為探討1999年集集地震震源區受到該次大地震釋放應力後，可能引起的應力調整作用，本研究綜合分析了集集地震震源區之地震活動、震源機制與地殼應力，並應用多種型態辨識方法由餘震分佈辨識斷層面，進而分析集集主餘震與已知斷層構造之關連性。
就地震活動分析而言，本研究使用地震群集法客觀篩選出1999年集集大地震之餘震資料，然後再應用震源疊合法，重新定位調整餘震活動，結果發現餘震活動顯著形成6個集集大地震前所沒有的群集現象。另由b值分析顯示集集大地震前之地震活動，較大規模的次數明顯不足，集集大地震後的餘震活動彌補了大量較大規模次數。藉由移動時窗方式之b值時序分析，本研究發現自1994年到集集大地震前b值有逐漸增大之線性特徵，此一異常現象是否為地震前兆，值得進一步深入探討。
就震源機制分析而言，集集地震震源區之較大與較深地震以逆斷層為主，較小與較淺地震則以走向滑移斷層為主。換言之，在集集震源區逆斷層機制主導大規模的地震，反應區域性的地殼應力型態。就空間分佈而言，車籠埔斷層東側之斷層上盤區域，主要受到逆斷層機制作用；在車籠埔斷層東南方與車籠埔斷層南北兩端的區域，主要受到走向滑移斷層機制作用；在車籠埔斷層中段與中央山脈東側則主要受到正斷層機制作用。各類震源機制隨著時間序列以近乎等比例方式發生，這意味著集集大地震引發大區域全面性的地體構造運動，因此三種震源機制個數都以大約相等比例的方式隨著時間觸發。
餘震分佈為辨識斷層面常用的傳統方法，本研究成功的應用多種型態辨識方法，包括地震群集法、震源疊合法、主分量分析與等面積投影法，分析餘震活動之空間分佈形貌，並搭配對比震源機制解已知的二個節面，藉以辨識斷層面。本研究總共辨識得出115個斷層面，提供後續斷層構造與地震活動相關性研究之寶貴線索，包括先存的斷層、新生的斷層或者是潛藏地下的盲斷層。
就地殼應力分析而言，藉由規模與震矩之經驗關係先由地震規模估算震矩值，再與震源機制結合而成經驗震矩張量，最後透過震矩張量總和方法，本研究分析1999年集集地震前後在震源區地殼應力之分佈與變化。結果顯示，雖然集集地震前後震源區之地震活動與震源機制分佈有很大差異，但是整體的地殼應力型態並無改變。用網格方式的應力估計結果，本研究為集集震源區建立了一個經驗震源機制、震矩張量與地殼應力資料庫。
綜合以上地震活動、震源機制與地殼應力分析的結果，本研究發現在走向滑移的構造環境下，伴隨地震之變形多集中於走向滑移斷層帶附近，地震的規模較小；在擠壓的構造環境下，伴隨地震之變形多分佈於廣大區域和比較分散的斷層帶上，甚至觸發先前存在的斷層構造重新活動，不但會引發大規模的地震，其影響範圍亦較廣泛。

In order to investigate stress adjustments in the source region of the 1999 Chi-Chi earthquake, the regional seismic activities, focal mechanisms, and crustal stresses are together analyzed in this study. In addition, we have identified the true fault planes by applying several methods of pattern recognition on aftershock distributions. The results are further used to examine their relevance to the known fault structures.
In the study of seismic activities, we first define the aftershocks of the Chi-Chi earthquake by the earthquake clustering method. The locations of aftershocks are then adjusted by the hypocenter collapsing method. The results show six distinct clusters of aftershocks. An analysis of the b value shows a lack of larger earthquakes before the Chi-Chi earthquake which was subsequently compensated by intensive its aftershocks. Using a moving window in time, we have observed that the b value gradually increases from 0.8 to 1.0 beginning 1994 until 1999. Whether this anomalously linear increase of the b value is a precursor of the Chi-Chi earthquake deserves further investigations.
In the study of focal mechanisms, we found the larger and deeper earthquakes often exhibit thrust faulting whereas smaller and shallower earthquakes exhibit strike-slip type. The fact that larger earthquakes are dominantly thrust faulting is a reflection of the regional crustal stress regimes in the Chi-Chi source. With respect to the Chelungpu fault, thrust faulting dominates the hanging wall areas to the east, strike-slip faulting near its southern and northern ends, and southeastern side, whereas normal faulting in its central part and to the eastern side of the Central Mountain Range. Furthermore, the relative ratios of the numbers of the three types of focal mechanism are almost constant throughout the three-year period after the Chi-Chi.
Regarding fault plane identification, a traditional way to effectively identify a fault plane is based on aftershock distributions. We have successfully applied several methods of pattern recognition, including earthquake clustering, hypocenter collapsing, principal component analysis, and equal area projection, to determine the true fault plane from the two nodal planes of a focal mechanism. The results of 115 identified fault planes are informative for subsequent studies on possible association of seismic activities with fault structures, including pre-existing faults, new fractures, or underground blind faults.
In the study of crustal stresses, we analyzed the stress patterns and changes of the source region due to the Chi-Chi earthquake by the method of moment tensor summation, where the moment tensor of an individual earthquake is constructed from its focal mechanism solution and a scalar moment derived from the moment-magnitude relationship. The results reveal that although the distributions of seismicity and focal-mechanism patterns are different before and after the 1999 Chi-Chi earthquake, the overall patterns of crustal stresses remain similar. A grid analysis of stress estimation has established a database of empirical focal mechanisms, moment tensors, and crustal stresses of the Chi-Chi source region.
In summary, the results of composite analyses of seismic activities, focal mechanisms, and crustal stresses can be summed up as follows. In a shear stress environment, the strike-slip deformation is usually concentrated around known faults, often induces earthquakes with small magnitudes. In a compressional stress environment, the thrust deformation often induces earthquakes which will distribute in broad areas and usually triggers complex types of focal mechanisms on pre-existing faults with significant magnitudes.

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