The , also called Nansei-Shotō Trench, is a 1398 km (868 mi) long

oceanic trench Oceanic trenches are prominent long, narrow topographic depressions of the ocean floor. They are typically wide and below the level of the surrounding oceanic floor, but can be thousands of kilometers in length. There are about of oceanic tren ...

located along the southeastern edge of Japan's

Ryukyu Islands The , also known as the or the , are a chain of Japanese islands that stretch southwest from Kyushu to Taiwan: the Ōsumi, Tokara, Amami, Okinawa, and Sakishima Islands (further divided into the Miyako and Yaeyama Islands), with Yona ...

in the

Philippine Sea The Philippine Sea is a marginal sea of the Western Pacific Ocean east of the Philippine archipelago (hence the name), the largest in the world, occupying an estimated surface area of . The Philippine Sea Plate forms the floor of the sea. Its ...

in the

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contine ...

, between northeastern

Taiwan Taiwan, officially the Republic of China (ROC), is a country in East Asia, at the junction of the East and South China Seas in the northwestern Pacific Ocean, with the People's Republic of China (PRC) to the northwest, Japan to the northe ...

and southern Japan. The trench has a maximum depth of 7460 m (24,476 ft). The trench is the result of oceanic crust of the

Philippine Plate The Philippine Sea Plate or the Philippine Plate is a tectonic plate comprising oceanic lithosphere that lies beneath the Philippine Sea, to the east of the Philippines. Most segments of the Philippines, including northern Luzon, are part of ...

obliquely subducting beneath the continental crust of the

Eurasian Plate The Eurasian Plate is a tectonic plate that includes most of the continent of Eurasia (a landmass consisting of the traditional continents of Europe and Asia), with the notable exceptions of the Indian subcontinent, the Arabian subcontinent and ...

at a rate of approximately 52 mm/yr. In conjunction with the adjacent

Nankai Trough The is a submarine trough located south of the Nankaidō region of Japan's island of Honshu, extending approximately offshore. The underlying fault, the ''Nankai megathrust,'' is the source of the devastating Nankai megathrust earthquakes, w ...

to the northeast, subduction of the Philippine plate has produced 34 volcanoes. The largest earthquake to have been recorded along the Ryukyu Trench, the

1968 Hyūga-nada earthquake The 1968 Hyūga-nada earthquake ( Japanese: 1968年日向灘地震) occurred on April 1 at . The earthquake had a magnitude of 7.5, and the epicenter was located in Hyūga-nada Sea, off the islands of Kyushu and Shikoku, Japan. The magnitude o ...

, was magnitude 7.5 and occurred along the northernmost part of the trench on 1 April 1968. This earthquake also produced a

tsunami A tsunami ( ; from ja, 津波, lit=harbour wave, ) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater exp ...

Ryukyu Trench and Ryukyu Arc structure near Taiwan

An east-west planar

seismic zone In seismology, a seismic zone or seismic belt is an area of seismicity potentially sharing a common cause. It may also be a region on a map for which a common areal rate of seismicity is assumed for the purpose of calculating probabilistic ground ...

associated with the Ryukyu Trench occurs off the east coast of Taiwan. This seismic zone is continuous laterally for 50 km and to 150 km depth. The

hypocenter In seismology, a hypocenter or hypocentre () is the point of origin of an earthquake or a subsurface nuclear explosion. A synonym is the focus of an earthquake. Earthquakes An earthquake's hypocenter is the position where the strain energy ...

s of earthquakes at this location outline a

Benioff zone Benioff is a surname. Notable people with the surname include: * David Benioff (born 1970), American writer, screenwriter and television producer * Hugo Benioff (1899–1968), American seismologist and academic ** Wadati–Benioff zone * Marc Benio ...

indicating that the Philippine Sea Plate is subducting at an angle of about 45° beneath the

Eurasian plate The Eurasian Plate is a tectonic plate that includes most of the continent of Eurasia (a landmass consisting of the traditional continents of Europe and Asia), with the notable exceptions of the Indian subcontinent, the Arabian subcontinent and ...

in this area; the dip of the slab changes dramatically from one end of the trench to the other as noted in the next section. Such depth and dip inferences of this area are consistent with the positions of the overlying Tatun and Chilung volcano groups of Taiwan. The region behind (N and NW of) the Ryukyu Arc is a bathymetric low known as the

Okinawa Trough The (also called , literally China-Ryukyu Border Trough ) is a seabed feature of the East China Sea. It is an active, initial back-arc rifting basin which has formed behind the Ryukyu arc-trench system in the West Pacific. It developed where t ...

