利用群組地震法探討阿拉斯加隱沒帶的震源特性

Abstract

斷層在中深部 (50 - 300 km) 高岩壓環境下的破裂機制目前尚有爭論。溫度、壓力與區域應力場的差別，均會對破裂機制產生不同的影響，進而決定斷層的破裂行為。連續深度剖面的震源參數提供了解不同深度破裂機制的機會。本研究分析地震波頻譜，計算深層地震包含拐角頻率、地震矩、應力降、地震波輻射能量、地震波幅射效率在內的震源參數，嘗試為深層地震的發生機制提供約束。 本研究利用群組地震法大幅減少逆推過程中的未知參數，有效分拐角頻率與路徑衰減對頻譜的影響。群組地震法的核心概念為建構多重震源-測站對能嚴謹地定義共同路徑Q與各自震源的fc，由觀測的資料同時決定震源與路徑衰減，以解決在頻譜上拐角頻率與路徑衰減之間的非線性關係。 本研究擷取IRIS資料中心144個分布於阿拉斯加隱沒帶的地震，規模介於3到5之間，震源深度90到170 km，並計算中深部地震動態與靜態的震源參數。結果顯示，拐角頻率與地震矩呈現清楚的逆相關，鞏固了逆推架構與結果的可信度。而拐角頻率、應力降、地震波輻射能量等各個震源參數的計算結果，符合地震理論的自我相似性規範。 阿拉斯加隱沒帶的震源特性具有高應力降 (18.34 ± 1.1 MPa) 與低地震波輻射效率 (0.27) 的特徵。暗示或許在阿拉斯加隱沒帶，深層地震的發生機制主導於熱失控模型。脫水脆化作用釋放出的水可能為熱失控模型的催化劑，而在地震波研究上僅看得出熱失控模型的特徵。本論文認為兩者是相輔相成的關係，共同為中深部地震的發生機制。

The Alaska subduction zone is known as an active subduction zone where earthquakes occur at high rate and widespread along the trench and down dip of the slab. In recent years, increasing numbers of seismic arrays have been installed in Alaska region that significantly increased aperture of observations. It provides an opportunity to systematically investigate depth-varying seismic source characteristics in subduction zone. The source characteristics can reflect the rupture behaviors of fault, but the differences in rupture behaviors as a function of depth are still notably debated. In this study, we employed a cluster-event method (CEM) to constrain the source parameters as well as along-path attenuation in the Alaska subduction zone. Neighborhood algorithm is applied to solve the nonlinear inverse problem. Using 40 stations from IRIS data management center, we analyzed 144 Alaska local earthquakes spreading over a depth range from surface to several hundred kilometers and a seismic magnitude range from 3 to 5 in 2012~2017. These source parameters are then converted to stress drop and radiated energy at different depths. The fc’s satisfy a self-similar scaling relationship with seismic moment of f_c∝〖M_0〗^(-3) with a mean stress drop of 18.34 ± 1.10 MPa in Madariaga’s form (Vs model). The lower radiation efficiency and higher stress drop might imply the shear heating instability and dehydration embrittlement as the same important faulting mechanisms for intermediate-depth earthquakes.