| 摘要: | 南投縣仁愛鄉在2021/09/13發生一起芮氏規模5.6，深度達到46.1公里的地震事件，從過去的地震活動分布顯示，南投地震發生在一群獨立且狹窄的深部地震群內，看起來與班尼奧夫帶沒有直接的關聯。為了瞭解異常深部地震群的發生機制與其幾何樣貌，並探討此異常深部地震群為碰撞造山的演化機制中，帶來什麼樣的暗示。我們使用密度分群法(DBSCAN, Kriegel et al., 2011)先對異常深部地震群進行分群，可以分為西群與東群，再分別對西群與東群使用雙差分定位法(HypoDD, Waldhauser and Ellsworth 2000)進行重新定位，得到更精確的震源位置。參考許多前人的研究，我們認為西群的發震機制是因為下部地殼有較強的剪切強度，岩石具有累積跟釋放應變能的能力，加上區域地震產生的裂隙提供管道讓流體能夠流滲至下部地殼，造成岩石的弱化與脆變，讓地震更容易發生，應力來源是已經隱沒到較深處的南中國海岩石圈的重力作用，加上緊跟在海洋岩石圈後頭的大陸性岩石圈，因密度較低產生向上的浮力。而東群的發震機制我們認為有兩種可能，第一種是鐵鎂質麻粒岩隨著歐亞板塊隱沒到更深處的位置，使得區域有較高的岩石剪切強度，而裂隙提供管道讓流體能夠弱化岩石與發生脆變。第二種則是由於榴輝岩化的發生，使得岩石相變脫水，導致地震的發生，應力來源為菲律賓海板塊所主導。
另外，南投地震為異常深部地震群內規模最大的事件，因此我們可以藉由地震波的體波(body wave)訊號對其進行分析，並研究波線路徑上可能經過的異常構造。
本研究從台灣陣列(Formosa Array)下載南投地震的波形資料，透過計算互相關函數得到北台灣S波相對到時差的特徵分布。我們發現在接近最北的三個測站KE01、KE06與VO05(震央方位角約30度)有明顯較快的S波到時，從波傳路徑順推的結果與速度構造模型顯示(Huang et al., 2014b)，波線在行徑的過程中經過異常的高速帶。根據Su et al. (2019)的研究，此異常高速帶為含水量較少的榴輝岩相變玄武岩所導致。
;The September 13, 2021 Nantou earthquake (ML 5.6, depth 46.1 km) occurred within a narrow zone of deep seismicity (35-80 km) beneath central Taiwan. This study aims to enhance our understanding of the seismic distribution within the deep seismic zone, investigate its underlying mechanism, and explore its tectonic implications. Additionally, the study examines variations in the S-wave arrival time from the Nantou earthquake.
To accomplish the first objective, events within the deep seismic zone from the CWBSN catalogue were sorted out and clustered using the density-based spatial clustering of applications with noise (DBSCAN, Kriegel et al., 2011). The two clusters thus derived were relocated using the double-difference algorithm (HypoDD) on a multiple events relocation scheme (Waldhauser and Ellsworth 2000). From a tectonic standpoint, we interpret the western cluster as being associated with the strong lower crust, characterized by mafic granulite and fault weakening induced by fluid. The source of stress is dominated by the slab pull of the South China Sea plate. As for the eastern cluster, two potential scenarios are proposed. First, as the Eurasian plate subducts deeper, the scenario may be similar to that of the western cluster. Secondly, the presence of eclogitization beneath the continental mountain root suggests the detachment of the oceanic lithosphere from the continental lithosphere. The source of stress is dominated by the collision of the Philippine Sea plate.
Regarding the second objective, S-wave waveforms from the Nantou earthquake, recorded by the Formosa Array, were subjected to cross-correlation analysis to determine the relative arrival time of the S-wave. The findings reveal a relatively faster arrival of the S-wave in northern Taiwan, which can be attributed to the H2 structure as indicated by Su et al. (2019). |
