利用尾波干涉法分析地震及應變資料探討2013瑞穗地震前後地殼變化

Temporal variations of crustal properties induced by the 2013 Ruisui, Taiwan earthquake from coda wave interferometry with ambient seismic and strain fields

戴汶珮(Wen-Pei Dai)

指導教授 ： 洪淑蕙

國立臺灣大學/理學院/地質科學研究所/碩士(2018年)

https://doi.org/10.6342/NTU201801299

摘要

本研究結合中央氣象局(CWBSN)與台灣寬頻地震網(BATS)所提供之三分量地震儀，以及中央研究院地球科學研究所(ASIES)的井下應變儀連續記錄的周遭噪訊，透過計算每天單站自我相關函數(Auto-Correlation Function, ACF)和任兩站間的交互相關函數(Cross-Correlation Function, CCF)分別得到測站底下和站間穩定的經驗格林函數(Empirical Green's Function, EGF)之時序變化，並結合尾波干涉法(coda interferometry)量測各天EGF直達表面波後的尾波相對到時和波形相關性的差異，用以研究發生於2013年10月31日花東縱谷北段，地震矩規模6.2的瑞穗地震前後，震源附近淺層地殼受到擾動所引起的速度及散射構造在時間和空間變化的分佈情形和其可能成因。
研究結果顯示在0.1-0.9Hz的頻段中，震後速度擾動有突降或上升的現象，且震前的波形相關性明顯下降，意味著斷層帶在震前的微弱擾動會影響地殼散射構造。根據尾波前表面波的群速度和相速度分析，顯示訊號能量主要在3-5秒之間，對3公里附近的剪力波速度最為敏感，因此比較推得的速度變化與3 km深度附近的同震靜態體積應變，顯示速度下降路徑大致集中在體積應變拉張區域。除了同震變化之外，亦發現井下應變儀連續記錄重建的EGF所求得的速度擾動變化，有一約4個月的週期性變化，推測應與瑞穗鄰近地區地下水位有所關聯。另在地震之外的時間段發現尾波能量異常，波形相關係數劇烈起伏的現象，其可能為噪訊源受到夏季季風的影響，使得相關係數值與速度擾動結果皆受到干擾，若使用井下地震儀則可避免此異常結果，獲得更精確、僅受到地震應變影響的速度擾動結果。

關鍵字

周遭噪訊 ； 交互相關函數 ； 經驗格林函數 ； 尾波干涉法 ； 同震速度變化 ； 同震體積應變

並列摘要

We use continuous three-component seismic and volumetric strain waveform data recorded by seismometers and borehole strainmeters, respectively, from CWBSN, BATS and AS of IES, to reconstruct daily variations of empirical Green’s functions (EGFs) from single-station auto-correlation functions (ACFs) and cross-correlation functions (CCFs) between pairs of stations. Coda wave interferometry is then employed to investigate temporal changes in crustal velocity and mechanical properties preceding and following the 2013 Mw 6.2 Ruisui Earthquake that struck the northern segment of the Longitudinal Valley in eastern Taiwan and investigate the possible physical causes for the changes.
By comparing the pre- and post-event coda waves extracted from the ACFs and CCFs, we present a strong case that not only coseismic velocity reduction but also preceding decorrelation of waveforms are explicitly revealed in both the seismic and strain CCFs filtered in the secondary microseism frequency band of 0.1-0.9 Hz. Dispersion analysis of surface waves before coda arrivals shows that signals are dominated in 3-5 s and most sensitive to shear wave velocity at about 3-km depth. We thus compare the obtained interstation path-average velocity perturbations with lateral variations of static volumetric strain induced by coseismic slip distribution and find that the regions showing pronounced coseismic velocity reductions often experience extentional strain changes. In addition to coseismic variations revealed in both the seismic and strain CCFs, quasi-periodic variations only appear in the strain derived EGFs, with a predominant cycle of about 4 months correlating with the groundwater fluctuations observed at Ruisui. During the season of summer monsoon between July and October, the extracted coda signals are anomalously weak and yield irregularly and largely fluctuating variations in the estimated velocity and waveform correlation coefficient. Such phenomena may be associated with the disturbance of very shallow structures affected by significant precipitation amounts and rapid rise of ground water level. The use of borehole seismometer can help prevent this obstruction and obtain more stable and accurate estimates in earthquake-induced velocity and structural perturbation.

並列關鍵字

ambient noise ； empirical Green’s function ； cross-correlation function ； coda wave interferometry ； coseismic velocity change ； coseismic volumetric strain

參考文獻

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