大屯山與台北盆地鄰近區域地下速度構造分析

Abstract

位於台灣北部地區的台北盆地主要是由山腳斷層的陷落活動與未固結之第四紀沈積層所形成的鬆軟沈積盆地。大屯火山群則緊鄰於台北盆地之北緣，而目前大屯火山群仍有很明顯的後火山活動現象。台北盆地地區的地震活動度相當低，但相較來說，在大屯火山群之區域則觀測到不少的微震活動。因此，瞭解台北盆地與大屯火山群地區的地震構造是相當重要的工作。 於本研究當中，為了提升分析結果之解析度，增加了波線覆蓋率，總計使用四個佈置於台北與大屯火山地區的地震觀測網資料，對地震波做波相定位，並與地震目錄整合以建立該地區之地震層析影像。此四個地震觀測網分別為兩個小區域臨時監測網與中央氣象局負責之CWBSN、TSMIP監測網。從1979年至2007年之所有地震目錄中，共選用了6861個至少被7個測站所記錄的地震事件，用以進行Vp 以及Vp/Vs的地下波速構造反演，並分析該區域其震源機制解之情況。 於最後層析影像結果中，在研究地區下呈現一低Vp/Vs比值的連續區域，與山腳斷層的地表分佈頗為一致，而於其上之高Vp/Vs比值區域則反應了鬆軟的台北盆地地區。此外，大屯火山群底下於淺部是低Vp/Vs比值、高Vp的區域，此即是反映了淺層已固結的堅硬火成岩體。 大屯火山地區之下的微震活動經重定位後可將其區分成兩群地震，此兩群地震分別位於七星山以及大油坑跟馬槽底下，而該區域地表則皆有頻繁的地熱活動現象。使用P波初動的方法得此區的震源機制皆是具有走向滑移分量的正斷層，但這些震源機制解之走向與傾角的差異極大，此種現象在火山活動地區極為常見。該區域之微震活動分佈可能是反應了大屯山底下的岩層破碎情況，並形成暢通的噴發管道路徑，以供氣體或者液體於大屯火山群底下流通。 因此，根據地震震源位置及其震源機制解的特性，本研究推論這些微震活動與金山斷層的關連性較小，而此兩群地震在空間分佈上的走向不太一樣，也可能暗示了大屯火山地區的地熱活動可能有多處來源。

The Taipei Basin with ‘soft’ deposit in the north of Taiwan has been produced by the motion of the Shanchiao fault. To the north, Tatun Volcano Group (TVG) which has post-volcanic activities is located adjacent to Taipei Basin. The local seismicity in Taipei Basin is very low. On the contrary, there are quite a few micro-earthquakes being detected in TVG. Therefore, it is very important to understand the seismotectonics of the Taipei Basin and TVG. In this research, in order to increase ray path coverage and improve the resolution, I picked the seismic arrival times and combined the earthquake catalogs from two local temporary seismic networks located in Taipei and Tatun region, the Central Weather Bureau Seismic Network (CWBSN) and the Taiwan Strong Motion Instrumentation Program (TSMIP) to establish a new seismic tomography image. Totally, 6861 events detected at more than 7 stations during periods from 1979 to 2007 were used to determine Vp and Vp/Vs structures as well as focal mechanisms. In our tomography results, the pattern of low Vp/Vs ratio in this region seems to coincide with the Shanchiao fault, and the region with high Vp/Vs ratio above it reveals that the effects of basin amplification affects about several kilometers. Besides, the shallow region with low Vp/Vs and high Vp beneath TVG should represent the solidified igneous rocks. The hypocenter distribution of micro-earthquakes beneath TVG can be classified into two groups. These two seismogenic zones at the shallow portion are coincident with the surface hydrothermal regions with hot spring and fumaroles. Those regions include Chishing Mountain, Matsao and Tayukeng. The focal mechanisms of those earthquakes are normal fault types with strike-slip component by using P wave first motions and these solutions have various strikes and dips even the events are close in time and spatial windows. This phenomenon is commonly observed in a volcanic area. Thus, those earthquakes may reflect the broken zone and unobstructed conduits exist beneath the TVG and the gas and liquid can pass through. So, based on the hypocenter distribution and focal mechanism solutions, it seems that these earthquakes are weakly correlated with the nearest fault - Chinshan fault. Besides, these two seismogenic zones incline to different directions and may imply that they are different passageways for the hydrothermal fluid and gas to ascend along from different sources.