利用二維有限差分法分析斷層帶轉換波相並與MiDAS資料進行比較

Abstract

光纖感測技術，或稱分布式聲學感測技術（Distributed Acoustic Sensing, DAS），藉由其高空間解析度及在多環境下的適應性，展現了相較於傳統地震儀的諸多優勢，並在地震學中逐漸成為一個熱門的研究領域。2020年起，花蓮地區啟動米崙斷層鑽井計畫（Milun fault Drilling and All-inclusive Sensing, MiDAS），透過此研究安裝在米崙斷層區域的光纖地震儀資料，並結合附近的傳統地震儀，我們針對傳遞至米崙斷層的地震體波到達斷層後轉換成在其兩側傳遞的轉換波相，或稱斷層帶轉換波相（Fault zone converted phase, FZCP）進行特徵分析。本研究使用二維有限差分模擬法（2-D Finite Difference Method, 2-D FDM）針對斷層進行模擬，目的為利用FZCP特徵重建米崙斷層的真實構造。在模擬中，我們針對多種不同的斷層結構及速度模型進行模擬。研究發現，當模型中置入花蓮地區的淺層速度模型時，將可清楚地在S波後方辨別出FZCP，類似的波相也在MiDAS的觀測資料中被觀測到。研究中還使用了傾斜疊加法，幫助我們提高辨別FZCP及計算其波速的能力。綜上所述，光纖地震儀為一種新型觀測工具，突破了傳統地震儀在解析細微結構上的限制，並透過FZCP及2-D FDM的綜合分析，本研究得以針對米崙斷層的真實構造提出了解釋模型。

Fiber optic sensing, also referred to as Distributed Acoustic Sensing (DAS), has emerged as a vibrant area within seismology due to its high spatial resolution and adaptability to diverse environments using fiber optic cables, offering advantages over traditional seismometers. This study leverages data from fiber optic seismographs deployed as part of the Milun fault Drilling and All-inclusive Sensing project (MiDAS) initiated in 2020 in the Hualien region, alongside nearby conventional seismometer stations. Through comprehensive waveform characterization analysis of seismic waves propagating to the Milun Fault, we specifically investigate converted phase resulting from the conversion of body waves propagating to both sides of the fault upon reaching it, known as fault zone converted phases (FZCP). This analysis incorporates 2D Finite Difference simulations to reconstruct the actual structure of the Milun Fault. The simulations integrate various fault structures and velocity models to replicate the fault's real conditions. Ultimately, we found that when the shallow velocity model of the Hualien region is included in the simulations, the FZCP becomes most prominent, appearing after the S waves. Similar wave phases are also identified in the MiDAS observational data. Moreover, the application of the Slant Stack method enhances our ability to identify FZCP and calculate their wave velocities. Overall, integrating fiber optic seismometers offers a pathway to overcome limitations in resolving subtle structures that were previously constrained by traditional seismometers. Through the combined analysis of fault zone converted phases, layer reflections, and 2-D Finite Difference Method simulations, our study provides valuable insights into the structural aspects of the Milun fault zone.