聯合接收函數與表面波頻散資料逆推高加索下方之岩石圈速度構造

Lithospheric Structure under the Caucasus Mountains from Joint Inversion of Receiver Functions and Rayleigh Wave Dispersion

李詩婷(Shih-Ting Li)

指導教授 ： 曾泰琳

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.2014.01834

摘要

高加索山脈為阿拉伯板塊向歐亞大陸板塊擠壓所產生的變形帶最北緣。本研究結合接收函數與表面波資料進行逆推，以求得喬治亞聯合地震網底下之速度構造。此地震網主要分布於大高加索中西部與小高加索，由11個中研院地球所於喬治亞境內設置之臨時寬頻地震儀以及4個喬治亞本國之寬頻測站所組成。研究方法可以抑制地層中不連續面深度與絕對速度之間的權衡問題（trade-off）。
本研究利用雙站法得到表面雷利波之相速度頻散曲線，週期5-80 s之結果顯示小高加索的相速度平均值比大高加索高出約0.25 km/s。在計算接收函數時，每個測站的遠震資料給予兩種不同頻寬的高斯低通濾波，並將Z分量對R分量做解迴旋以求得接收函數波形。由於某些測站接收的資料隨不同方位角與波線參數有所變化，我們先集中討論來自後方位角90° 到120°區間的地震資料。
聯合逆推法所得的大高加索西部之莫荷面（Moho）深度約為40-50 km，並且大高加索中部的莫荷面深度有由西向東變深的趨勢，其深度也與H-κ stacking所得到的結果一致。小高加索的速度梯度範圍較寬，約落於45-65 km之間，莫荷面位置較不明顯，若以絕對速度值Vs = 4.2 km/s定義為莫荷面位置，相較於前人文獻對於此區域的測量結果稍淺2-5 km。此外，我們在TRLT測站下方約5 km深處觀測到一明顯低速層，此測站位於火山高原並且附近有活躍的微地震，雖然低速帶特徵類似安地斯山脈中部Altiplano-Puna火山區域的研究結果，未來仍需更多資料證實此低速層是否與地底下之岩漿體有關。

關鍵字

高加索 ； 地殼構造 ； 接收函數 ； 大陸碰撞 ； 表面波

並列摘要

The Caucasus mountain belts may be considered as the northernmost boundary of the collision created by the impinging of Arabia to Eurasia. In this study, we jointly invert receiver functions and surface wave data to estimate the velocity structure under a combined seismic network in Georgia covering the central-western part of the Greater and Lesser Caucasus. This seismic network consists of 10 broadband stations from the Institute of Earth Sciences, Academia Sinica of Taiwan and the other 4 from the Georgia Seismic Center. The method can obtain the detail velocity structure with little trade-off between absolute velocity and depth of the discontinuity.
　　For the surface wave constraints, we extract surface Rayleigh wave phase dispersion using two-station method and the results indicate that the phase velocity under the Lesser Caucasus is higher than Great Caucasus by 0.25 km/s in average. As for the receiver functions, we select teleseismic earthquakes and deconvolve Z from R components for each station using different Gaussian filters. Because of the azimuthal variations and ray parameter-dependent variations are found at some stations, we first focus on the earthquakes with back azimuth from 90° to 120°.
　　The Moho depth obtained by joint inversion is about 40-50 km for the stations in the Greater Caucasus, and the depth increases toward the eastern part of the array under the central Great Caucasus. The results are consistent with the depths obtained using the H-κ stacking technique. As for the Lesser Caucasus, the boundary of Moho is less obvious, with a wider zone of gradient between 45 and 65 km. The Moho depth is 2-5 km shallower than the previous estimates using a threshold value Vs = 4.2 km/s as the definition. Interestingly, we discover a very thin but sharp low-velocity layer at the depth of 5 km beneath station TRLT, which is located on the Javakheti volcanic plateau with active earthquake swarms nearby. More evidences are required to confirm whether the low-velocity layer is associated with a magma chamber underneath, similar to the findings from the receiver function studies in the Altiplano-Puna volcanic region, Central Andes.

並列關鍵字

Caucasus ； crustal structure ； receiver function ； continental collision ； surface wave

參考文獻

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