臺灣地區近地表之震波非均向性研究

On the Near-surface Seismic Anisotropy of Taiwan

陳力維(Li-Wei Chen)

指導教授 ： 龔源成

共同指導教授 ： 梁文宗(Wen-Tzong Liang)

國立臺灣大學/理學院/地質科學研究所/碩士(2014年)

https://doi.org/10.6342/NTU.2014.01845

摘要

本研究使用井下-地表地震儀資料，以尾波交互相關法建立垂直及水平經驗格林函數，量測近地表(< 400m) 33組震波非均向性快軸方向及強度，並且同時獲得31筆Vp、33筆均向性Vs、31筆Vp/Vs比值。
過去研究臺灣地殼內的震波非均向性以觀測地震剪力波分離及表面波層析成像為主，由於此二法分別受限於路徑平均效應及缺乏高頻資料，無法解析近地表的震波非均向性。中央氣象局從2011年陸續建置了30個地表-井下地震觀測站，配合臺灣寬頻地震網既有的4個地表-井下測站，提供了難得的機會得以全面了解臺灣近地表震波特性。
結果顯示，臺灣近地表的震波非均向性強度變化大(~ 0-30%)且和區域地質高度相關，在空間分佈上呈現兩種不同的形態：(1)臺灣的山脈區域，震波非均向性快軸方向平行於山脈的走向，與地層內傾斜的橫向均向構造相關。 (2)西部平原和縱谷地區富含沖積物的區域，震波非均向性的快軸方向平行於最大主應力方向，由岩層中的節理或裂隙所主導，而震波非均向性強度與井體中地層的岩性相關。普遍來說，位於變質岩帶的量測強度最大，位於沉積岩區域的量測強度次之，而年輕未固結的沖積層、膠結鬆軟的沉積岩、還有火成岩區域的量測強度為最小。
臺灣近地表的震波速度及Vp/Vs比值量測與井體中地層的岩性高度相關。測站位於年輕且未固結的沖積層或階地堆積層之中，由於膠結疏鬆的岩層中孔隙率高，含水飽和程度高，所得震波速度慢且Vp/Vs比值高。而測站位於中央山脈區域的變質岩帶，整體而言震波速度快且Vp/Vs比值低。

關鍵字

地震尾波 ； 震波干涉法 ； 震波非均向性

並列摘要

We measure the near-surface Vs, Vp and Vs azimuthal anisotropy of Taiwan by using the Empirical Green’s Functions (EGF) between the newly deployed borehole seismic array and their corresponding overhead surface stations.
Since the inter-station distances of the borehole-surface station pairs are less than 400m, relatively high frequency EGFs are required to accurately measure the azimuthal anisotropy. We compared EGFs derived from two approaches: (1) deconvolution of earthquake signals, and (2) cross-correlation of earthquake coda waves from local earthquakes (ML>4). While both approaches may provide robust high frequency EGFs (2-8 hz), the application of deconvolution method is limited by the nearby seismicity, thus, most of our results are derived by the coda cross-correlation method.
We derived EGFs from 34 borehole-surface station pairs. We first correct for the borehole sensor orientation, we then measured the Vs azimuthal anisotropy, and evaluate the shallow Vp, Vs and Vp/Vs at each borehole site.
In more than 30 derived EGFs, we have found clear cos2θ patterns of Vs azimuthal dependence. These results represent direct measurements for the near-surface seismic anisotropy, and they are strongly correlated with the surface geology. In general, the fast axis of Vs anisotropy is sub-parallel to the mountain strike in the mountain area, and sub-parallel to the trend of maximum compression stress, suggesting that the observed anisotropy is likely related to the orogeny-induced fabrics in mountain ranges and stress-aligned cracks in western plains, respectively. The strong near-surface anisotropy also implies that delay times contributed by the shallow crust might be underestimated in studies of the shear-wave splitting measurements.

並列關鍵字

earthquake code waves ； seismic anisotropy ； seismic interferometry

參考文獻

陳勇全 (2004), 六龜地區礫岩沈積環境與潮州斷層之研究.

Kuo‐Chen, H., F. T. Wu, D. Okaya, B. S. Huang, and W. T. Liang (2009), SKS/SKKS splitting and Taiwan orogeny, Geophysical Research Letters, 36(12).

Huang, T., Y. Chen, Y. Gung, L. Chiao, W. Liang, and S. Lee (2013), Resolving the crustal seismic anisotropy of Taiwan using ambient seismic noises, paper presented at AGU Fall Meeting Abstracts.

Alford, R. (1986), Shear data in the presence of azimuthal anisotropy: Dilley Texas, paper presented at 1986 SEG Annual Meeting, Society of Exploration Geophysicists.

Bos, A. G., W. Spakman, and M. C. Nyst (2003), Surface deformation and tectonic setting of Taiwan inferred from a GPS velocity field, Journal of Geophysical Research: Solid Earth (1978–2012), 108(B10).