| dc.description.abstract | GPS (Globe Positioning System) is widely used to measure crustal deformation in recent years. Besides tectonic movement, environmental factors also contribute a considerable portion of measured deformation. The influence of environmental factors on altitude variation is significant. Especially, Taiwan is an island easily affected by tides, air temperature, air pressure and water-storage loading etc. The surveyed altitude data with continuous GPS stations built by IES (Institute of Earth Sciences in Academia Sinica) and MOI (Ministry of Interior) from January 1994 to middle of 1999 were used in this study.10 local weather observation stations that are adjacent to the GPS stations we also collected. Additional, we used ground water leveling data of Laupi to analyze if height of S23R is affected by ground water change.
We apply three different methods to understand the relationship between continuous altitude variations and environmental factors. (1) We separated data into IES, MOI and five local regions then employed PCA (Principle Component Analysis) to understand principle modes of height variation in time series. (2) We employed FFT (Fast Fourier Transform) to know cycling change of each station. (3) First, we used simple regression and statistic test to interpret influence of particular factor. Then we employed forward selecting and MRA (multiple regressions analysis) to build up the influenced model of environmental factors.
Conclusion of this study can be summarized as follows: Except in north region, annual and semiannual period can be analyzed in GPS height variation by PCA. Because data lengths are not long enough, we can only detect 14, 28 days and else smaller than 200 days periods signal by FFT. Approximately, the effect of environment factors listed according to magnitude include Sa tide, air temperature, Mf tide, air pressure, relative humility, Ssa tide, rainfall and Mm tide. Average amplitude affected by Sa, Ssa, Mm, Mf tides are 4.56, 3.39, 2.19, 5.46mm, respectively, and 0.44mm/℃, -0.40mm/mb by air temperature and pressure. It means ocean tide correction still is not enough either in island or near coast. Concrete, soil thermal expansion and atmosphere loading may cause vertical displacement. There are other geophysical sources that are not considered in this study should be investigated in future. | en_US |