| Summary: | 碩士 === 國立成功大學 === 地球科學系碩博士班 === 95 === Postseismic slip from large earthquake often reflects frictional properties of the fault zone. Frictional properties of fault can be used to understand the rupture process and the earthquake mechanism. The Chengkung earthquake of magnitude Mw 6.8 occurred in eastern Taiwan on December 10th 2003, resulting from rupture of the Chihshang fault. According to the distribution of the earthquakes, the Chihshang fault is a high-angle reverse fault. It is rapidly creeping and capable of producing large earthquake. We used 11 continuous Global Positioning System （CGPS） data and Bernese software v4.2 to obtain the precise station coordinates relative to Paisha, Penghu in the period of 5 months after the Chengkung earthquake. The largest horizontal and vertical postseismic displacements are 85.8 and 83.7 mm, respectively. This research is to understand the frictional properties of the Chihshang fault by analysing postseismic slip of the Chengkung earthquake.
Afterslip was determined by inverting CGPS data using 3D elastic dislocation model. The results showed that the slip was focused on shallow part of fault （0 - 8 km）. The maximum afterslip is 641 mm at the depth of 4 km. The geodetic moment is 0.85×1026 dyne-cm which is far greater than aftershock which is 4.69×1024 dyne-cm, indicating that afterslip occurred aseismically. We estimate coseismic slip by 3D elastic dislocation model, and estimate afterslip and σ（a - b） （σ is normal stress; a and b are frictional parameters） by rate-and-state friction model. We found that the coseismic slip distribution must be quite compact in order to generate a stress perturbation which is large enough to generate the rapid afterslip. According the distribution of σ（a - b）, the value of σ（a - b） at north part of the Chihshang fault is large, indicating this section is stable sliding. The small value of σ（a - b） on the shallow part of the fault, and the lateral heterogeneity in frictional parameters result in a localization of afterslip. The results correlated well between two models. The areas of the coseismic surface rupture are controlled by the distribution of σ（a - b） near surface. In the stress increase region, the rupture will be occurred on the area with σ（a - b） value close to zero but not on the area with large σ（a - b）.
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