A thin low-velocity layer that continues from the surface down to at least 135 km depth has been detected within the subducted slab through the analysis of a three-component short-period set of digital data from seismic stations in southern Taiwan. Two distinct phases in the first few seconds of the P-waves generated by nine intermediate-depth earthquakes are observed at one station (TAW) located just at the intermediate up-dip of the Wadati-Benioff zone. Detailed analyses of these phases show the faster one is a refracted wave from the Moho of the subducting plate while the subsequent phase is the direct wave propagated within the subducting crust. These two distinct phases are not observed at any other stations, even at Station HEN which is only 20 km from the plate boundary. These phenomena suggest that the intermediate-depth earthquakes occurred in the low-velocity layer of the subducting crust and that the thickness of the low-velocity layer is less than 15 km. The continuity of such a thin low-velocity layer down to at least 135 km is hard to explain by the metastable persistence of dry gabbro at depths less than 60 km. Instead, however, the low-velocity layer might be largely attributed to the metastable persistence of hydro phases and/or fluid produced by a series of dehydration reactions during subduction.
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