RESEARCH
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My main research interests include seismic imaging using earthquake waveforms and ambient noise, lithospheric structure and deformation, earthquake rupture processes, array analysis, and geophysical inversion methods.

Building the community velocity model in the Sichuan-Yunnan region, Southwest China

Since 2017, with the support of the China Seismic Experimental Site (CSES) of China Earthquake Administration (CEA), we have started to build the multi-scale community velocity model (CVM) in the Sichuan-Yunnan region, Southwest China. We are aiming to construct CVMs of four different scales in Sichuan and Yunnan, namely the regional block scale, fault zone scale, fault scale, and urban scale, with its lateral resolution about 10-50km, 1-5km, 0.1-0.5km and 1km, respectively. We first used the point-wise joint inversion of ambient noise surface wave dispersion, teleseismic Rayleigh wave ZH amplitude ratio and P-wave receiver function data from 114 permanent stations to a 3-D shear wave velocity model of the crust and uppermost mantle in the Sichuan-Yunnan area (Yang, Yao et al., 2020)  (https://github.com/yanyangg/SW_China_Vs_model). Then based on this initial 3-D reference model, we further constructed the first version of the regional CVM in Sichuan & Yunnan, that is, SWChinaCVM-1.0 (https://github.com/liuyingustc/SWChinaCVM), from joint inversion of dense body wave and surface wave travel times. The lateral resolution of this model can reach 50km in most regions. 



 With the dense arrays in the fault zone regions and basins, we are now starting to build high resolution models of other scales, in order to form the real multi-scale CVM in Sichuan and Yunnan.


Direct Inversion of 3D Radially Anisotropic Vs Structure from Surface Wave Traveltime Data

A novel method is implemented to invert Rayleigh and Love wave dispersion curves of all paths jointly for 3-D shear wave velocity and radial anisotropy parameter simultaneously without intermediate steps. This new approach is based on our previous method of direct inversion of 3-D isotropic Vs model (DSurfTomo, Fang, Yao, et al., 2015). Conventional approach requires the computation of division Vsh over Vsv after inversion, which is sometimes unstable. The new parameterization in our approve allows for direct inversion of the 3-D radial anisotropy parameter (Vsh/Vsv). Therefore, spatial smoothing can be directly applied to  (Vsh/Vsv), which ensures a more stable inversion of (Vsh/Vsv). We use the method to derive high-precision crustal shear wave velocity and radial anisotropy models around the eastern Himalayan syntaxis using ambient noise dispersion data (5-40s). Results show the crust can be divided into several subregions with different rigidity and preferred mineral alignment orientation depth-dependently. 

The DRadiSurfTomo code, surface wave dispersion data, and the resulting model files in our paper (Hu, Yao, Huang, 2020, JGR) can be downloaded online (https://doi.org/10.5281/zenodo.3592528). 

Direct Inversion of 3D Azimuthally Anisotropic Vs Structure from Surface Wave Traveltime Data

 Azimuthal anisotropy retrieved from surface waves is important for constraining depthvarying deformation patterns in the crust and upper mantle. We present a direct inversion technique for the three‐dimensional shear wave speed azimuthal anisotropy based on mixed‐path surface wave traveltime data. This new method includes two steps: (1) inversion for the 3‐D isotropic Vsv model directly from Rayleigh wave traveltimes and (2) joint inversion for both 3‐D Vsv azimuthal anisotropy and additional 3‐D isotropic Vsv perturbation. The joint inversion can significantly mitigate the trade‐off between strong heterogeneity and anisotropy. With frequency‐dependent ray tracing based on 2‐D isotropic phase speed maps, the new method takes into account the ray‐bending effect on surface wave propagation. (Reference:Liu, C., Yao, H.*, Yang, H.‐Y.*, et al, 2019, JGR, https://doi.org/10.1029/2018JB016920)

DAzimSurfTomo package Download: https://github.com/Chuanming-Liu/DAzimSurfTomo

New seismic tomographic methods and joint inversion

1. We have developed a new joint inversion methods for better imaging 3-D Vp and Vs crust structure using direct inversion of surface wave travel times (based on frequency-dependent ray tracing) and body wave travel times. (Fang, Zhang, Yao, et al., 2016). With a new parameterization approach, we have proposed a new method of directly invert for more reliable 3-D Vp/Vs models using both body and surface wave travel times (Fang, Yao , Zhang et al., 2019, GJI).

