臺灣地震與長微震之動態誘發

Dynamic triggering of earthquakes and tremors in Taiwan

葉庭禎(Ting-Chen Yeh)

指導教授 ： 胡植慶

共同指導教授 ： 陳卉瑄

國立臺灣大學/理學院/地質科學研究所/碩士(2011年)

https://doi.org/10.6342/NTU.2011.01577

摘要

深層非火山長微震 (non-volcanic tremors) 的發現為科學家開啟了一個新的研究視野，透過非火山長微震的研究能藉以了解在孕震帶深處複雜的斷層變形行為。在研究長微震的期間，科學家也發現低頻地震 (low frequency earthquakes) 的訊號同時出現，研究結果顯示長微震的物理模形為透過脆性破裂釋放輻射地震能。能夠造成長微震這種特殊訊號的環境通常認為與具有高流體液壓的區域斷層或是非均質的韌性剪切帶有關。在臺灣 Peng and Chao (2008) 與 Chao et al. (2011) 發現了長微震被遠距地震觸發的訊號，他們認為觸發長微震動態應力的門檻值約為 7–8 kPa。而本研究藉以分析地震波形來瞭解動態觸發地震與長微震之觸發因子有何異同。根據本研究的結果，在蘇門答臘地區自 2000 年到 2010 年期間 Mw ≧ 7.0 的地震中，一共有四個地震觸發台灣之長微震，三個地震觸發台灣的地震活動，我們利用表面波的振幅來計算動態應力值，其結果顯示觸發長微震與觸發地震的最小動態應力值分別為 2.86 kPa 和 0.0454 kPa，故我們推測淺部的地震與深部的長微震對於遠距地震帶來的動態應力反應並不相同。此外，遠距地震之表面波振幅與其波形之背景雜訊為主要影響長微震的觸發因子。根據研究結果，台灣觸發長微震之震央局限在中央山脈南部的一小區域，此區域在孕震帶深部有群震的現象與較高的Vp/Vs值。我們推測，因為此環境的條件，使得有效正應力相當低，故該區的斷層帶非常脆弱，所以只要微小的動態應力擾動則可以觸發長微震，故認為台灣長微震的活動與流體的活動高度相關。

關鍵字

動態觸發 ； 觸發長微震 ； 觸發地震

並列摘要

The discovery of deep-seated tremor opened up a new window into the complex processes below the base of the seismogenic zone. As tremor and associated low frequency earthquakes appear to represent shear failure capable of producing seismic radiation, a brittle failure model is generally proposed as the physical mechanism. It is possible that very high fluid pressure on discrete fault patches or distinct lithologic heterogeneity facilitate slip in an otherwise ductile shear regime. Tremor in Taiwan was triggered by teleseismic surface waves from distant events [Peng and Chao, 2008; Tang et al., 2010; Chao et al.,
in review, 2011], where a threshold of dynamic stress 7–8 kPa is inferred. Built on the previous research, in this study we refine the triggering threshold and further explore the similarity/difference in triggering response of tremor and earthquakes. This research illuminates the differences in response of shallow and deep portions of the fault to dynamic stress changes induced by distant earthquakes. We found 4 of 21 Sumatra Mw ≧ 7.0 earthquakes since 2000 triggered tremor, while 3 events triggered small earthquake within the surface wave train. The dynamic stress calculation indicates that the triggered stress induced by surface waves need to be above 2.86 kPa and 0.0454 kPa for immediate triggering of tremor and earthquakes, respectively. The tremors are confined in a small area underneath south of Central Range, below the seismogenic zone where the earthquake swarms and higher Vp/Vs ratio are located. This suggests the possible connection between tremor activity and fluid process. The dominant role of small shear-stress perturbations in stimulating the tremor but not crustal earthquakes indicates a very weak fault zone with extremely low effective normal stress. Likely due to near lithostatic pore pressures in the tremor source region.

並列關鍵字

dynamic triggering ； triggered tremor ； triggered earthquakes

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