[S07P-02] モンゴル下の上部マントル3次元構造：アムールプレート西縁境界との関係
Multi-mode phase speeds of Love and Rayleigh waves are used to construct a new radially anisotropic S wave model in the middle Eurasian and Mongolian regions. Our dataset includes seismic waveforms of over 2151 teleseismic events with a magnitude greater than 5.8 from 2009 to 2021, recorded at permanent and temporary broad-band seismic stations in and around Mongolia. The multi-mode dispersion curves of Love and Rayleigh waves were extracted using the nonlinear waveform fitting method for individual seismograms. We then reconstructed phase speed maps for each mode and frequency, incorporating finite-frequency effects of surface waves. Finally, localized multi-mode dispersion curves derived from the phase speed maps were inverted for local anisotropic 1-D S-wave speed profiles, which eventually form a 3-D shear wave model.
Our new model exhibits significant lateral variations of S wave speeds at 70-100 km depth beneath Mongolia, i.e., slow anomalies in the tectonically active western Mongolia and fast anomalies in stable eastern Mongolia. In the radial anisotropy model, SH waves are faster than SV waves in almost the entire Mongolian lithosphere above 100 km depth, except for the northern Altay Mountains region. The Hangay Dome region is characterized by significantly slower velocities, indicating the asthenospheric upwelling that causes the uplifting of this region. This study reveals distinct lateral variations of S wave speeds across the boundary between the Amurian and Eurasian plates, characterized by the prominent fast anomaly of the dipping lithosphere at the western edge of the Amurian plate, which may indicate the delamination or dripping of the Amurian lithosphere in its western margin.
Our new model exhibits significant lateral variations of S wave speeds at 70-100 km depth beneath Mongolia, i.e., slow anomalies in the tectonically active western Mongolia and fast anomalies in stable eastern Mongolia. In the radial anisotropy model, SH waves are faster than SV waves in almost the entire Mongolian lithosphere above 100 km depth, except for the northern Altay Mountains region. The Hangay Dome region is characterized by significantly slower velocities, indicating the asthenospheric upwelling that causes the uplifting of this region. This study reveals distinct lateral variations of S wave speeds across the boundary between the Amurian and Eurasian plates, characterized by the prominent fast anomaly of the dipping lithosphere at the western edge of the Amurian plate, which may indicate the delamination or dripping of the Amurian lithosphere in its western margin.