Imaging Crust-to-Upper Mantle Seismic Velocity Structures by Joint Inversion of Multiple Datasets: Cases of Vietnam and Taiwan

• Main Authors: Hsin-Hua Huang, 黃信樺
• Other Authors: Yih-Min Wu
• Format: Others
• Language: en_US
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/33312411724570758302

博士 === 國立臺灣大學 === 地質科學研究所 === 101 === Seismic tomography technique has been widely applied to different tectonic environments around the world and proved as a powerful tool to retrieving the information of Earth’s interior. The understanding of complex tectonic processes largely relies on how extensive and accurate structural imaging we can resolve, and is therefore of primary importance to proceed. In this dissertation, we attempted to improve our imaging ability in a scale of crust-to-upper mantle by the joint inversion with multiple datasets, including various phases of seismic waves and geotechnical data, to better clarify the tectonic interpretations for the Vietnam and the Taiwan region. The dissertation mainly consists of three projects. In the first, with a newly deployed portable broadband array and the local seismic network in northern Vietnam, we used the crustal P-wave (Pg) and uppermost-mantle head-wave (Pn) data to obtain the first local 3-D P-wave velocity model and the related Moho depth variation. Our results show a good correlation with the surface geology and major structures, and reveal that the RRSZ is likely a lithospheric-scale structure penetrating to the uppermost mantle with mantle thermal anomalies. The northern Vietnam in general appears to possess a weak crust and hot upper mantle with a long and complex thermo-tectonic history probably induced by the past delamination. In the second project, we studied the velocity structures of Taiwan orogen by jointly using the seismological and geotechnical data with a highly dense seismic network. Although the previous tomographic achievements in Taiwan region have been fruitful, most of them were P-wave velocities (Vp), or with Vp/Vs ratio jointly, but few for S-wave velocities (Vs). However, to better unravel the tectonic complexity of Taiwan, interpreting with all three indicators could be crucial. But models from different studies are hard to compare and verify to each other due to different datasets and inversion settings. Therefore, we attempted to provide a new set of Vp, Vs, and Vp/Vs models in comparable resolution and internal consistency by an elaborate joint-inversion scheme; in which a special constraint by the borehole logging data was also imposed on the near-surface part of model where is usually poor-resolved by ordinary inversion. Derived models show tremendous changes of shallow velocities than previous studies and copious information with mutual verifications of Vp, Vs, and Vp/Vs, rendering us to clearly delineate the west-dipping interaction between the Eurasian lithosphere and the subducting Philippine Sea plate, and the southward evolution of the Coastal Range and forearc basement. To further expand the model resolution to the deeper depth to explore the slab geometry beneath Taiwan region (especially for north of 23°N latitude), in the third project we measured and combined the teleseismic data into tomographic inversion. Rather than the direct joint inversion, we adopted a two-step strategy to invert an accurate local model first by local data only, and then implement a nonlinear joint inversion with the teleseismic data together (without fixing the ray-incident points at the model bottom). Results show that the deep extension of the eastward subducting Eurasian plate can be retrieved readily to ca. 200 km deep with a plausible slab deflection around the latitude 23.2°N. We then constructed a 3-D schematic model accordingly for Taiwan region. We ultimately summarized the achievements for respective projects, and their prospections for future works. In addition, two side projects done in my Ph.D period are also briefly introduced and supplemented: One is studying the out-of-ordinary Jiasian, Taiwan earthquake (Mw=6.3) for its tectonic implications, and the other is using the spatial seismicity and stress pattern to explore and construct the northern-Taiwan kinematic model.