Normal-mode synthetics and application – Surface-wave tomography beneath southern Taiwan
| Autor: | Hsin-Ying Yang, 楊欣穎 |
|---|---|
| Rok vydání: | 2012 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 100 Normal mode theory, which compromises accuracy and efficiency in computation, allows us to investigate wave propagations in a spherically symmetric earth model. This theory has been widely used in exploring the resonant frequencies of the Earth excited by large earthquakes, and is able to provide insights into seismic structures of the Earth. In this thesis, we focus on synthetic waveforms expressed by normal-mode summation as well as application to surface-wave tomography. A new derivation of displacement in response to seismic sources in terms of the modal summation of the Earth is presented. Starting from the well-known formal expression of normal-mode summation, we obtain the explicit expressions of displacements by means of generalized spherical harmonics and coordinate transformation. Moreover, numerical experiments of normal-mode synthetics are conducted to investigate the influences of self-gravitation, attenuation, radial anisotropy, density and P-wave speed on seismic waveforms. We develop a wavelet-based measurement of surface-wave phase velocities and then apply this method to a linear temporary array with ~140 km span in southern Taiwan to obtain shear-wave speed structures in the crust and uppermost mantle. Synthetic tests using waveforms in both spherically symmetric and 3D heterogeneous models verify the reliability of our measurement approach. Our seismic images reveal similar features in the crust to those constrained by local body-wave travel-time tomography, while yielding more robust features down to the depth of 70 km. Our model particularly improves the resolution beneath western Taiwan, where the lower crust is characterized by high seismic speed. This feature is probably pertain to magmatic underplating as a consequence of the opening of the South China Sea and has remained unscathed during arc-continent collision. Our tomography model provides new reliable seismological evidence for the tectonic evolution of Taiwan as a result of the convergence between Eurasian and Philippine Sea Plates. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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