[S-IT29] 東アジアにおけるジオダイナミクスの新展開

[SIT29-01] 東アジア地域のマントル構造とダイナミクス

★招待講演

*趙 大鵬¹ (1.東北大学大学院理学研究科附属地震・噴火予知研究観測センター)

キーワード：東アジア、地震波トモグラフィー、マントルダイナミクス

In the past a few years we have used new methods of seismic imaging to study the 3-D seismic structure and anisotropy of the mantle beneath East Asia, which provides new insights into seismotectonics, magmatism and mantle dynamics (e.g., Zhao et al., 2016, 2017a; Zhao, 2017, 2019; Ma et al., 2019; Wei and Zhao, 2020). High-resolution images of seismic tomography and receiver functions of the Asian region are obtained, revealing significant lateral heterogeneities in the crust and upper mantle, which are caused by active plate subductions and continental orogeny. A significant advance in the seismic imaging is tomographic inversions for three-dimensional distribution of seismic anisotropy in the crust and mantle, which provides important new information on the lithospheric deformation and mantle convection associated with the continental orogeny and plate subductions. The intraplate volcanism in Northeast Asia is caused by hot and wet upwelling flows in the big mantle wedge above the stagnant Pacific slab in the mantle transition zone (MTZ). The age distribution of the subducting Pacific slab beneath East Asia is estimated, shedding new light on the evolution of the Pacific slab, as well as the East Asian tectonics during the Late Mesozoic to the Cenozoic (Liu et al., 2017). The nucleation of great earthquakes, such as the 2008 Wenchuan earthquake (M 8.0), the 2011 Tohoku-oki earthquake (M 9.0) and the 2015 Nepal earthquake (M 7.9), is controlled by structural heterogeneities in and around the seismogenic fault zones. It is considered that fluids are involved in the nucleation and rupture processes of all types of earthquakes. The cause of deep earthquakes is still not very clear, though transformational faulting triggered by metastable olivine transforming to spinel in the cold, stressed core of the subducting slab is a viable mechanism, and a metastable olivine wedge is revealed within the western Pacific subducting slab at the MTZ depths. The 2015 Bonin deep earthquake (M 7.9, ∼670 km depth) occurred at the MTZ bottom within the vertical Pacific slab which is penetrating into the lower mantle. This very unusual deep event was caused by joint effects of several factors, including the slab’s fast deep subduction, slab tearing and thermal variation, stress changes and phase transformations in the slab, and complex interactions between the slab and the ambient mantle (Zhao et al., 2017b).
References
Liu, X., D. Zhao, S. Li, W. Wei (2017). Age of the subducting Pacific slab beneath East Asia and its geodynamic implications. Earth Planet. Sci. Lett. 464, 166-174.
Ma, J., Y. Tian, D. Zhao, C. Liu, T. Liu (2019). Mantle dynamics of Western Pacific and East Asia: New insights from P-wave anisotropic tomography. Geochem. Geophys. Geosyst. 20, 3628-3658.
Wei, W., D. Zhao (2020). Intraplate volcanism and mantle dynamics of Mainland China: New constraints from shear-wave tomography. J. Asian Earth Sci. 188, 104103.
Zhao, D., S. Yu, X. Liu (2016). Seismic anisotropy tomography: New insight into subduction dynamics. Gondwana Res. 33, 24-43.
Zhao, D. (2017). Big mantle wedge, anisotropy, slabs and earthquakes beneath the Japan Sea. Phys. Earth Planet. Inter. 270, 9-28.
Zhao, D., Y. Isozaki, S. Maruyama (2017a). Seismic imaging of the Asian orogens and subduction zones. J. Asian Earth Sci. 145, 349-367.
Zhao, D., M. Fujisawa, G. Toyokuni (2017b). Tomography of the subducting Pacific slab and the 2015 Bonin deepest earthquake (Mw 7.9). Scientific Reports 7, 44487, doi:10.1038/srep44487.
Zhao, D. (2019). Importance of later phases in seismic tomography. Phys. Earth Planet. Inter. 296, 106314.