Seismological evidence for a low-velocity layer within the subducted slab of southern Taiwan

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 analys is of a three-component short-period set of digital data from seismic stati ons in southern Taiwan. Two distinct phases in the first few seconds of the P-waves generated by nine intermediate-depth earthquakes are observed at o ne station (TAW) located just at the intermediate up-dip of the Wadati-Beni off zone. Detailed analyses of these phases show the faster one is a refrac ted wave from the Moho of the subducting plate while the subsequent phase i s the direct wave propagated within the subducting crust. These two distinc t 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 int ermediate-depth earthquakes occurred in the low-velocity layer of the subdu cting crust and that the thickness of the low-velocity layer is less than 1 5 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 depth s less than 60 km. Instead, however, the low-velocity layer might be largel y attributed to the metastable persistence of hydro phases and/or fluid pro duced by a series of dehydration reactions during subduction. (C) 1999 Else vier Science B.V. All rights reserved.