臺灣博碩士論文加值系統

本研究分析2014年中央氣象局和國家地震工程研究中心的寬頻地震資料，來偵測古坑斷層的顫動(tremor)分布情形以探討古坑斷層為何具有高地殼變形但低地震能量釋放的特性。本研究提出了新的顫動偵測程序，包含2-5Hz濾波、降噪、交對比、振幅限制、時頻圖頻譜分析做顫動篩選，並利用格點搜尋與距離加權法做顫動定位。利用此分析程序，本研究在古坑斷層帶上找到34個顫動事件，這些是事件主要發生在2014年第160天之後，而且深度幾乎完全侷限在深度6公里之下。根據地震與顫動的分布，古坑斷層被可被分為3個區域：（一）、無地震也無顫動的區域，深度在0-6公里；（二）、地震與顫動混合的孕震區，深度在6-19公里；（三）、僅有顫動但無地震的深部滑移區，深度在19公里以下。本研究認為第一區為斷層鎖定區，第二區為速度弱化區，包含黏滑行為與條件穩定滑動及顫動行為，第三區則為速度強化與部分速度弱化強度較低的區域，主要以穩定滑移為主。同時本研究也根據2012-2016的全球定位系統(GPS)記錄進行一維彈性錯位模型的反演，結果顯示古坑斷層的鎖定深度在6公里深處，此深度與地震和顫動分布的上邊界相同，證實前述的推論。另外，本研究也發現顫動的出現與否和斷層是否活動有著高度相關性，跟地震的相關性則不明顯，但這不代表顫動是造成斷層活動的主因，因為顫動所釋放的能量並不足以對地表速度變化造成太大的影響，換言之，這些產生顫動的區域都不大且其摩擦參數a-b→0-，因此本研究推測古坑斷層的孕震帶目前可能主要處於條件穩定滑動的狀態。本研究所找到的顫動訊號多屬二分鐘內的事件，且可與微震發生於同一區域，與一般隱沒帶觀測的長微震特徵不同，我們認為這種型態的顫動可能普遍存在於每一個地震能量釋放率較低的斷層。由於這種訊號可用來指示斷層面上活動區域，因此在分析孕震帶如鎖定深度與地栓(asperity)分布位置時上扮演著非常重要的角色。

We have analyzed seismic waveform recorded by broadband stations deployed by the Central Weather Bureau (CWB) and National Center for Research on Earthquake Engineering (NCREE) during 2014 to detect ambient tremors along the Gukeng Fault. The objective of this study is to unveil why the fault has distinct interseismic deformation but with low seismic activity. In this study, we developed a new procedure for detecting short duration ambient tremors including bandpass filtering between 2-5Hz, noise reduction, normalized cross correlation of waveform envelop, amplitude constraint, and waveform spectrogram analyses. The tremors were located by applying grid-search and distance-weighting methods. Totally 34 ambient tremor events were detected in the Gukeng Fault zone based on our new approaches. Our results show that Gukeng Fault zone can be separated into three areas based on the distributions of tremors an earthquakes: the area I with depth of 0-6 km has few tremors and earthquakes; the area II with depth of 6-19 km has both tremors and earthquakes, and the area III with depth below 19 km has only a few tremors but no earthquakes at all. We interpreted the area I as the locking zone and the area II as a velocity-weakening zone with stick-slip sliding, tremors and conditionally stable sliding. The area III may represent a zone of velocity strengthening with a few patches exhibiting velocity neutral or slightly velocity weakening. By applying one-dimensional elastic dislocation model to the GPS data during 2012-2016, we found that the locking depth of the Gukeng Fault is at 6 km, which is same as the upper bound of the distribution of the tremors and earthquakes. This finding confirms our early inference. In addition, we found that the temporal motion of the Gukeng Fault clearly shows high correlation with the occurrence of the tremors but not with the seismicity. However, this finding does not imply the tremors we found are responsible for the fault movement because the energy of tremors is too small to account for the surface displacement across the fault. Thus, we suggest that major part of the seismogenic zone of the Gukeng Fault may be under conditionally stable sliding. As the tremors we found usually have duration under 2 minutes, which implies scattered small patches with friction parameter a-b→0- and is quite different the long-duration tremors found in subduction zones. We believe the short duration type tremors may overwhelm in those faults with less seismic efficiency. Because these short-duration tremors can act as good indicators of fault motion when lack of earthquakes, they can play a very important role in understanding the characteristics in seismogenic zone such as fault locking depth and the distribution of fault asperities.

誌謝 I
中文摘要 III
Abstract IV
目錄 VI
圖目錄 VIII
表目錄 X
第一章 緒論 1
1.1 研究動機與目的 1
1.2 前人研究 8
1.2.1 顫動(tremor)簡介 8
1.2.2 偵測顫動的方法 9
1.2.3 臺灣過去關於的顫動研究 11
1.3 研究區域介紹 18
第二章 研究資料與方法 21
2.1 研究資料 21
2.2 研究方法簡介 23
2.3 建立古坑斷層帶網格模型 24
2.4 建立S波走時差模型 26
2.5 資料前處理 27
2.6 顫動分析 29
2.6.1正規化交對比與門檻設定 29
2.6.2 顫動的振幅門檻設定 39
2.6.3 時頻分析 41
2.7 顫動定位 43
2.7.1 計算測站對的延遲時間是否介於格點的走時差範圍 43
2.7.2 利用兩測站對延遲時間與格點搜尋找出可能的顫動源 43
2.7.3 計算座標群的平均位置 44
2.7.4 利用距離修正平均座標的位置 44
第三章 研究結果 50
3.1 分析結果 50
3.2 顫動時間分布 53
3.3 顫動空間分布 55
第四章 討論 57
4.1 顫動與地震分布的關係 57
4.2 古坑斷層滑移穩定性探討 59
4.3 顫動分布與GPS逆推古坑斷層鎖定深度的關係 63
4.4 顫動及地震發生時間與斷層活動的關係 66
4.5 本研究方法的優點與未來工作 69
第五章 結論 70
參考文獻 71
附錄A 顫動事件列表與波形紀錄 77

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