以雙差分地震層析成像法探討台灣東部縱谷斷層系統構造

台灣位處歐亞板塊及菲律賓海板塊的板塊聚合處，劇烈的地殼變形作用之下在台灣東部形成碰撞縫合帶─縱谷斷層系統，此斷層系統不僅是台灣重要的地質構造單元，同時亦為災害性地震的好發地，如2013年的瑞穗地震、2014年的鳳林地震及2018年的花蓮地震，皆造成建築倒塌與人員傷亡等重大災害。即便如此，目前對於詳細的縱谷斷層系統的了解仍然有限。由於這些近期中大型地震序列多發生在未知或過去了解有限的構造上，提供了過去沒有記錄到的額外地震資料。加上自2012年開始，中央氣象局新一代的地震觀測網已排除過去遠距傳輸延遲可能造成的時間誤差，大幅提升資料的準確性。因此，我們利用2012年後的近期地震資料與雙差分地震層析成像法進行台灣東部地震層析成像的反演，加強解析與探討縱谷斷層系統的地下斷層幾何構造，並與Kuo-Chen et al. (2012)與Huang et al. (2014)的速度模型比較，發現本研究的速度模型與地表斷層的形貌更為吻合之外，在平行縱谷走向的剖面中，於利吉層出露位置下方發現相應的低速構造，也在奇美斷層的位置發現一個高速體切穿淺層的低速體，在垂直縱谷走向的剖面中，看到更細微的構造邊界並且觀察到縱谷北段西傾的地震分佈切穿一個高速體，推測此西傾構造並非發育在岩性邊界。後續研究可以結合波形交相關計算求取更精確的相對走時，進一步提升反演模型的精準度。

關鍵字

縱谷斷層系統；三維速度模型；雙差分地震層析成像法

並列摘要

Taiwan is located at the plate boundary between the Eurasian plate and the Philippine Sea plate, where severe oblique collision is ongoing and results in a suture zone with a complicated Longitudinal Valley fault system (LVFS) in eastern Taiwan. This fault system is not only an important tectonic division but also capable of producing frequent damaging earthquakes, such as 2013 Ruisui earthquake, 2014 Fenglin earthquake, 2018 Hualien earthquake, etc. However, the understanding of the detailed LVFS fault geometry and segmentation at depths is still limited. The recent earthquakes ruptured several blind faults that were not identified in the past and provided more information than before for investigating the fault system. In addition, a new generation of Central Weather Bureau seismic network which eliminates telemetry delay issues also provides data with better time accuracy since 2012. Therefore, this study is aimed to revisit the LVFS at higher resolution with the new dataset from January 2012 to June 2019 and double difference tomography method. Comparing with the result from Kuo-Chen et al. (2012) and Huang et al. (2014), our results demonstrate that the new imaging show more detailed and sharpened features in correlation with geological units and LVFS. In NNE-SSW cross-sections, we find a low velocity structure beneath Lichi Melange and a relative high velocity structure near Chimei fault. In EES-WWN cross-sections, we can see a west-dipping seismic zone cutting through a high velocity body, which implies a fault structure rather than a lithological boundary. Waveform cross-correlation technique to calculate relative arrival times can be further combined to help improve and derive an even finer velocity model in the future.

並列關鍵字

Longitudinal Valley fault ； velocity model ； double-difference tomography

參考文獻

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以雙差分地震層析成像法探討台灣東部縱谷斷層系統構造

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