This research involves the study of the lateral and radial distributions of anomalous S-wave speeds (Vs) in the transition zone beneath the northern Philippine Sea and the back-arc region of the Tonga subduction zone. In both regions, pronounced anomalies of fast P-wave speeds (Vp) have been taken as evidence for remnants of subducted pacific lithosphere that remain stagnant, at least temporarily, in the transition zone of the mantle. Results from the proposed research will offer a rare opportunity to compare the distribution and magnitude of anomalies in both Vs and Vp, forming constraints for understanding the nature of the anomalous bodies. This work is motivated by two observations. First, scattered, deep-focus seismicity off the inclined Tonga Wadati-Benioff zone lacks a corresponding anomaly in Vp, suggesting that the effect of cold temperature, which should raise Vp, is counteracted by compositional or mineralogical effects. Second, Vs may be highly anisotropic near the source regions of this scattered seismicity in the transition zone. Thus, links will be sought between the deep seismicity and anomalies in Poisson's ratio or in anisotropy of Vs.
