Anisotropic Crustal Structure of Taiwan Orogen Constrained by Ambient Seismic Noises
Date Issued
2015
Date
2015
Author(s)
Huang, Tzu-Ying
Abstract
We apply the ambient noise tomography (ANT) to Taiwan. In ANT, the path coverage is directly provided by the available inter-station paths. The high-density seismic stations in Taiwan thus offer a great opportunity for high resolution tomography with ANT. Besides two major seismic networks, Central Weather Bureau Seismic Network (CWBSN), and Broadband Array in Taiwan for Seismology (BATS), we have also incorporated the continuous broad-band data from three east-west linear arrays of the TAiwan Integrated GEodynamics Research (TAIGER) project. With above permanent and temporary seismic networks, we have achieved unprecedented path coverage of surface wave study in Taiwan. In particular, the unique geometry of TAIGER arrays allows us to largely improve the lateral resolution of the NNE-SSW Taiwan tectonic trend. We construct 2-D surface wave velocity maps form 4 to 20 sec using a wavelet-based multi-scale inversion technique. Patterns of lateral variations of our shorter period (<10 seconds) model demonstrate very good correlation with the surficial geology, whereas the overall structure, albeit with much better resolution in the shallow depth, is generally consistent with previously established body wave models. The absence of the source of Bouguer gravity anomaly in our model implying that it is likely caused by a deeper mountain root. We also investigate the crustal azimuthal anisotropy of Taiwan using seismic ambient noises. The reliability of the pattern of the resulting anisotropy is supported by the synthetic test and experiments of various azimuthal weighting schemes in 2-D. With iterative approaches, we then report 3-D seismic anisotropic tomography of Taiwan that shows a nearly 90° rotation of anisotropic fabrics across 10-20 km depth consistent with the presence of two layers of deformation. The upper crust is dominated by collision-related compressional deformation, whereas the lower crust, or the crust of the subducted Eurasian plate, is dominated by convergence-parallel shear deformation. This lower crustal shearing is interpreted as driven by the continuous sinking of the Eurasian mantle lithosphere when the surface of the plate is coupled with the orogen. The two-layer deformation redefines the role of subduction in the formation of the Taiwan mountain belt.
Subjects
ambient noise tomography
surface wave
wavelet-based multi-scale inversion
Bouguer gravity anomaly
crustal seismic anisotropy
Taiwan orogen
Type
thesis
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