A seismic profile of the upper mantle along the southwestern edge of the Philippine Sea plate using short-period array data

Abstract

We investigated the structure of the P-wave velocity (Vp) in the upper mantle beneath the southwestern edge of the Philippine Sea plate using traveltimes and waveforms from a dense, short-period seismic array of approximately 75 seismographs in Taiwan. Using the seismograms from 39 shallow-focus earthquakes, a composite seismic profile was constructed for a distance range of up to 4000 km. We picked the traveltimes of the first arrivals to construct a smoothly varying model for Vp which also served as a basis for static corrections. After band-pass filtering and deconvolution to remove effects of source-time functions and attenuation, we modelled the waveforms and triplicated arrivals in order to constrain the nature of the upper-mantle discontinuities. Finally, after slant stacking, the entire observed wavefield was downward continued to obtain a regional model TS.406. This model includes a small low-velocity zone between depths of 105 and 190 km, and then a gradual increase in Vp down to the depth of 406 km. At this discontinuity, Vp increases by 5.5 per cent. The P-wave velocity then varies linearly in the transition zone with a moderate gradient of 0.0027 km s-1 km-1 until the base of the transition zone at 662 km, where Vp increases by 5.4 per cent. When compared with other models for the Philippine Sea region and average global models, model TS.406 has lower velocities above a depth of 300 km and then faster Vp in the upper part of the transition zone. For example, above 300 km the TS.406 model is slower than IASP91 by 1.5 per cent, but in the upper transition zone (406 to 571 km), it is faster by 1.2 per cent. Thus, the moderate-amplitude anomaly of fast Vp in the middle of the transition zone reported by tomographic studies seems to occur at shallower depths. Furthermore, unlike the pronounced Northern Philippine Sea anomaly, Vp near the bottom of the transition zone is close to the global average under the southwestern edge of the Philippine Sea.