本研究分析1999年台灣集集地震之震後變形，使用台灣中部以及東部橫跨地震後14年的GPS連續觀測與定期觀測結果，進行時間序列分析得到集集地震震後地表位移。我們透過彈性半空間斷層錯位模型與體積應變模型，分別考慮車籠埔斷層以及台灣深部流變構造的幾何形態，建立斷層模型與黏彈性模型之格林函數，並且利用震後地表速度作為資料進行反演，計算斷層面上的震後滑移速率，以及下部地殼的應變速率隨時間的演進，藉此估算斷層面上的速度－狀態相關摩擦係數（a–b）及下部地殼的有效黏滯係數。我們發現，若加入考慮下部地殼的應變，將會得到截然不同的斷層滑移分布。反演結果顯示車籠埔斷層南段的斷層滑移速率，在集集地震震後初期可達到1 m/yr，並且在震後四年內快速衰減至低於0.1 m/yr，並估算出斷層面上的速度－狀態相關摩擦係數（a–b）約為9.4e-3，與前人研究所認為的量值相符合。然而，在有效黏滯係數的估算上，本研究使用岩石試驗參數所估算的有效黏滯係數比前人所認為的大了約10倍，可能代表實驗室與現地環境溫壓條件的不同，導致此計算仍有相當程度的不確定性。若是考慮同震庫倫應力變化，有效黏滯係數皆大約由1e18 Pa•s遞增至1e19 Pa•s，與前人研究所認為的範圍差異不大。未來若能使用更符合現地環境狀態的岩石試驗參數，此分析方法將能更有效地評估震後下部地殼與上部地函之間的黏滯係數隨時間的變化情形。

We use GPS observations in central and eastern Taiwan to derive the postseismic deformation following the 1999 Mw 7.6 Chi-Chi, Taiwan earthquake across 14 years. Combining 3D half-space dislocation model and finite-volume strain model with the geometries of the Chelungpu fault and the rheological structure beneath Taiwan, we calculate the slip rate on the fault and the strain rate in the lower crust by linear inversion. The comparison between the afterslip model and the model combining with strain in the lower crust shows the different slip distribution. The inversion shows that the slip rate on the southern segment of the Chelungpu fault decays from 1 m/yr to 0.1 m/yr during the four years after the mainshock. The steady-state friction property （a–b）is about 9.4e-3, which is in good agreement with several previous studies. However, our estimation on the effective viscosity by the laboratory-measured parameters fails to agree with the general values, which may cause by the different pressure-temperature conditions between the experiment and in-situ environment. If we constrain the calculation by the Coulomb stress change from the coseismic slip model of Chi-Chi earthquake, the effective viscosity varies from about 1e18 Pa•s to 1e19 Pa•s during the 14 years after the mainshock, which falls within a reasonable range.

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