View source for Ryukyu Trench

{{short description|Oceanic trench along the southeastern edge of Japan's Ryukyu Islands in the Pacific Ocean}}
[[File:Okinawa trench topographic.png|thumb|300px|Red line indicates the bathymetric low of the Ryukyu Trench]]
The {{nihongo|'''Ryukyu Trench'''|琉球海溝|''Ryūkyū kaikō''}}, also called '''Nansei-Shotō Trench''', is a 1398&nbsp;km (868&nbsp;mi)<ref name="oceana">{{cite web|url=http://oceana.org/en/explore/marine-places/ryukyu-trench|title=Ryukyu Trench|work=Marine Places|publisher=Oceana.org|accessdate=3 March 2012|archive-date=13 November 2013|archive-url=https://web.archive.org/web/20131113155509/http://oceana.org/en/explore/marine-places/ryukyu-trench|url-status=dead}}</ref> long [[oceanic trench]] located along the southeastern edge of [[Japan]]'s [[Ryukyu Islands]] in the [[Philippine Sea]] in the [[Pacific Ocean]], between northeastern [[Taiwan]] and southern Japan. The trench has a maximum depth of 7460&nbsp;m (24,476&nbsp;ft).<ref name=oceana /> The trench is the result of oceanic crust of the [[Philippine Plate]] obliquely subducting beneath the continental crust of the [[Eurasian plate]]<ref name="encyclopedia">{{cite web|url=http://www.encyclopedia.com|title=Ryukyu Trench|last=Allaby|first=Alissa|author2=Michael Allaby |year=1999|publisher=Encyclopedia.com|accessdate=3 March 2012}}</ref> at a rate of approximately 52&nbsp;mm/yr.<ref name="northend">{{cite journal|last=Nishiwaza|first=Azusa|author2=Kentaro Kaneda|author3=Mitsuhiro Oikawa|title=Seismic Structure of the Northern End of the Ryukyu Trench Subduction Zone, Southeast of Kyushu, Japan|journal=Earth, Planets and Space|date=2009 |volume=61|issue=8|pages=37–40|doi=10.1186/BF03352942 |bibcode=2009EP&S...61E..37N |doi-access=free }}</ref> In conjunction with the adjacent [[Nankai Trough]] to the northeast, subduction of the Philippine plate has produced 34 volcanoes.<ref name="volcano">{{cite journal|last=Chang-Hwa|first=Chang-Hwa|year=2003|title=The Caldera Eruptions in Ryukyu Arc: As Inferred the Thermal Anomaly in Kyushu|journal=Journal of the Balneological Society of Japan|publisher=Science Links Japan|volume=53|issue=3|pages=90–91|url=http://sciencelinks.jp/j-east/article/200403/000020040304A0063980.php|accessdate=3 Mar 2012|archive-date=15 August 2011|archive-url=https://web.archive.org/web/20110815130337/http://sciencelinks.jp/j-east/article/200403/000020040304A0063980.php|url-status=dead}}</ref>
The largest earthquake to have been recorded along the Ryukyu Trench, the [[1968 Hyūga-nada earthquake]],
was magnitude 7.5 and occurred along the northernmost part of the trench<ref name=northend /> on 1 April 1968.<ref name="quake">{{cite journal|last=Yuji|first=Yagi|author2=M. Kikuchi|author3=T. Sagiya|title=Co-seismic slip, post-seismic slip, and aftershocks associated with two large earthquakes in 1996 in Hyuga-nada, Japan|journal=Earth, Planets and Space|volume=53|issue=8|pages=793–803|doi=10.1186/BF03351677|year=2001|bibcode=2001EP&S...53..793Y |doi-access=free}}</ref> This earthquake also produced a [[tsunami]].

== Ryukyu Trench and Ryukyu Arc structure near Taiwan ==
[[File:Map00148 (28286522445).jpg|thumb|Undersea geographic features of the western Pacific]]
An east-west planar [[seismic zone]] associated with the Ryukyu Trench occurs off the east coast of Taiwan.<ref name=survey>{{cite web|title=Cenozoic Plate Tectonic Setting|url=http://www.moeacgs.gov.tw/english/twgeol/twgeol_ptsetting_02.jsp|publisher=CENTRAL GEOLOGICAL SURVEY, MOEA|accessdate=3 March 2012|archive-date=24 May 2011|archive-url=https://web.archive.org/web/20110524173114/http://www.moeacgs.gov.tw/english/twgeol/twgeol_ptsetting_02.jsp|url-status=dead}}</ref> This seismic zone is continuous laterally for 50&nbsp;km and to 150&nbsp;km depth. The [[hypocenter]]s of earthquakes at this location outline a [[Wadati–Benioff zone]] indicating that the [[Philippine Sea plate]] is subducting at an angle of about 45° beneath the [[Eurasian plate]] in this area; the [[Strike and dip|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 Volcano Group|Tatun]] and [[Chilung Volcano Group|Chilung volcano group]]s of Taiwan.<ref name=survey />

The region behind (N and NW of) the Ryukyu Arc is a bathymetric low known as the [[Okinawa Trough]]. The [[Yilan Plain]] of Taiwan could be the westward continuation of this trough, but the Yilan Plain sits on the forearc side of the Ryukyu Trench system.<ref name=survey /> 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&nbsp;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.<ref name=survey />

== Seismic structure ==
[[Ocean bottom seismography]] methods combined with earthquake studies of the [[Wadati–Benioff zone]] 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&nbsp;km, and steeper at deeper depths, reaching 70° below about 70&nbsp;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&nbsp;km depth.<ref name=crust />

[[Ocean bottom seismography]] studies of the Ryukyu trench provide insight into the [[P wave]] 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.<ref name=crust>{{cite journal|last=Kodaira|first=S|author2=T. Iwasaki |author3=T. Urabe |author4=T. Kanazawa |author5=F. Egloff |author6=J. Makris |author7=H. Shimamura |title=Crustal structure across the middle Ryukyu trench obtained from ocean bottom seismographic data|journal=Tectonophysics|date=15 October 1996|volume=263|issue=1–4|pages=39–60|doi=10.1016/S0040-1951(96)00025-X|bibcode = 1996Tectp.263...39K }}</ref> 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&nbsp;km/s, 2.8-2.9&nbsp;km/s, 3.5&nbsp;km/s, and 4.5–5&nbsp;km/s. In the area of this transect, the wedge reaches a thickness of 9&nbsp;km at 50&nbsp;km from the trench. Beneath the wedge are several seismic layers within the oceanic crust.

Separate [[ocean bottom seismography]] and [[reflection seismology|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&nbsp;km) low velocity (4–5&nbsp;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&nbsp;km/s) that coincides with the location of the {{M|w|link=y}} 7.5 1968 Hyuganada earthquake.<ref name=northend /> 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 {{M|w}} 8.0.<ref name=northend /> The exact mechanism by which the subduction of paleo-arc crust prevents sufficient stress build up for a larger earthquake is unknown.

== See also ==
*[[Ring of Fire]]
*[[Ryukyu Islands]]
*[[1968 Hyūga-nada earthquake]]
*[[Okinawa Trough]]

== References ==
{{Reflist}}

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{{East Asia plates}}

[[Category:Oceanic trenches of the Philippine Sea]]
[[Category:Geology of Japan]]
[[Category:Geology of Taiwan]]
[[Category:Natural history of the Bonin Islands]]
[[Category:Natural history of the Ryukyu Islands]]
[[Category:Landforms of Taiwan]]
[[Category:Subduction zones]]