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. 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; 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 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. This may indicate that the Ilan 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.
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