大别-苏鲁及其邻近地区基于背景 噪声的勒夫波群速度成像

Love wave group velocity tomography in Dabie-Sulu and its adjacent areas from seismic ambient noise

  • 摘要: 大别-苏鲁造山带是中国大陆东部地区最重要的构造之一. 为了研究该地区的地壳上地幔速度结构,本文收集了国家数字地震台网和中国区域地震台网的山东、河南、安徽、江苏和湖北等省的144个宽频带地震台连续两年(2009年5月—2011年5月)的水平向地震记录(E分量和N分量)数据,首先对台站对之间E-E,E-N,N-N和N-E分量进行互相关,然后分别对这4个互相关分量采用相位权重叠加法进行叠加,最后旋转到横向分量(T-T)获得勒夫波经验格林函数(EGF);用频时分析(FTAN)方法获得4000余条勒夫波群速度频散曲线,并进一步反演得到了周期为6—40s的勒夫波群速度分布图.结果表明,周期为6—10s的勒夫波群速度分布与地表构造特征相吻合.大别造山带、苏鲁造山带、湖北西部隆起均表现为高速;华北盆地发育,表现为大面积的低速;江汉盆地、南襄盆地、合肥盆地等因其规模不同而显示不同程度的低速.在周期为6—30s的勒夫波群速度分布图上,大别和苏鲁地区均显示高速,已有的研究结果中地壳的低速并没有得到反映.其原因一方面可能与勒夫波群速度纵向分辨不高有关,另一方面高压变质岩深度分布可能比已有研究结果给出的要深. 6—30 s的分布图上郯庐断裂带及其邻近地区表现为不同程度的高速,可能与该地区白垩纪以来处于拉张构造体制,地幔物质受到扰动,造成物质上涌有关.

     

    Abstract: The Dabie-Sulu orogenic belt is one of the major tectonic zones in eastern China Continent. In order to obtain the velocity structure of the crust and upper mantle, we collected the continuous horizontal components seismograms recorded by Shandong, Henan, Anhui, Jiangsu, Hubei seismography networks and China Broadband Digital Seismography Network during May 2009 through May 2011.Firstly, component cross-correlations (E-E, E-N, N-N, N-E) for every station pairs were performed; secondly the four component cross-correlation functions were stacked by using the phase-weight stacking method; thirdly the components (E-E, E-N, N-N, N-E) were rotated into the radial (R) and transverse (T) components to obtain empirical Green’s functions of Love waves; finally more than 4000 Love wave group velocity dispersion curves are got by frequency-time analysis (FTAN), and the Love wave group velocity maps in the period of 6—40 s were reconstructed. The group velocity maps from 6 s to 10 s are well consistent with surface geological features. The results reveal high velocity anomalies in Dabie orogenic belt, Sulu orogenic belt and western Hubei uplift. North China exhibits low velocity due to its basin structure; Jianghan, Nanxiang and Hefei basins also show low velocity structures. The Love wave group velocity maps at the period of 6—30 s indicate that there are high velocity anomalies in Dabie and Sulu orogenic belts, while the low velocity anomalies in the middle crust in the previous studies cannot be found in this study. On one hand, it may be due to poor depth resolution of Love waves group velocity; on the other hand, this is probably because the metamorphic rock zone extends deeper than that in the previous studies. High velocity anomalies appear in Tanlu fault zone and its adjacent regions at the group velocity maps in the periods of 6—30 s, which may be caused by the upwelling of the mantle material and the mass exchange between the lower crust and upper mantle in the state of extension since the Cretaceous.

     

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