Three-dimensional Elastic and Anelastic Structure of the Lowermost Mantle Beneath the Western Pacific From Finite-Frequency Tomography

We build a finite-frequency tomography method that uses traveltime and amplitude data for obtaining 3-D maps of shear velocity (Vs) and seismic attenuation (measured with the quality factor Q). We then apply this method to recover the 3-D Vs and Q structures in the lowermost mantle beneath the western edge of the Pacific large low shear velocity province, using the S and ScS phases for 31 earthquakes that occurred underneath the vicinity of Tonga and Fiji regions. Our data set consists of the transverse components of 1,341 traces from the Japanese F-net seismic station network. The waveform data are applied with a band-pass filter in the period range of 12.5-200 s, corresponding to the frequency range of 0.005-0.08 Hz. Both Vs and Q are lower than those in the Preliminary reference Earth model (PREM) in the bottom depth range, with the lowest part being situated at the center of the region we sample. This feature is robust across a variety of inversion configurations. We then estimate possible temperature anomalies in this region from the obtained V-s and Q structures. Discrepancy between the temperature anomalies predicted by these two quantities suggests that explaining simultaneously V-s and Q anomalies in this region requires both temperature and chemical anomalies. Assuming that Q anomalies are a reliable proxy for temperature and that compositional anomalies primarily consist of an excess in iron oxide, we propose a possible thermal and compositional structure for this region.