| 摘要: | 山腳斷層目前列為第二類活動斷層，緊鄰著臺灣人口密度最高的臺北盆地、大屯火山群以及核一、核二電廠，因此從地震安全的角度下，瞭解山腳斷層活動特性及鄰近地區的地下構造極為重要。但因此區域缺乏中、大規模地震，地震活動度相對較低，所以本研究使用區域內密集微震監測網，測站總數超過40個(由CWB、NCREE、TVO、IES、ITRI等單位負責)，而資料期間從2015/04/06至2017/12/31，共1493筆地震資料。使用雙差分層析影像法進行逆推(double-difference tomography)，同時為了確認資料的可信度，除了觀看波線密度分布，也使用DWS方法以及棋盤格測試法做解析度討論，發現因地震分佈極為不均勻，使高解析區多位在山腳斷層北段及大屯火山區，以及深度15公里內。三維速度構造結果方面，可以得出位於地表Vp、Vs呈現高速異常值，判斷應為火成岩分佈所致，而大屯山區呈現低Vp、Vs、Vp/Vs異常值，研判可能為高度裂隙帶及熱蒸氣分佈區域，地震分佈結果，在地熱區可以發現地震有群聚效應，而縱切剖面部分在深度4~8km處則看出一東北西南傾的孕震帶。;The study area is the most populated region, where the Taipei Basin, Tatun volcanic zone, the active fault-Shanchiao fault, and two nuclear power plants (Chinshan and Kuosheng) all lie in, in northern Taiwan. Understanding the underground velocity structure and the seismicity in this area is essential. However, the local seismicity is relatively low, and the lack of medium and large earthquakes along the Shanchiao fault. By contrast, the activity of micro-earthquakes in the Tatun volcanic zone is notable. In order to improve the resolution and increase ray path coverage for better understanding the complex region, we used a dense micro-earthquake monitoring network with more than 40 stations. Including real-time broadband station and dozens of stations operated by the other five organizations (CWB, TVO, IES, NCREE, ITRI). During 2015/04/06~2017/12/31, 1493 events detected by this network were used to invert the P wave velocity (Vp) and P wave to S wave velocity ratio (Vp/Vs) structures in the study area. After the preliminary locating of events, we applied the double-difference tomography (tomoDD), which is a method to solve for the 3-D velocity structure and more precise earthquake relocation. At the same time, in order to confirm the credibility of the data, we use the Derivative Weight Sum (DWS) method and the checkerboard test method for the resolution analysis. Because the seismic distribution is extremely uneven, the high-resolution area is mostly within 15 km at the northern part of the Shanchiao fault and the Tatun volcanic zone. In terms of three-dimensional velocity construction results, it can be concluded that the high-level anomaly values of Vp and Vs at the surface may be caused by the distribution of igneous rocks. The Tatun Volcano areas exhibit low Vp, Vs and Vp/Vs anomaly values, and the judgment may be high fractured and crack density. At the hot vapor distribution area, the seismic distribution results show that the earthquake has a clustering effect in the geothermal area. The longitudinal section at a depth of 4 to 8 km can see a northeastern-southwestern dip. The relocated seismicity and underground velocity structure can provide more details about the seismogenic and tectonic relations between the Shanchiao fault and the Tatun Volcano area. |
