利用周遭噪訊層析成像探討臺灣淺部地殼構造

Shallow Seismic Structure of Taiwan Constrained by Ambient Seismic Noises

簡志傑(Chih-Chieh Chien)

指導教授 ： 龔源成

國立臺灣大學/理學院/地質科學研究所/碩士(2017年)

https://doi.org/10.6342/NTU201702613

摘要

本研究利用周遭噪訊層析成像，自噪訊中萃取短週期(3-7秒)雷利波計算經驗格林函數，檢視臺灣本島淺部地殼(<10公里)剪力波速度構造及震波非均向性。震波資料來自2006-2007年間六個不同觀測網，為了增加波線覆蓋程度，首次計算寬頻與短週期資料之間的交互相關函數，並透過多重尺度小波逆推，建構臺灣淺部地殼三維均向與方位非均向性剪力波速度模型。
均向速度方面，整體淺部地殼的剪力波速度側向變化強烈，且速度分布與地表地質十分吻合，高速異常帶主要分布在中央山脈，速度約3.0-3.5 km/s，低速異常帶分布在桃園臺地、彰化平原、嘉南平原與花東縱谷，速度約1.5-2.3 km/s，其中位在嘉南平原的低速異常帶可延續至深度5-7公里。非均向性部分，地殼中的非均向性大致可分為(1)主要反映岩層中裂隙，導致快軸方向平行最大主應力型態與(2)受面理差異排列主導的構造相關型態。研究成果顯示，臺灣淺部地殼的剪力波方位非均向性快軸方向主要沿著造山帶走向，與近地表非均向性量測成果相比，我們認為快軸方向平行最大主應力的非均向性主要侷限在深度1公里以內。

關鍵字

周遭噪訊層析成像 ； 剪力波方位非均向性 ； 臺灣淺部地殼地震地體構造

並列摘要

In this study, we aim to better constrain the shallow seismic structure (<10 km) of Taiwan by utilizing the short period (3-7 seconds) empirical Green’s functions (EGFs) of Rayleigh waves extracted from ambient seismic noise. To improve the data coverage, continuous data recorded by 6 seismic networks during the time period from 2006 to 2007 are used to derive the EGFs between all the cross-network station pairs. With this unprecedented data set, we are able to build up high resolution 3-D Vs and Vs azimuthal anisotropy models for the shallow crust of Taiwan, using a wavelet-based multi-scale inversion technique.
Robust features both in Vs and Vs anisotropy variations of the shallow crust are obtained in the resulting 3-D models. The strong Vs lateral variations are closely related to the surface geology, and the patterns of Vs anisotropy are mostly dominated by structure-related anisotropy, with fast polarization direction generally parallel to the mountain ranges. With the new results, and our recent studies with broad-band EGFs and borehole noise interferometry, we conclude that the stress-aligned anisotropy is likely confined in the near-surface depths (< ~1 km).

並列關鍵字

ambient seismic noise tomography ； Vs azimuthal anisotropy ； shallow seismic structure of Taiwan

參考文獻

Chen, K.-X., Chen, P.-F., Chen, L.-W., Yao, H., Fang, H., and Su, P.-L., 2016, South Ilan Plain High-Resolution 3-D S-Wave Velocity from Ambient Noise Tomography: Terrestrial, Atmospheric and Oceanic Sciences, v. 27, no. 3, p. 375.

Bensen, G. D., Ritzwoller, M. H., Barmin, M. P., Levshin, A. L., Lin, F., Moschetti, M. P., Shapiro, N. M., and Yang, Y., 2007, Processing seismic ambient noise data to obtain reliable broad-band surface wave dispersion measurements: Geophysical Journal International, v. 169, no. 3, p. 1239-1260.

Brocher, T. M., and Christensen, N. I., 1990, Seismic anisotropy due to preferred mineral orientation observed in shallow crustal rocks in southern Alaska: Geology, v. 18, no. 8, p. 737-740.

Chang, C.-P., Chang, T.-Y., Angelier, J., Kao, H., Lee, J.-C., and Yu, S.-B., 2003, Strain and stress field in Taiwan oblique convergent system: constraints from GPS observation and tectonic data: Earth and Planetary Science Letters, v. 214, no. 1-2, p. 115-127.

Chang, E. T. Y., Liang, W.-T., and Tsai, Y.-B., 2009, Seismic shear wave splitting in upper crust characterized by Taiwan tectonic convergence: Geophysical Journal International, v. 177, no. 3, p. 1256-1264.