利用經驗格林函數探討重複地震破裂特性

Investigation of Rupture Process of Small Repeating Earthquakes

陳奕尹(Chen I-Yin)

指導教授 ： 陳卉瑄 ； 金亞伊

國立臺灣師範大學/理學院/地球科學系/碩士(2017年)

https://doi.org/10.6345/NTNU202202811

摘要

斷層面上的滑移量分布，反應單一地震發生時，斷層面破裂的動力行為，因此本研究利用利用重複地震的波形相似特性，以有限斷層逆推法計算中小規模地震的震源破裂模型，目的在了解斷層嵌塊中滑移量的空間分布特徵及其控制因子。本研究使用重複地震序列中規模較小的事件作為經驗格林函數(eGf)，對較大的目標事件（target event）解迴旋，以求得震源時間函數及進行滑移量逆推，除了討論重複地震的震源特性和其復發週期的關係，並嘗試理解平均和最大滑移量與地震規模的相依性。
本研究選用一個規模3.4-3.5的準週期性序列（Quasi-periodic Sequence, SQ），發震週期約三年；另一個為非週期性序列（Aperiodic Sequence, SA），發震週期由數分鐘至三年左右不等，研究中發現，SQ中每個事件的最大滑移量主要集中於兩個斷層嵌塊，破裂的區域在空間分布上高度重疊；SA在2009年規模6.9的花蓮地震後數分鐘至數十分鐘內，連續發了三個規模4.0-4.6的事件，由於受到震後滑移速率影響，其滑移分布呈現各自不同的破裂位置與型態，具有高度的異質性。不同週期特性的重複地震序列表現了迥異的破裂行為，可能說明當滑移速率改變重複地震之週期特徵時，也影響了斷層嵌塊本身的異質性，這兩個序列的比較，說明了震源特性和地震的週期行為的相依性。
此外，我們從全台灣規模3以上的重複地震目錄共62個序列中，篩選出13個序列、共23個規模3.3-4.9的事件進行逆推計算，發現其符合以圓形斷層（circular fault）為基本假設，推導之滑移量與地震矩的關係。本研究計算之平均滑移量約1.74-26.85 cm，其與規模呈現D ∝ Mo2/5線性關係，近似於理論平均滑移量得到之D ∝ Mo1/3的線性關係，且平均滑移量略大於理論平均滑移量，反映了在計算過程中，由於考慮可信度較高的破裂面積，導致滑移量比理論值高；而逆推之平均滑移量與理論滑移量差距越大，代表滑移量在斷層嵌塊上的集中程度越高。在不同地區跨規模的滑移量與地震矩的分析，我們發現規模2的重複地震事件表現出與地震矩的弱關聯性，本研究推論其與無震滑移的貢獻度有關，規模越小的重複地震事件，周圍無震滑移區域的範圍越大，使滑移量在斷層嵌塊上高度集中，亦導致逆推得到的地震滑移量未呈現D ∝ Mo1/3的線性關係。

關鍵字

斷層嵌塊 ； 重複地震 ； 經驗格林函數

並列摘要

Using small earthquakes as empirical Green’s functions (eGf), we apply a finite-source inversion to demonstrate a link between slip heterogeneity and earthquake recurrence of two repeating sequences in eastern Taiwan. The M3.4–3.5 quasi-periodic repeating earthquakes that are characterized by 3-yr recurrence interval reveal overlapped slip concentrations. These obtained models show peak slip of 6.70-17.48 cm. Under the influence of nearby M6.9 event, the M4.0-4.6 repeating earthquakes that are separated only by 4-88 mins, reveal an aperiodic manner. Inferred slip behaviors perform not only a distinct rupture characteristic without overlap in the slip areas, but an inconsistent slip distribution compared with pre-M6.9 repeating earthquake. The variation of rupture behavior suggests the temporal change of loading rate caused by the nearby large earthquake may change the slip heterogeneity in a repeatedly ruptured asperity. We also study repeating earthquake sequences with a greater range of magnitude for slip inversion. Slip heterogeneity, as a result of heterogeneous stress accumulation on a fault, may rely on the stress condition in each subarea of the fault plane, whereas stress condition inside the asperity controls where the peak slip takes place. A collection of slip heterogeneity in different events therefore, will have potential application in monitoring the temporal evolution of stress state.

並列關鍵字

asperity ； repeating earthquake ； empirical Green’s function

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