The island of Taiwan (Formosa) sits on the boundary between two tectonic plates the Eurasian and the Philippine Sea. To the south of the Island the Manila Trench, in which the Eurasian Plate is being subducted beneath the Philippine Sea Plate, extends southward to the Philippines. To the east the Ryuku Trench, in which the Philippine Sea Plate is being subducted beneath the Eurasian, extends northeast to Japan. Taiwan forms part of two island-arcs, the Ryukyu Arc, which is formed volcanically from material which is subducted in the Ryuku Trench, melted by the heat of the Earth's interior, then rises through the overlying Eurasian plate to form volcanic islands, and the Luzon Arc on which melted material from the subducting Eurasian Plate is rising through the overlying Philippine Sea Plate.
In a forthcoming paper in the journal Tectonophysics, a team of scientists lead by Kamil Ustaszewski of the Lithosphere Dynamics Section at the Helmholtz Centre Potsdam German Research Centre for Geosciences describe how they built up a map of the geological structure underneath Taiwan, using Earthquake tomography, a technique that uses vibrational waves created by natural earthquakes (of which there are quite a lot in Taiwan) to create a three dimensional picture in the same way that a sonar uses artificially generated sound waves.
Three dimensional map of the structure beneath Taiwan; the Moho is the Mohorovičić Discontinuity, which forms the boundary between the Crust and the Upper Mantle. Scale bar is in kilometers. From Ustaszewski et al. (2012).
This revealed that the Eurasian Plate is folded sharply beneath the Island along a north-south line bisecting the island and sinking into the Earth at close to 90°. The Philippine Sea Plate is moving past this, being pushed northwards by an extension centre (ocean ridge) to the south. Towards the north end of the island the Philippine Sea Plate is itself being subducted, at a less steep angle, beneath an accretionary prism made up of material that is not easily subducted, such as old volcanic islands that have been dragged along by the moving plate. This prism has become annealed to the Eurasian plate, forming an new section of continental lithosphere, complete with an emerging Mohorovičić Discontinuity, the upper part of this being covered by sediment, is indistinguishable from the older Eurasian crust, leading to the illusion of two tectonic plates being subducted beneath one-another in the same place.