. The

Yilan Plain Yilan Plain (), also called the Lanyang Plain (), or historically Kabalan (), Kapsulan (), Komalan () is a plain in Yilan County, Taiwan. The plain has an alluvial fan which formed by Lanyang River. The plain was formed in the shape of nearl ...

of Taiwan could be the westward continuation of this trough, but the Yilan Plain sits on the forearc side of the Ryukyu Trench system. This may indicate that the Yilan Plain represents a former spreading centre that sits trench-ward of the current spreading centre and volcanic arc. Near 122°E (about 100 km East of the Taiwan Coast), the Ryukyu Arc is displaced to the north relative to the eastern extent of the arc. One hypothesis is that a north trending dextral transform faults has displaced this section of the arc to the north. A competing hypothesis claims that no transform fault motion is involved in the displacement, but rather the trench is continuous up to the northeast continental margin of Taiwan. A third hypothesis maintains that the trench is continuous through the continental margin right up to the northeastern Taiwan coastline, also without the existence of a dextral north-south trending fault.

Seismic structure

Ocean bottom seismography methods combined with earthquake studies of the

constrain the dip angle of the Philippine Sea Plate along the Ryukyu trench. In the Northern part of the Ryukyu trench, the dip of the Philippine Sea Plate is shallow at shallow depth, reaching only about 11° in the first 50 km, and steeper at deeper depths, reaching 70° below about 70 km. In contrast, the slab dip in the central and southern parts of the Ryukyu trench is more gentle, reaching only 40-50° at 70 km depth. Ocean bottom seismography studies of the Ryukyu trench provide insight into the

P-wave A P wave (primary wave or pressure wave) is one of the two main types of elastic body waves, called seismic waves in seismology. P waves travel faster than other seismic waves and hence are the first signal from an earthquake to arrive at an ...

velocity structure of the area. In the northern part of the trench, several transects have been studied, including a profile of the back arc region parallel to the trench, a transect spanning the trench, fore arc and back arc region, and a transect spanning the Ryukyu volcanic arc. The transect perpendicular to the length of the trench images many distinct velocity layers. The sedimentary wedge created by subduction has four distinct layers with p-wave velocities of 1.8 km/s, 2.8-2.9 km/s, 3.5 km/s, and 4.5–5 km/s. In the area of this transect, the wedge reaches a thickness of 9 km at 50 km from the trench. Beneath the wedge are several seismic layers within the oceanic crust. Separate ocean bottom seismography and multi-channel seismic studies provide insight into the structure of the northern end of the Ryukyu trench region. Features of note include a thick (7–12 km) low velocity (4–5 km/s) zone on the landward side of the trench, the existence of subducting paleo-arc crust near the top of the trench in contrast to simple oceanic crust located at the middle of the trench, and a zone in which the Philippine Plate subducts beneath low P-wave velocity material (Vp = 5 km/s) that coincides with the location of the 7.5 1968 Hyuganada earthquake. It has been hypothesized that the above structural heterogeneity, in particular the subducting paleo-arc crust and its associated bathymetric highs, is one reason why earthquakes in this region are not larger i.e. exceeding 8.0. The exact mechanism by which the subduction of paleo-arc crust prevents sufficient stress build up for a larger earthquake is unknown.

References

{{East Asia plates Oceanic trenches of the Philippine Sea Geology of Japan Geology of Taiwan Natural history of the Bonin Islands Natural history of the Ryukyu Islands Landforms of Taiwan Subduction zones

Ryukyu Trench and Ryukyu Arc structure near Taiwan

Seismic structure

See also

References