Azusa Shito

Upper mantle transition zone structure beneath the Philippine Sea Region

Seismic structure of the upper mantle transition zone beneath the Philippine Sea region is studied using waveform modeling. Models AZ41 and AZ40 were derived to represent the structure beneath the north central part and northwestern regions, respectively. AZ41 has high-velocity anomalies from 510 km to 660 km 3% faster than a global model ak135, a depression of the ‘660’ discontinuity to 690 km, and a small (+2%) discontinuity at 510 km. Model AZ40 is the same as AZ41 except there is no depression of the ‘660’. On the other hand, the waveforms which sampled the southern region can be explained with the global model ak135. These results support the interpretation that subducted Pacific lithosphere is stagnant above the ‘660’ in the northern part, whereas in the southern part it penetrates into the lower mantle. In accord with earlier suggestions, the different configuration of the subducted slab suggested above could be due to the migration of the trench.

Evolution of the oceanic lithosphere inferred from Po/So waves traveling in the Philippine Sea Plate

Po/So waves are characterized by their high-frequency content and long-duration travel over great distances (up to 3000 km) through the oceanic lithosphere. Po/So waves are developed by the multiple forward scattering of P- and S-waves due to small-scale stochastic random heterogeneities. To study the nature of these heterogeneities, Po/So waves are analyzed in the Philippine Sea Plate, which consists of three regions with different lithospheric ages. In the Philippine Sea Plate, Po/So waves propagate in the youngest region (15 Ma) and propagate more effectively in older regions. We investigate the mechanism of this propagation efficiency using numerical Finite Difference Method simulations of 2-D seismic wave propagation. The results of this study demonstrate that the increase in propagation efficiency of Po/So waves depends on the age of the oceanic lithosphere, and this relationship can be qualitatively explained by thickening of the oceanic lithosphere including small-scale heterogeneities and a reduction in the intrinsic attenuation. These small-scale heterogeneities may form continuously in oceanic lithosphere from the time of its formation at a spreading ridge, via the solidification of melts distributed in the asthenosphere.