琉球隱沒帶最南端地殼應力於慢滑移事件期間之變化

Abstract

本研究聚焦於隱沒帶中相鄰大地震滑動區域的慢滑移事件，這類慢地震事件能量釋放速率極低，但其引發的應力轉移可類比於大地震。Radiguet et al. (2016)在墨西哥觀察到一例由慢滑移應力轉移引發的規模7.3地震，而 Chen et al. (2018)則在台灣東部花蓮外海記錄到三次慢滑移事件。研究主要探討慢滑移活動期間及其周圍地殼應力的時間同步變化。為此，我們選取Chen et al. (2018)所定義的三個慢滑移事件滑動範圍為研究區域，使用地震累積規模分布（b 值）時間序列作為軸差應力隨時間變化的指標，並基於震源機制解計算主應力軸方向的時間序列。最後，我們以庫倫應力模型估算應力影響範圍，以期觀察這三次慢滑移事件所引發的應力變化。 在地震事件選取方面，本研究以慢滑移事件滑動範圍為空間條件，深度範圍則限定為班氏帶上下 10 公里。我們使用中央氣象署寬頻地震網（CWASN）2003至2017年重定位地震目錄(Wu et al., 2008)，涵蓋 67,852 起地震，並以 Mc 值 2.2 為下限，採用固定 300 個事件的多尺度時間窗進行最大概似估計以計算 b 值的時間變化。Mc值則依據最大曲率法進行計算。此外，本研究以台灣寬頻地震網（BATS）中 286 個震源機制解，以固定 35 個事件的多尺度時間窗估算應力方向，並以拔靴法計算不確定性。 研究結果顯示，研究區域背景 b 值介於 0.8 至 1.2 之間。於三次慢滑移事件期間，均觀察到 b 值的極小值，且此期間 b 值平均值下降亦伴隨地震事件數增加，這一趨勢與 Dascher-Cousineau and Bürgmann (2024)的統計結果一致。應力主軸方向在第二次慢滑移事件期間顯現顯著變化（傾角增加 10 度，方位角增加 30 度），但未符合 Warren-Smith et al. (2019)的觀察結果，推測此差異源於區域構造的不同。此外，慢滑移期間地震事件位置也集中於庫倫應力模型預測的應力增加區域，此發現與前人研究(Radiguet et al., 2016)中觀察到的地震與應力上升區域重疊現象相似。

This study primarily focuses on slow slip events (SSEs), which occur adjacent to megathrust earthquake rupture zones in subduction zones. Despite releasing energy at a much slower rate, SSEs can induce stress transfers comparable to those triggered by major earthquakes. Radiguet et al. (2016) observed an SSE-induced stress transfer leading to a magnitude 7.3 earthquake in Mexico, while Chen et al. (2018) documented three SSE occurrences together with earthquake swarms offshore eastern Taiwan. This research examined the temporal synchronization among SSE, seismic activities, and crustal stress variations in the surrounding area of SSEs. To this end, we adopted the three SSE rupture zones identified by Chen et al. (2018) and used the temporal distribution of the Gutenberg-Richter b-value for an indicator of the changes in differential stress over time. Additionally, we calculated the time series of principal stressaxes orientations from focal mechanism solutions, and used a Coulomb failure stress model as a reference for the spatial extent of stress impacts. Our aim was to detect stress variations associated with these three SSEs. The earthquakes included in this study were limited to the SSE source zones, with depths constrained to ±10 km of the Benioff zone. We relocated 67,852 earthquake events from the Central Weather Administration’s Broadband Seismic Network (CWASN) from 2003 to 2017 (Wu et al., 2008). A magnitude of completeness (Mc) of 2.2 was applied for the detemimation of b-values using a maximum likelihood estimation in a series of fixed 100-events moving time window. The Mc was determined using the maximum curvature method with an adjustment of 0.2 plus, and standard deviations for the b-values were calculated by Shi and Bolt’s method (1982). Additionally, we employed 286 focal mechanisms from the Broadband Array in Taiwan for Seismology (BATS) to estimate the orientations and plunges in the principal stress axes within a series of fixed 35-events moving time window, with uncertainties assessed by bootstrapping. Results indicate that background b-values in the study area range from 0.8 to 1.2. During each SSE, the minimal b-values were observed, accompanied by a decrease in b-value and an increase in earthquake occurrences. This aligns with statistical recent findings by Dascher-Cousineau and Bürgmann (2024). Significant changes in stress orientation were only observed during the second SSE, with a 10-degree dip increase and a 30-degree azimuth increase, which was not similar to the shifts observed by Warren-Smith et al. (2019) and likely due to differences in tectonic structures. Earthquakes during the SSEs also occurred within the increase zones of Coulomb failure stress, consistent with Radiguet et al. (2016) who reported overlap between the coseismic slip and Coulomb stress increase areas.