利用接收函數及表面波頻散之聯合逆推法探討西藏Hi-CLIMB陣列下之地體構造

Abstract

西藏為印度與歐亞兩個大陸板塊經過約5千萬年的碰撞而生成的高原，是全球地殼最厚、平均海拔最高且面積最廣之地質區塊。本研究中，我們將接收函數與頻散曲線兩種地震資料結合在一起來逆推，以求得南北向的Hi-CLIMB線性陣列下方之速度構造。接收函數對垂直速度變化為較敏感，但面臨地層速度與厚度之間的權衡 (trade-off) 問題，表面波資料的加入可以有效增加絕對平均S波速度 (VS) 的約束，得到更可靠的解。對於表面波頻散資料，我們直接採用Lai (2009) 以雙站法所量測的雷利波相速度值；至於接收函數，我們將Z分量對R分量進行解迴旋計算接收函數波形，並採用高斯低通濾波 (Gaussian filter width) 為1.0與2.5之兩種不同頻寬，最後將各測站之頻散曲線與依據方位角以及慢度疊加後之接收函數同時進行逆推。研究結果顯示拉薩地區之莫荷面深度約為75公里，上部地函80至120公里深之平均速度約為4.6公里/秒。相對的，羌塘地區之莫荷面較淺，約為65公里，而上部地函平均速度約為4.3公里/秒，比拉薩地區明顯慢了約7%，此南北上部地函的速度差異比Hung et al. (2011) 在層析成像中的結果5%更強。莫荷面沿著Hi-CLIMB剖面的變化與前人研究Tseng et al. (2009) 和Nowack et al. (2010) 一致。我們從結果中還觀察到一層5至10公里厚且較連續的低速帶分布在Hi-CLIMB下方15至20公里處，伴隨著斷續出現的30公里深低速層，此外還有一個明顯的低速層在50公里下部地殼分布於雅魯藏布縫合帶附近。此區域包含三種深度的低速帶，中間層較不明確，但整體與Hung et al. (2011) 的地殼低速層尺度相符，推測可能與隆格爾裂谷 (Lunggar rift) 有所關連。

Tibetan Plateau is a product of the continental collision between India and Eurasia beginning about 50 million years ago. The crust under Tibet has been greatly thickened and the Moho depth is the deepest of the world. In this study, we use joint inversion of receiver functions and surface wave dispersion curves to estimate the velocity structure under each seismic station along a north-south array of Hi-CLIMB experiment. Receiver functions highlight the P-to-S conversions generated by layered discontinuities under a station of sub-vertical waves, thus the results are sensitive to depth of velocity contrast in fine structures. The trade-off between absolute velocity and depth of the discontinuity is eliminated by including independent constraints from the surface wave dispersion. For the surface wave constraints, we use phase velocities of the fundemantal Rayleigh wave measured under Hi-CLIMB by Lai (2009) using two-station method. As for the receiver functions, we select teleseismic earthquakes and deconvolve Z from R components for each station for two different frequency conditions (Gaussian width of 1.0 and 2.5) . Because of the azimuthal variations, we focus on the earthquakes coming from the southeast quadrant. The results show that the average depth of Moho undersouthern-central Lhasa terrane is about 75km, and the corresponding shear wave velocity (VS) in the upper mantle is about 4.6 km/s at the depth between 80 and 120 km. In contrary, the average Moho depth beneath Qiangtang terraneis about 10 km shallower and the upper mantle VS is slower by nearly 7% (i.e., VS of ~ 4.3 km/s) . Such contrast in VS between the two terranes is also observed in the finite frequency tomography by Hung et al. (2010, 2011) but the intensity is more prominent in our model. The variation of Moho depths along the array is in good agreement with the previous estimates using virtual seismic reflection profiling of SsPmp (Tseng et al., 2009) and Gaussian beam imaging (Nowack et al., 2010) . We also detect several thin layers (~5-10 km) of low velocity zone at different depths within the Tibetan crust. The shallowest one is at the depth of about 15-20 km, which extends almost continuously under Lhasa and Qiangtang terranes accompanied with some snatchy low velocity layers at depth of ~30 km. Another low velocity layer is clearly identified at the depth of about 50 km beneath the southernmost Lhasa near the Yarlung-Zanbo suture. In this region, the crustal structure contains a total of three low velocity layers that coincide with the crustal-scale, low-velocity anomaly in the tomography (Hung et al., 2011) and could be associated with the active Lunggar rift.