09:45 〜 10:00
[SIT23-10] Inversion of waveform data for radial profiles of shear velocity and attenuation of the lowermost mantle beneath the western Pacific
キーワード：lowermost mantle, waveform inversion, anelastic structure
The existence of large low shear velocity provinces (LLSVPs) in the lowermost mantle is widely known by seismological global-scale studies. To understand the nature of these features, we investigate the elastic and anelastic structure of the lowermost mantle at the western edge of the Pacific LLSVP by inverting a collection of S and ScS waveforms. The transverse component data were obtained from F-net for 31 deep earthquakes beneath Tonga and Fiji, filtered between 12.5 and 200 s. We observe a regional variation of S and ScS arrival times and amplitude ratios, according to which we divide our region of interest into three subregions. For each of these subregions, we then perform 1D (depth-dependent) waveform inversions simultaneously for radial profiles of shear wave velocity (VS) and seismic quality factor (Q). In figure, models for all three subregions (#1-3) show low VS (a) and low Q (b) structures from 2000 km depth down to the core–mantle boundary. We further find that VS and Q in the central subregion, sampling the Caroline plume, are substantially lower than in the surrounding regions, whatever the depth. In the central subregion, VS-anomalies with respect to PREM (dVS) and Q are about −2.5 per cent and 216 at a depth of 2850 km, and −0.6 per cent and 263 at a depth of 2000 km. By contrast, in the two other regions, dVS and Q are −2.2 per cent and 261 at a depth of 2850 km, and −0.3 per cent and 291 at a depth of 2000 km. At depths greater than ∼2500 km, these differences may indicate lateral variations in temperature of ∼100 K within the Pacific LLSVP. At shallower depths, they may be due to temperature difference between the Caroline plume and its surroundings, and possibly to a small fraction of iron-rich material entrained by the plume.