[S-IT16] Deep Earth Sciences

[SIT16-P03] Upper mantle structure beneath Mongolia and its surrounding regions from multi-mode surface waves: Implications for the western margin of Amur plate

*Baigalimaa Ganbat^1,3, Kazunori Yoshizawa^1,2, Demberel Sodnomsambuu³, Ulziibat Munkhuu³ (1.Dept Natural History Sciences, Graduate School of Science, Hokkaido University, 2.Dept Earth & Planetary Sciences, Faculty of Science, Hokkaido University, 3.Institute of Astronomy and Geophysics of Mongolia)

Keywords:Surface wave, tomography, 3D S-wave structure, Mongolia, Amur plate

A new radially anisotropic shear wave speed model in the central Eurasia and Mongolia regions is constructed using multi-mode phase speeds of Love and Rayleigh waves. Our dataset includes seismic waveforms of over 2151 teleseismic events (M>5.8) from 2009 to 2021, recorded at permanent and temporary stations in Mongolia and neighboring GSN (Global Seismograph Network) stations. In this study, we used the three-stage inversion technique developed by Yoshizawa & Kennett (2004). At first, we performed fully nonlinear waveform fitting for individual seismograms to extract the multi-mode phase speeds of Love and Rayleigh waves based on the Neighborhood Algorithms (Yoshizawa & Kennett, 2002a; Yoshizawa & Ekström, 2010). In the second stage, we retrieved multi-mode phase speed maps using the linearized inversion based on the method of surface-wave tomography incorporating approximate effects of finite frequency (Yoshizawa & Kennett, 2002b). In the third stage, localized multi-mode dispersion curves derived from these multi-mode phase speed maps were used to construct the local 1-D S-wave profiles, including radial anisotropy, which eventually formed a radially anisotropic 3-D shear wave model beneath central Eurasia around Mongolia. Our new model suggests significant lateral variations of the S wave model at 70-100 km depth beneath Mongolia, i.e., slow anomalies in the tectonically active western Mongolia and fast anomalies in stable eastern Mongolia. The radial anisotropy model shows a faster SH wave speed than SV in almost the entire Mongolian lithosphere above 100 km depth, except for the northern edge of the Altay Mountains region. The Hangay Dome and Hovsgol rift, especially the northeast of the Hangay dome, is generally characterized by velocities lower than in other regions. A slow wave-speed area with the anisotropic parameter ξ < 1 beneath the northeast edge of the Hangay dome might reflect the mantle upwelling resulting in the uplifting process of the Hangay region. The high-velocity anomalies in Middle Gobi are still prominent at 150 km depth, while the Hentiy mountain and South Gobi have essentially disappeared by that depth. The study reveals distinct lateral variations of shear wave speeds near the expected boundary between the Eurasian and Amur plates, especially near the northern, western, and southern edges of the Amur plate.