2. We have developed a joint inversion method (using the Neighborhood Algorithm) that combines dispersion data and Rayleigh wave ZH ratios for the inversion of crustal Vs and Vp/Vs.  (Yuan, Yao, Qin, 2016, Chin. J. Geophys.). In addition, we have developed an iterative linearized inversion approach for 1-D crustal Vs structure as well as interfaces using surface wave dispersion, Rayleigh wave ZH ratio and P wave receiver functions. (Zhang P. & Yao, 2017, Earthquake Science)

3. We have developed a linear array ambient noise adjoint tomography strategy for imaging high-resolution crustal Vs structure. (Zhang C., Yao, et al., 2018, JGR)

Direct Inversion of 3D isotropic Vs Structure from Ambient Noise and Surface Wave Traveltime Data

We proposed a method to invert surface wave dispersion (traveltime) data directly for 3-D variations of shear wave speed, that is, without the intermediate step of phase or group velocity maps, using frequency-dependent ray tracing (Fang, Yao*, et al., 2015, GJI). A fast marching method is used to compute, at each period, surface wave travel times and ray paths between sources and receivers. This avoids the assumption of great-circle propagation that is used in most surface wave tomographic studies, but which is not appropriate in complex media. We represent the 3-D shear wave speed model by means of 1-D profiles beneath grid points and the wave speed model is estimated with an iteratively reweighted least squares algorithm, and upon iteration the surface wave ray paths and the data sensitivity matrix are updated using the newly obtained wave speed model. This method has been applied to Taipei Basin in Taiwan (Fang, Yao, et al., 2015, GJI), Hefei urban area (Li, Yao et al., 2016, SRL), fault zone regions (Gu et al., 2019, PAGEOPH), a shale gas production field in SW China (Liu et al., 2018, JAG), Taiwan Strait (Zhang, Yao et al., 2018 JGR), and many other regions. 

(DSurfTomo Package Download: https://github.com/HongjianFang/DSurfTomo)

Earthquake Rupture Imaging from Compressive Sensing and Back Projection

1. We developed compressive sensing (CS) techniques in the frequency domain (Yao et al., 2011, GRL; Yin & Yao, 2016, GJI) and wavelet domain (Fang, Yao, Zhang, 2018, GJI) to image frequency-dependent seismic radiation and rupture processes of great earthquakes. For the four largest megathrust earthquakes in the past 10 years. Our results reveal generally low-frequency radiation closer to the trench at shallower depths and high-frequency radiation farther from the trench at greater depths (Yao et al., 2011, GRL; Yao et al., 2013, PNAS; Yin et al., 2016, GRL, ...). Together with coseismic slip models and early aftershock locations, our results suggest depth-varying frictional properties at the subducting plate interfaces.

2. We developed an iterative backprojection method with subevent signal stripping to determine the distribution of subevents (large energy bursts) during the earthquake rupture.We also relocate the subevents initially determined by iterative backprojection using the traveltime shifts from subevent waveform cross-correlation, which provides more accurate subevent locations and source times. Subevents location results for the 2011 Mw 9.0 Tohoku-Oki Earthquake (Yao et al., 2012, GJI, PDF download)

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Copyright © 2015 Huajian Yao All Rights Reserved
Email: hjyao @ ustc.edu.cn Tel: +86 - 551 - 63607201 Add: 96 Jinzhai Road, Hefei, Anhui Province, 230026, China