Current research interests:
1. Full waveform inversion of seismic structure of the crust and upper mantle using earthquake data
Adjoint tomography, an advanced full waveform inversion based on spectral-element and adjoint methods, helps obtain high-fidelity seismic images of the Earth’s interior with the more accurate theory and numerical method, i.e., 3-D finite-frequency theory and spectral-element method. It is particularly effective in assimilating large seismic waveform data sets to update low-resolution 3-D models iteratively to converge to an optimal high-resolution model that can better predict the waveforms. Below is an example of adjoint tomography model evolution for the study region of East Asia (Chen et al., 2015, JGR).
We are currently working on using adjoint tomography to develop 3-D anisotropic wavespeed models of the crust and mantle beneath a couple study regions including East Asia and Southwest Europe. Our main research objective focuses on providing robust seismological constraints of the lithosphere, asthenosphere, and partial melt in the crust and upper mantle by better recovering the patterns and the strength of seismic wavespeed anomalies. These constraints can help test the hypotheses regarding the tectonic evolution of different landform features, for example, the Tibetan Plateau, the Hangai Dome in central Mongolia, the Changbaishan Volcano in northeast China, and the Betics, the Gibraltar Arc, the Alboran Sea, the Rif, and the Atlas Mountains in the western Mediterranean. Our latest publication (Chen et al., 2017, Nature Communications) highlights the new finding beneath the Tibetan Plateau, a T-shaped high wave speed structure beneath South-Central Tibet, interpreted as an upper-mantle remnant from earlier lithospheric foundering, spatially correlated with ultrapotassic and adakitic magmatism, supporting the hypothesis of convective removal of thickened Tibetan lithosphere causing major uplift of Southern Tibet during the Oligocene.
Chen, M., Niu, F., Tromp, J., Lenardic, A., Lee, C.-T. A., Cao, W., and Ribeiro, J. (2017), Lithospheric foundering and underthrusting imaged beneath Tibet, Nature Communications, doi:10.1038/ncomms15659. [link] [PDF download]
Chen, M., Niu, F., Liu, Q., and Tromp, J. (2015), Mantle-driven uplift of Hangai Dome: New seismic constraints from adjoint tomography, Geophys. Res. Lett., 42, doi:10.1002/2015GL065018. [link]
Chen, M., Niu, F., Liu, Q., Tromp, J. and Zheng, X. (2015), Multiparameter adjoint tomography of the crust and upper mantle beneath East Asia: 1. Model construction and comparisons, J. Geophys. Res., doi:10.1002/2014JB011638. [link]
2. Ambient noise adjoint tomography of the crust and uppermost mantle
Ambient noise has been a novel source of data aside from earthquakes. In the region lacking earthquake records but with dense arrays, the ambient noise interferometry waveforms provide the key to resolve the subsurface structure at a resolution determined by the inter-station spacing and array aperture. The array deployments in China, Japan, Europe and U.S. serve such purposes of providing ambient noise data in addition to earthquake data for better constraining the crust and uppermost mantle structure. Traditional ambient noise tomography requires intermediate steps of measuring dispersion curves by as- suming 1-D layered model between station pairs and constructing group and/or phase velocity maps. Different from traditional tomography, ambient adjoint tomography (Chen et al., 2014, GRL) incorporates a spectral-element method (SEM) and takes empirical Green’s functions (EGFs) of Rayleigh waves from ambient noise interferometry as the direct observation. The frequency-dependent traveltime misfits between SEM synthetic Green’s functions and EGFs are minimized with a preconditioned conjugate gradient method, meanwhile the 3-D model gets improved iteratively utilizing 3-D finite-frequency kernels. The ambient adjoint tomography serves a better approach to more accurately recover the strong wave speed anomaly pattern and amplitude in the crust and uppermost mantle at regional scale. Our study of SE Tibet in particular reveals a more complex and disconnected low wave speed zones indicative of partial melt in the mid-lower crust, contrasting with the pervasive narrow zone from the channel flow models.
Liu, Y., Niu, F., Chen, M., and Yang, W. (2017), 3-D crustal and uppermost mantle structure beneath NE China revealed by ambient noise adjoint tomography, Earth and Planetary Science Letters, doi:10.1016/j.epsl.2016.12.029. [link]
Xing, G., Niu, F., Chen, M., Yang, Y. (2016), Effects of shallow density structure on the inversion for crustal shear wavespeeds in surface wave tomography, Geophysical Journal International, doi: 10.1093/gji/ggw064. [link]
Chen, M., Huang, H., Yao, H., van der Hilst, R. D. and Niu, F. (2014), Low wave speed zones in the crust beneath the SE Tibet revealed by ambient noise adjoint tomography, Geophys. Res. Lett., Vol. 41, No.2, 334-340, doi:10.1002/2013GL058476. [link]