| Summary: | 碩士 === 國立中央大學 === 地球物理研究所 === 92 === GPS time series analysis and modeling studies of crustal deformation in southwestern Taiwan
Min-Chien Tsai
Abstract
Global Position System (GPS) has become an efficient tool for studying crustal deformation and geodynamics. In the event of a major earthquake, continuous GPS data are useful for researches on interseismic, coseismic and postseismic deformations. Combining with results from near-fault dense-spacing GPS surveys, we may detect locations and deformations of blind faults and estimate the fault-slip rates. These important information can be used for earthquakes potential evaluation. The southwestern Taiwan is an active tectonic area. Frequently occurred earthquakes indicate that this area now is in a period of highly seismic activities. In this study, we use GPS time series analysis to correct 1993-2002 velocity field in southwestern Taiwan. In order to understand the active tectonic structures in this area, we assume an appropriate fault model in elastic half-space based on geological, earthquake, and seismic reflection data, to invert for optimal fault geometry and slip rate.
Stacking of power spectral densities from continuous GPS data in southwestern Taiwan, we found the slopes of spectra (spectral index) are -0.61, -0.62, and -0.55, for the E, N, U components, respectively. This result and former studies indicate the continuous GPS data are best described as a combination of white noise and flicker noise. Hence we use 「white noise + flicker noise model」as noise model for GPS time series analysis in this study. The observed motion of long-period recording continuous GPS station can be described by some model parameters such as linear rate, annual periodic and semi-annual periodic motions, coseismic offsets, postseismic rate change, and exponential decay after earthquakes. The maximum likelihood estimator (MLE) is utilized to estimate the amplitudes of white and flicker noise in the postfit residuals of each time series. The full covariance matrix is the sum of the white and flicker noise covariance matrices; it can be employed to model time series data again. By iterating foregoing steps, we can obtain the cleaned time series data and final model parameters. When the postfit residuals show a random distribution, it means the corrections are essentially complete. The mean RMS values of resdiuals which represent for precision of continuous GPS observations are 3.2mm, 2.2mm, and 9.7mm in the east, north and up components, respectively. We use the results from GPS time series analysis to estimate the 1993-2002 velocity field of southwestern Taiwan. With respect to Paisha (S01R) of Penghu , the horizontal velocity field shows obviously decrease from west to the east of Jiuchiunken－Muchiliao－Liuchia fault, and even decrease to nearly zero at the Chianan coastal area. On the other hand, the velocities increase southward and their directions become southwesterly to the south of Tsochen fault. It reaches to 50 mm/yr in the Kaohsiung－Pingtung area. This implies the complexity of structures in southwestern Taiwan.
Based on the corrected velocity field, we assume two fault models in the inversion. The basis of model fault geometry is the thin-skinned theory that proposed a decollment underneath the fold-thrust belt in western Taiwan. The difference between the two models is one with a decollment only, and the other has a thrust fault extending to the surface from decollment. The results of inversion by grid search and bootstrap method indicate the surface traces of the thrust fault is near the Jiuchiunken－Muchiliao－Liuchia fault, but the slip rates on the extending fault plane are relatively small. In other words, the model without extending fault is enough to describe the GPS observations on the surface. When the surface projection for the western upper edge of decollment is very close to Chuko fault, average slip rate on decollment is 44.6 mm/yr. The optimal depth and dip are 5 km and 0°, respectively. It is a sub-horizontal decollment. However, there is a high correlation between two model parameters, position and depth. Down to 15 km, the depth of decollment becomes larger when the western edge of decollment moves further east. Hsu et al.（2003） used a GPS surface velocity field of Taiwan for the time period between 1993 and 1999 to infer interseismic slip rates on subsurface faults, their result show there exists a nearly horizontal decollment at a depth of 10 km and the slip rate on decollment is 40 mm/yr, consistent with our conclusion from this study.
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