Variations of attenuation along backarc axis of the SW Okinawa trough: effect of the Eurasian plate

The edge of the subduction zone is thought to be warm and vigorous in lateral flow because of its additional exposure to the asthenosphere. An opposite example may be found in NE Taiwan where the northwestward subducting Philippine Sea plate is impinging sideways against the Eurasian plate. This is where subduction transforms to collision, creating an edge complex with a lot of unique features. In this study we measured Q values of P waves that travel the mantle wedge corner in hope to depict how attenuation may reflect the subduction-collision transformation. It is very hard to pin down a precise Q with the tradeoff between corner frequency and t*. We demonstrate a new technique to determine the Q values along 3 profiles straddling the backarc basin Okinawa trough. Q values increase from 100 at typical backarc to > 400 at the edge of the subducting Philippine slab. We infer that the mantle wedge is blocked by the ~100 km thick Eurasian continental lithosphere on the western end and that a vertical thermal boundary layer may have formed to strongly reduce the attenuation.