臺灣東部縱谷地區斷層活動特性

Abstract

台灣東部花東縱谷是歐亞板塊和菲律賓海板塊的板塊縫合帶，具有高度的地震活動。前人研究透過地質調查、大地測量和同震地表變形特徵等分析將縱谷斷層劃分為數個斷層段，分別是嶺頂、瑞穗、池上、奇美、利吉。本研究分析縱谷地區地震活動度，根據地震震源分佈以及構造斷層的空間相關性，本研究將地震分為四個地震群（N1、N2、S1和S2）來討論可能對應的斷層區段的發震特性。 本研究使用中央氣象局的地震目錄和中央研究院地球科學所AutoBATS的震源機制解，分析1995年至2022年的台灣地區的地震。研究結果可分為兩部分：（1）計算b值、z值和地震頻率進行地震參數的統計分析；（2）擬合每個地震群相關的斷層面，討論最大主應力方向與斷層面，以計算斷層活動角，進階討論與構造應力的比值(最大與最小主應力R-ratio)的關係，從而檢測斷層活動狀態。 第一部分顯示了在分析時段中，縱谷斷層中段S1區域相對較低地震活動。而位於N1、N2和S2的分群展現較高的地震活動，且1995年至2022年之總體b值均介於0.85～0.87。S1群於1995年至2022年累積之地震資料顯示b值低至0.73，然而若將2022年之地震資料排除，b值會回升至0.857，顯示S1群在2022年大規模地震之比例明顯高於過往，可能肇因於小規模之地震資料尚未完備。 第二部分結果顯示北段N1和N2地震群呈現連續向西傾斜之擬合斷層面，當摩擦係數（µ）假設為0.75時，大部分規模大於5.5之主要地震事件處於有利活動的狀態。縱谷南段S1和S2地震群則呈現不同的擬合結果。南段東側的S2地震群顯示了池上斷層向東傾斜的鏟狀斷層面，西側S1地震群在中央山脈下方顯示了一個陡峭且向西傾斜的斷層面，當µ = 0.75時，2022年關山-池上地震事件的主震處於不利活動的狀態。 透過係數測試，當µ降至0.59時，上述2022年之地震事件將轉為有利活動的狀態，推測地層荷重改變或較高的孔隙水壓等會降低摩擦係數之條件皆可能是觸發中央山脈斷層活動的關鍵作用。

The Longitudinal Valley is a suture zone between the Eurasian Plate and the Philippine Sea Plate and exhibits a high seismicity in eastern Taiwan. Previous researches and field surveys have led to the division of the Longitudinal Valley Fault (LVF, eastern side of the Longitudinal Valley) into several segments, namely Lingding, Rueyshui, Chihshang, Chimei, Lichi. According to the distribution of earthquakes and their spatial correlation with tectonic faults, this study divides earthquakes into four clusters (N1, N2, S1, and S2) and discusses their seismogenic characteristics accordingly. This study employed the earthquake catalog from the Central Weather Bureau and the focal mechanism solutions provided by AutoBATS (Institute of Earth Sciences, Academia Sinica) for the period of 1995 to 2022. The results were conducted from: (1) A comprehensive analysis of the b-value, z-value, and earthquake frequency; (2) Fitting fault planes associated with each group, and applying the stress state inverted by MSATSI to the fault planes in order to calculate the fault activation angles and the tectonic-stress ratio (maximum over minimum stresses, R-ratio), thereby examining fault activity. The first part of the study reveals a relatively low seismicity in the middle segment of the LVF(S1). The N1, N2 and S2, exhibit higher seismicity and share a similar b-value of 0.85~0.87, on the other hand, the S1 group display a lower b-value of 0.73 while if we move out the earthquakes in 2022, the b-value of S1 will raise to 0.857. It shows that the proportion of large-scale earthquakes in the S1 group in 2022 is significantly higher than in the past, which may be due to the incomplete data of small-scale earthquakes in the catalog. The second part of the study reveals the fault planes at northern groups N1 and N2 beneath the Central Range and the Coastal Range, and most of the main earthquake events are in the favorable orientations when the friction coefficient (µ) is set to 0.75. In contrast, the southern groups S1 and S2 present different convergent results. S2 shows a clearly east-dipping listric fault plane of Chihshang fault, S1 demonstrates a steep and west-dipping fault plane beneath the Central Range with the mainshocks of 2022 Guanshan-Chihshang earthquake events in unfavorable orientation when µ = 0.75. Our study revealed that the 2022 earthquake sequence resulted from the reactivation of the fault of the fault on the western side of the Longitudinal Valley. mechanically, this reactivation shall occur when the friction coefficient decreases to as low as 0.59. This finding suggests that either reducing strata loading or increasing fluid pressure may play a crucial role in triggering the faulting on the western side of the Longitudinal Valley.