北越 Ma River斷層帶孕震構造機制之探討研究

The Seismogenic Structures beneath the Ma River Fault, Vietnam: Insights from Studys of Focal Mechanisms

吳瑋哲(Wei-Jhe Wu)

指導教授 ： 陳朝輝

國立中正大學/理學院/地球與環境科學系地震研究所/碩士(2013年)

摘要

小地震 (M<4) 的震源機制解常被用於了解斷層構造上 (Shearer, 1998)，但由於小地震的能量較小，訊雜比 (signal-noise ratio) 較低，在一般情況下，能有效且準確撿拾 P 波及 S 波的測站，相較於大地震來的少。在有效撿拾波形測站分布不均，且訊雜比較低的情況下，所獲得的震源機制解將因其不確定性
(uncertainties)，大大的降低所能利用的程度，因此如何加入一些地震波的資訊去有效的提升震源機制解的準確度，為本研究所要努力的目標。國立中正大學與越南地質科學院合作，在 2009 年於北越 Ma River 斷層帶設置 12 部臨時寬頻地震儀 (Güralp CMG-6TD)，本研究將由地震學的觀測與分析方法來了解北越 Ma River 斷層帶的地震活動情形與地殼地震構造，並利用其所記錄的微震資料配合波形反演之方法進行詳細之斷層面解分析。本研究採用 Zhu and Helmberger (1996) 所提出的 “cut and paste” (CAP) 方法將全波段的地震波分成體波與表面波，並且獨立的透過這些波段進行波形擬合來找尋最佳震源機制解。而獲得震源機制解的方法是使用非線性全域逆推方法－格點搜索法 (grid search)，將所有可能的解模擬出合成波相並與觀察到的真實波形進行比對，找出最小的失配 (misfit) 值視為其最佳解，並允許各波段作時間的位移。然而本研究認為 CAP 方法的準確度，主要受控於地震定位的準確性，因此本研究並對於地震事件進行雙差分重新定位，有效的降低地震定位上的誤差，進而提升震源機制解的準確度。初步結果顯示， Ma River 斷層的斷層機制大多以走向滑移斷層(右移)及正斷層機制為主，本研究將透過所獲得的結果，進一步分析 Ma River 斷層的孕震構造。

關鍵字

Ma River斷層 ； 越南 ； 震源機制 ； 應力反演

並列摘要

The focal mechanisms of small earthquakes (M＜4) are commonly used to infer
seismogeneric structure of a fault, giving constrain on the characteristics of regional stress field, and furthermore to test stress-triggering hypotheses. However, the uncertainties in obtaining focal mechanisms often weaken the robustness of the results.Therefore, to determine an accurate focal mechanism for a small earthquake has became an important task in observational seismology. Since the year of 2009, The National Chung Cheng University has carried out a project with the Institute of Geological Science, Vietnam Academy of Science and Technology to deploy 12 temporary broadband seismic stations around/near the Ma River fault zone to record microearthquakes occur in the northern Vietnam. By using the high quality waveform data recorded by the network, we are able to perform waveform inversion to obtain fault plane solutions more precisely and have better knowledge on the characteristics of seismogeneric process of the Ma River fault. This study adopted the “cut and paste” (CAP) method which was developed by Zhao and Helmberger [1994]. The major advantage of the CAP method is allowing us to cut waveforms into segments,such as Pnl (body waves) and surface waves, and further use each segment to invert focal mechanism independently. During the inversion, each focal mechanism is obtained by applying a direct grid search through all possible solutions to find the global minimum of misfit between observation waveform and synthetics, which is incorporating with time shifts technique simultaneously with finding the best solution.We have relocated 212 events by applying the double-difference relocation algorithm,and results indicate that earthquake clusters are located between the Ma River fault and the SoLan fault in the northern Vietnam. Especially, we find a cluster occurs in
the northern part of the Ma River fault where the strike of the fault is bending toward to the northwestern direction. One significant result in our research is that a more realistic 1-D P wave velocity model for the study region is also obtained, which can provide more accuracy in determining earthquake locations for the future work. The CAP technique is also proved that it can be used to obtain precise focal mechanisms of small earthquakes efficiently. Our results all show highly correlation with local geological structures and also indicate complex segmentation of the Ma River fault.

並列關鍵字

Ma River Fault ； Vietnam ； focal mechaism ； stress inversion

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