Ryukyu Trench: Difference between revisions
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[[File:Okinawa trench topographic.png|thumb|300px|Red line indicates the bathymetric low of the Ryukyu Trench]] |
[[File:Okinawa trench topographic.png|thumb|300px|Red line indicates the bathymetric low of the Ryukyu Trench]] |
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The {{nihongo|'''Ryukyu Trench'''|琉球海溝|''Ryūkyū kaikō''}}, also called '''Nansei-Shotō Trench''', is a 1398 kilometer (868 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}}</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 meters (24,476 ft).<ref name=oceana /> The trench is the result of oceanic crust of the [[Philippine |
The {{nihongo|'''Ryukyu Trench'''|琉球海溝|''Ryūkyū kaikō''}}, also called '''Nansei-Shotō Trench''', is a 1398 kilometer (868 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}}</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 meters (24,476 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|coauthors=Michael Allaby|year=1999|publisher=Encyclopedia.com|accessdate=3 March 2012}}</ref> at a rate of approximately 52 mm/yr .<ref name="northend">{{cite journal|last=Nishiwaza|first=Azusa|coauthors=Kentaro Kaneda and Mitsuhiro Oikawa|title=Seismic Structure of the Northern End of the Ryukyu Trench [[Subduction Zone]], Southeast of Kyushu, Japan|journal=Earth, Planets and Space|volume=61|issue=8|pages=37–40|url=http://www.terrapub.co.jp/journals/EPS/pdf/2009e/6108e037.pdf}}</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}}</ref> |
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The largest earthquake to have been recorded along the Ryukyu Trench, the [[1968 Hyūga-nada earthquake]], |
The largest earthquake to have been recorded along the Ryukyu Trench, the [[1968 Hyūga-nada earthquake]], |
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was magnitude 7.5 and occurred along the northernmost part of the trench <ref name=northend /> on April 1st 1968.<ref name="quake">{{cite journal|last=Yuji|first=Yagi|coauthors=M. Kikuchi and 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|pages=793–803|url=http://iisee.kenken.go.jp/staff/yagi/pdf/EPS2001_hyuga.pdf|accessdate=3 Mar. 2012}}</ref> This earthquake also produced a [[tsunami]]. |
was magnitude 7.5 and occurred along the northernmost part of the trench <ref name=northend /> on April 1st 1968.<ref name="quake">{{cite journal|last=Yuji|first=Yagi|coauthors=M. Kikuchi and 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|pages=793–803|url=http://iisee.kenken.go.jp/staff/yagi/pdf/EPS2001_hyuga.pdf|accessdate=3 Mar. 2012}}</ref> This earthquake also produced a [[tsunami]]. |
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Revision as of 04:13, 20 August 2012
The Ryukyu Trench (琉球海溝, Ryūkyū kaikō), also called Nansei-Shotō Trench, is a 1398 kilometer (868 mi)[1] 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 meters (24,476 ft).[1] The trench is the result of oceanic crust of the Philippine Plate obliquely subducting beneath the continental crust of the Eurasian Plate[2] at a rate of approximately 52 mm/yr .[3] In conjunction with the adjacent Nankai Trough to the northeast, subduction of the Philippine plate has produced 34 volcanoes.[4] 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 [3] on April 1st 1968.[5] This earthquake also produced a tsunami.
Ryukyu Trench and Ryukyu Arc Structure Near Taiwan
An East-West planar seismic zone associated with the Ryukyu trench occurs off the East Coast of Taiwan.[6] This seismic zone is continuous laterally for 50 km and to 150 km depth. The hypocenters of earthquakes at this location outline a Benioff zone indicating that the Philippine Sea Plate is subducting at an angle of about 45° beneath the Eurasian plate in this area; it should be noted that 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.[6]
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.[6] This may indicate that the Ilan Plain represents a former spreading center that sits trench-ward of the current spreading center 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.[6]
Seismic Structure
Ocean bottom seismography studies of the Ryukyu trench provide insight into the P-wave velocity structure of the area. Several transects (see accompanying images) 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. [7] With respect to the transect perpendicular to the length of the trench, many distinct velocity layers are imaged. The sedimentary wedge created by subduction has 4 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. The wedge reaches a thickness of 9 km at 50 km from the trench. Beneath the wedge are several layers of oceanic crust (see accompanying figures).
Ocean bottom seismography methods combined with earthquake studies can the dip angle of the Philippine Sea Plate along the Ryukyu trench. Whereas the dip in the Northern part of the Ryukyu trench is shallow at shallow depth, reaching only about 11° in the first 50 km, the dip near the middle of the trench is steeper at 40°-50° (at 70 km depth), and the dip in the northern part of the trench at depth (70 km) is inferred from earthquakes describing a Benioff zone to be 70°.[7]
Separate ocean bottom sesimography and multi-channel seismic profile studies provide insight into the seismic 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 Mw 7.5 1968 Hyuganada earthquake.[3] 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 Mw 8.0.[3] The exact mechanism by which the subduction of paleo-arc crust prevents sufficient stress build up for a larger earthquake is unknown.
See also
References
- ^ a b "Ryukyu Trench". Marine Places. Oceana.org. Retrieved 3 March 2012.
- ^ Allaby, Alissa (1999). "Ryukyu Trench". Encyclopedia.com. Retrieved 3 March 2012.
{{cite web}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ a b c d Nishiwaza, Azusa. "Seismic Structure of the Northern End of the Ryukyu Trench [[Subduction Zone]], Southeast of Kyushu, Japan" (PDF). Earth, Planets and Space. 61 (8): 37–40.
{{cite journal}}: URL–wikilink conflict (help); Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Chang-Hwa, Chang-Hwa (2003). "The Caldera Eruptions in Ryukyu Arc: As Inferred the Thermal Anomaly in Kyushu". Journal of the Balneological Society of Japan. 53 (3). Science Links Japan: 90–91. Retrieved 3 Mar. 2012.
{{cite journal}}: Check date values in:|accessdate=(help) - ^ Yuji, Yagi. "Co-seismic slip, post-seismic slip, and aftershocks associated with two large earthquakes in 1996 in Hyuga-nada, Japan" (PDF). Earth, Planets and Space. 53: 793–803. Retrieved 3 Mar. 2012.
{{cite journal}}: Check date values in:|accessdate=(help); Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ a b c d "Cenozoic Plate Tectonic Setting". CENTRAL GEOLOGICAL SURVEY, MOEA. Retrieved 3 March 2012.
- ^ a b Kodaira, S (15). "Crustal structure across the middle Ryukyu trench obtained from ocean bottom seismographic data". Tectonophysics. 263 (1–4): 39–60. doi:10.1016/S0040-1951(96)00025-X. Retrieved 4 March 2012.
{{cite journal}}: Check date values in:|date=and|year=/|date=mismatch (help); Unknown parameter|coauthors=ignored (|author=suggested) (help); Unknown parameter|month=ignored (help)