| 摘要: | 台灣車籠埔斷層鑽井計畫（Taiwan Chelungpu-fault Drilling Progarm，TCDP）於2004年12月在台中大坑完成達兩公里的深井鑽探，並且在深約1,111公尺處發現了新鮮的斷層滑動面。一般認為，這個滑脫面即為當初1999年9月21日集集地震所造成的。 一般我們所知，地震的能量可以分為三個主要的部份：輻射能、破裂能以及摩擦能，其中的摩擦能的往往因為轉變為熱能釋放而比較難以估算，而破裂能和斷層破裂的瞬間相關，且這部分的能量在地震總能量中佔有相當重要的機制。於是，我們在斷層滑脫面取出來斷層泥的岩芯來加以研究，總共有12公分（principal slip zone，PSZ），並對最底部厚約2公分的主要活動面（major slip zone，MSZ）做資料的分析（Ma et al, 2006）。分析指出車籠埔斷層的顆粒結構趨勢為N(D)=0.0045D-2.3（N(D)為顆粒密度，D為顆粒直徑）；換句話說，車籠埔斷層的破裂趨勢(fractal dimension)為2.3，約大於一般破裂力學(fracture dynamic theory)的平均值1.8到2.0之間。 此外，我們還發現在MSZ的顆粒結構中，顆粒直徑小於100nm的部分，特性和100nm以上的顆粒有明顯的不同，其顆粒接近於圓形而比較不像一般自然界破裂的不規則形狀，而且這些小於100nm的顆粒只發現在這12公分斷層泥的弱面之中，其他的層面並沒有發現這樣的顆粒。 計算完破裂趨勢之後，進一步的計算破裂能量，希望藉著能量的分布趨勢，來了解集集地震以及車籠埔斷層上所發生的歷史地震之相關特性和分析。The Taiwan Chelungpu Drilling Project (TCDP) drilled several holes penetrating the Chelungpu fault, and identified a 12-cm slip layer near the depth of 1136 m of Hole-C. This primary slip zone (PSZ) had been recognized as the major slip zone of Chi-chi earthquake on September 21st 1999, with several layers related to historical events along the Chelungpu fault. We examined the fault gouge related to the PSZ to give further exploration on the fracture energy, which is related to the surface area of the broken particles, to understand the possible partition of the energy for the most recent event and historical events. Previous studies in observation of the particle, related to the Chi-Chi earthquake, show that the grains smaller than 100 nm, having the spherical shape, and did not follow the power law. We further made the similar studies for the layers in the PSZ and made the comparison of the observations of these layers. We cut the PSZ into 10 samples with the dimension of 2cm * 2cm. By observation, we defined 16 different layers, which might be related to different historical events, in the PSZ. In the observation of scanning electron microscope (SEM), we obtain the relationship between particle diameter D and particle density N(D), and make a power-law of grain size distribution for each layer. In the weak layers, which might be associated with the most recent Chi-Chi event, we found the fractural dimension of about 2.2 and 2.3. Other layers have the fractural dimension of near 2.0, and with no observation of spherical grain of less than 100nm. It suggests the existence of the spherical grains might be resulted from dynamic processing during faulting, and would be disappeared after healing. According to the grain size distribution, we calculated the fracture energy from total surface area of the grains by EG=SλGC (S is surface area (m2), λ is grain roughness, GC is specific energy (Jm-2)). In the weak layer, the surface fracture energy is about 2.93-4.38MJm-2, other layers have relatively lower values of about 0.88MJm-2. Further studies on the comparison of the surface fracture energy, and chemical composition will be made to understand the dynamic process of fauling. |
