利用周遭噪訊層析成像探討臺灣淺部地殼構造

Abstract

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。

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).