters beneath ground level in Saitama Prefecture. It is not clear where the MTL goes from there, but it is thought that it passes beneath Kashima or Ōarai on the Pacific coast in Ibaraki Prefecture . The MTL passes south of Awaji Island as it crosses the Kii Channel from Shikoku to the Kii Peninsula . On the Kii Peninsula , it passes north of the Kinokawa River, from Wakayama on

966-485: The NUVEL-1A model may need further revision. 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

1008-583: 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

1050-528: The Nankai Trough (where the heat flow is one of the highest in the region) reveal that sediments there only reach pre-oil window to early oil window thermal maturities. However, the trough is potentially a major source of hydrocarbon fuel, in the form of methane clathrate. Nevertheless as of 2014 there is no commercial exploitation. At depth in the ocean bottoms, in some cases water can form an ice-like solid structure that has methane trapped in its crystalline lattice, forming gas hydrates . The source of water for

1092-414: 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,

1134-620: The accumulation of gas hydrates are the sand-rich areas of the trough (Collett, 2002). Well coring indicates the presence of at least three gas hydrate zones. Krason, 1994, estimated that there are 0.42 to 4.2×10 cubic meters of methane within the gas hydrates. Seismically, the high bottom simulating reflectors are considered indicative of gas hydrates (Colwell et al., 2004). Methane-rich horizons have been identified as areas of higher attenuation of sonic frequencies (10 to 20 kHz) and only slight attenuation of seismic frequencies (30 to 110 Hz) (Matsushima, 2006). The Nankai accretionary complex

1176-412: The destruction of World War II. The region is well known for its citrus varieties and orchards. Much of the coast consists of networks of small rias into which flow very steep and rapid streams characterised by numerous high waterfalls . Forestry and fishing were the traditional economic mainstays of the region and remain important even today despite a declining population and labour force. The region

1218-533: The eastern foothills of the Kongō Range , connecting to the shear zone along the east side of the Ikoma Mountains between Nara Prefecture and Osaka Prefecture. The MTL is known to pass through the region between Kunisaki Peninsula and Saganoseki Peninsula in Ōita Prefecture . However, because the Ōno River basin south of Ōita is covered by late Mesozoic sedimentary rock, the geological continuity in

1260-404: The formation of gas hydrates frequently comes from the dewatering of a subducting slab as well as the overriding plate (Muramatsu et al., 2006). Gas hydrates nearest the trough appear to be sourced mainly from dewatering associated with subduction, while with increasing distance from the trough the sourcing is more a result of lateral movement of methane enriched waters (Muramatsu et al., 2006). This

1302-525: The most recent in the 16th century. It is expected that seismic activities will continue in the future. The boundaries of the regions are still being discussed, so the regions are not well defined. The Great Hanshin earthquake of 1995 occurred on the Nojima Fault , a branch of the MTL. Approximately 6,434 people lost their lives; about 4,600 of them were from Kobe. It caused approximately ten trillion yen ($ 100 billion) in damage, 2.5% of Japan's GDP at

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1344-410: 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

1386-461: The smectite, converting it to illite clay (Steurer et al., 2003). 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

1428-516: The southern foothills of the Sanuki Mountains , at the northern border of Tokushima Prefecture. The Japan Median Tectonic Line is visible in forms of several outcrops and fault saddles in Ina city, Ōshika village, and Iida city in southern Nagano Prefecture. The historic Akiha Kodo pilgrimage route ( Japan National Route 152 ) between Suwa-taisha shrine in central Nagano Prefecture and

1470-473: The subducted seafloor, including backarc basin crust of the Shikoku Basin , as well as due to serpentization of uppermost mantle beneath the overriding plate (Obana et al., 2006). Recent large scale earthquakes resulting from subduction along the Nankai Trough have occurred in areas of large scale increases in the dip angle of the subducting plate (Hori et al., 2004). Drill cores from the seaward edge of

1512-778: The time. The MTL goes from Shizuoka Prefecture to Nagano Prefecture . At Nagano prefecture, it lies between the Ina mountains and the Akaishi mountains. Then, it goes to Chino-shi, the northern part of Nagano prefecture. The MTL goes from Shimonita in Gunma Prefecture to the Hiki hill district in Saitama Prefecture , and then continues down to the Kantō Plain . In the central Kanto Plain, boring tests have shown that

1554-414: 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

1596-653: The west coast of Ehime Prefecture . It continues eastward past the city of Saijō and along the foothills of the Ishizuchi Range. From Miyoshi in Tokushima Prefecture it passes along the northern side of the Yoshino River until it arrives at the city of Tokushima on the east coast of the island. There are active shear zones to the north of the MTL: north of Iyo and, in eastern Shikoku, along

1638-600: The west coast past Gojō in Nara Prefecture (where the river is called the Yoshino River) to Mie Prefecture on the east coast. There is an active shear zone north of the river in the southern foothills of the Izumi Mountains, which separate Wakayama Prefecture from Osaka Prefecture . East of Nara Prefecture, shear zones along the MTL are less active. The MTL takes a northern turn and passes along

1680-659: The westward direction is unclear. According to one theory, the MTL may turn south around the Ōno River, connecting Usuki in Ōita Prefecture to the Yatsushiro tectonic line in Kumamoto Prefecture . According to another theory, it may connect Ōita directly to the Kumamoto tectonic line. After crossing the Hōyo Strait from Kyushu to Shikoku, the MTL passes north of the Sadamisaki Peninsula to land at Iyo on

1722-500: Was concluded by Kanda et al., 2004, through inversion analysis of seismic data. Historically, the most recent large-scale earthquake to occur in the Nankai Trough was in 1944 off the Kii Peninsula . Using recent ocean bottom seismograph studies, it has been determined that most of the seismicity occurs near the trough axis (Obana et al., 2006). Along the western area of the Nankai Trough, seismicity appears to be related to irregularities in crustal structure such as fractures generated from

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1764-513: Was determined by drilling a series of boreholes and measuring the concentration, as well as radiometric age determination of the halogen elements iodine, bromine, and chlorine (Tomaru et al., 2007). The age determination of the iodine indicated multiple methane sources. It has been estimated that convergent margins may contain up to two-thirds of the total gas hydrate volume on the Earth (Kastner, 2001). The Nankai Trough has been described as containing

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