2016年臺灣美濃地震之地震構造研究

Abstract

地震序列的餘震分布是了解地震構造的一項重要資料，因此，本研究藉由美濃地震探討其地震構造，以及地震序列的發震原因。為了解析美濃地震序列的空間分布，本研究從中央氣象局地震觀測網蒐集了1182筆美濃地震序列之地震記錄，為了瞭解更細微的地震空間分布，本研究使用了三維速度模型與雙差分定位法將地震序列進行重新定位。定位後再加入中央氣象局自由場強地動觀測網之地震資料求取P波初動解，進而探討美濃地震序列的地震構造。本研究總共求得14組P波初動解，震源機制結果顯示，美濃地震之震源深度為15.3公里，可能的斷層面之震源參數為strike = 256°、dip = 8°、rake = -18°。本研究發現在主震周圍的餘震主要以平移斷層為主，推論可能由於主震破裂的平移斷層分量造成餘震產生相似的震源機制；另外，在主震破裂面上盤的東側有正斷層發生，發震原因可能為主震同震滑移時，上盤向西方向滑移後，上盤處的後半部伸張而產生正斷層；主震震央西方向的餘震群則受到主震同震滑移以西方方向破裂之影響，造成台南盆地底下深部的地震活動。基於前人研究結果，本研究發現美濃地震位於基盤岩石與上覆沉積層之間的邊界附近，此邊界附近的震源分布呈現階梯狀形貌，可能為基盤斷層，並由北往南漸深，最後和一個向東隱沒的構造相連結，顯示美濃地震的發生原因，應為此基盤-沉積層邊界附近的階梯狀構造與隱沒構造之間的地震活動。另外，在主震震央北方存在兩個線性地震構造，一個以近水平面分布在深度8公里附近；一個大約分布在10-15公里之間，稍微向南方傾斜。這兩個線性地震構造，符合前人所推論的階梯狀構造中震源的分布情況，且位在庫倫應力的正值上，為主震所觸發，因此，美濃地震的同震滑移觸發了階梯狀構造附近的地震活動，可能為基盤斷層的再活動所造成之地震活動。

The aftershock distribution of the earthquake sequence is an important information for understanding the seismogenic structure. To know the details of the distribution of the earthquake sequence, I relocated the earthquake sequence using the double-difference earthquake location program with three-dimension velocity model. I collected 1182 earthquake records from the Central Weather Bureau Seismic Network (CWBSN) to relocate the earthquake sequence. Then, to improve the focal mechanism solutions, I further integrated earthquake records from Free-field Strong Motion Network to determine the P-first motions. Consequently, I obtained 14 first-motion focal mechanism solution. The mainshock have a combination of the thrusting and left-lateral strike-slip fault type, including parameters as below：focal depth 15.3 km, and a possible fault plane of strike 256°, dip 8°, and rake -18°. It is possible that the left-lateral slip of the mainshock caused the similar focal mechanism near the mainshock. I detected that there were normal faulting occurring on the eastern part of the hanging wall. I referred that eastern part of the hanging wall was extensive after the mainshock ruptured. In addition, the seismicity activity beneath the Tainan basin result from the east-oriented rupture directivity on the rupture plane. Based on the previous studies, I found that the source of the Meinong earthquake occurred near the basement-cover interface. The distribution of the hypocenters neared the basement-cover interface imaged by the step-like structure. According to previous studies, the step-like structure may be the basement fault and gradually deepened from NW to SE, finally connecting with a subducted structure. The result indicated that the occurrence of Meinong earthquake should be the seismic activity in the transition zone between the step-like structure and the subducted structure. The relocated seismicity revealed that two linear seismogenic structures in the northern part of the earthquake sequence. One linear seismogenic structure distributed at a depth of 8 km with flat distribution, the other distributed in the range of 10-15 km dipping to the south. Both of the structures near the step-like structure may be triggered by the mainshock of Meinong earthquake in the results of Coulomb failure stress. I inferred that the two linear seismogenic structures corresponded with the step-like structure and was likely to cause the reactivation of the basement fault.