岩石的流變性質對於研究地球岩石圈內之應力演變及地表形變與地震週期間的關連上是項很重要的控制量。本研究計畫主要利用由GPS及InSAR所觀測的地表運動資料，研究美國西部山間地震帶（Intermountain seismic belt，ISB）幾個活動正斷層區域中岩石圈的流變特性。資料方面主要是由美國EarthScope PBO大型計畫所建立超過100站的連續 GPS觀測網觀測所得，並佐以其他相關地球物理資料如地震與重力等。研究中將利用二維的黏彈解析模型[Politz, 2003]及地表運動之觀測對岩石圈的流變結構進行分析，並進一步嘗試以三維有限元素法（Pylith）來推算複雜地體系統中如火山及斷層等構造間的應力交互作用。這項研究計畫將首先利用ISB中的兩個歷史大地震，1959年Hebgen Lake地震（規模 7.5）與1983年Borah Peak地震（規模7.3），之震後形變量來反演岩石圈之流變結構。由於此斷層帶緊臨黃石火山系統的西北部，故將使用側向變化的二維流變模型[Politz, 2003]來探討此火山系統的流變特性以及火山活動（如岩漿遷移）與地震間的時空關連。計畫的第二階段將根據GPS觀測的地表運動及古地震資料（至~6000年前），對沿美國落磯山脈東側長370公里的Wasatch斷層帶的應變與應力的時空演化進行模式分析，探討此斷層之地震週期性及其對時變地震災害估算的影響。研究中除了使用上述之二維黏彈解析模型外，也將嘗試以有限元素法對三維流變模式進行分析。特別注意的是，這個計畫的研究區域涵蓋比較簡單的地體系統；而且計畫主持人對此區域有十年以上之研究經驗，故在執行上比直接研究台灣地區有效率。此計畫後我會將所獲得的流變模式分析之經驗與結果，利用在複雜的台灣弧陸碰撞及造山系統上。這將對台灣地區的地體動力研究提供基本且重要的參考流變模型。 The rheology of viscously deforming rocks at depth is of fundamental importance when trying to understand stress evolution of the earthquake cycle and therefore time-dependent seismic hazard along active faults. The highlight of this research project will be on the study of kinematics and dynamics of continental deformation in the Intermountain seismic belt (ISB) of the western United States, with application to a more complex tectonic regime of the Taiwan area. Continuous GPS measurements together with other constraints from seismic and gravity data will be used to model the properties of lithospheric rheology. Both semi-analytic and numerical finite-element modeling methods will be employed to evaluate the effect of the tectonic complexity and time-dependent stress inter-actions among various geological structures. This two-year project will first focus on the study of postseismic deformation of the 1959 Ms=7.5 Hebgen Lake and 1983 Ms=7.3 Borah Peak earthquakes, two largest historic normal-faulting earthquakes of the western U.S. interior (Fig. 1). A 2-D semi-analytic viscoelastic modeling method [Politz, 2003] will be employed to evaluate the lithospheric rheologic structure based on time-dependent changes in crustal motion determined by measurements from GPS arrays deployed by the EarthScope PBO (Fig. 1). Note that the proximity of the Hebgen Lake fault zone to the Yellowstone volcanic system will be addressed in this project for studying the mechanism of interaction between earthquake faulting and volcanic fields. Furthermore, contemporary crustal deformation along the 370 km-long Wasatch fault zone, Utah, measured also by GPS stations of EarthScope PBO, will be modeled for elastic and viscoelastic properties for the continental lithosphere. A finite element modeling method (Pylith) will then be employed to evaluate the stress evolution due to viscoelastic relaxation of the lithosphere, and its relation to earthquake cycle based on the time history of paleoearthquakes of the Wasatch fault zone will be addresses. This numerical modeling method can be applied to study the postseimic deformation of the 1999 Chi-Chi earthquake and its implications to the temporal evolution of lithospheric strength in the Taiwan deforming region. 研究期間：9908 ~ 10007
