青藏高原及其周边区域上地幔顶部Pn和Sn波Q值成像研究

In Tibetan Plateau, uppermost mantle Pn- and Sn-waves attenuation, similar to crustal Lg-wave attenuation, is one of research forefronts in seismology.The correlation between crust and uppermost mantle attenuations indicates the crust-mantle weak layers, revealing the crust-mantle decoupling in the plate-driven crust-mantle deformations. Following the research work regarding crustal Lg-wave attenuation, we investigate the propagation characteristice of the uppermost mantle Pn- and Sn-phases in Tibetan Plateau. Based on the regional seismic data in Qinghai-Tibet Plateau and its surrounding regions, we calculate the amplitude spectra for both Pn- and Sn-waves, and then extract the dual-station and dual-event data. According to the previous crustal model (e.g., CRUST2.0 and other new results), we construct the propagation geometries for the uppermost mantle phases. At individual frequencies, we obtain the regional Q values, the source spectral functions, the source radiation patterns, and site responses by using a multi-parameter joint inversion scheme. We construct a broadband high-resolution tomographic attenuation models for the uppermost mantle phases, Pn between 0.5 and 10.0 Hz and Sn between 0.1 and 10.0 Hz, respectively. We use a statistic method to obtain the average uppermost mantle Q values for the different geological blocks in and around Tibetan Plateau. The low Q overlaps between the uppermost mantle Pn and Sn and the crustal Lg (which we have already obtained) indicate the weak layers between crust and mantle, suggesting the regions with crust-mantle decoupling. These results will benefit understanding the thickening and uplift of the Qinghai-Tibet Plateau.

青藏高原上地幔顶部Pn波强衰减、Sn波不发育区与地壳Lg波衰减的调查是地震学研究的前沿。地壳-上地幔顶部地震波衰减结构的相关性能够指示壳幔之间软弱层的分布，揭示板块驱动壳幔变形模式中壳幔的耦合-解耦关系。继地壳Lg波衰减成像研究之后，开展青藏高原地区上地幔顶部Pn波Sn波的震相特征调查。利用青藏高原及其周边地区丰富的区域地震资料，计算Pn和Sn波振幅谱，提取双台和双事件振幅谱比值数据。根据已有的地壳模型，构建上地幔顶部震相的传播几何。实现区域Q值、源谱函数、辐射花样和台基响应等多参量联合反演，建立青藏高原及其周边区域宽频带高分辨率的上地幔顶部Pn波(0.5-10.0 Hz)、Sn波(0.1-10.0 Hz)Q值成像模型。统计分析青藏高原及其周边区域不同地质块体的上地幔顶部Pn和Sn波Q值，并与已有的地壳Lg波Q值比较，确定壳幔耦合型式。为解析青藏高原增厚和隆升提供新的地震学约束。

根据22,760条宽频带垂直分量数字地震记录，研究了青藏高原地区上地幔Pn波的几何扩散和衰减。利用对数域内抛物线型几何扩散函数，从观测的Pn波振幅谱数据中分离几何扩散，从而定量地测量上地幔顶部的Pn波衰减。在青藏高原中部和东部观测到的Pn波的强烈衰减可能表明壳幔热活动强烈。在青藏高原中部，共轭断裂带之下可能存在一个热的上地幔。根据26,272条宽频带切向分量数字地震记录，研究了青藏高原地区上地幔Sn波的几何扩散和衰减。利用IASPI91速度模型，计算Sn波到达时间，以0.7 km/s的群速度窗口截取Sn波列，并测量0.05-10.0 Hz的振幅谱。我们建立了一套基于密集的多台多事件，频率独立的，衰减与源谱函数联合反演的上地幔顶部Sn波Q值成像方法。获得了上地幔剪切波Q值。在青藏高原中部地区，Sn波高度衰减。青藏高原周边地区，衰减相对较低。例如，喜马拉雅山、塔里木、柴达木、鄂尔多斯和四川盆地等。这些低衰减区域可能是印度板块和欧亚板块碰撞过程中古老地壳碎片沉积的地方。低Q值区域主要分布在青藏高原中部和四川盆地的西部边缘，可能预示着地壳和上地幔可能发生强烈的热运动。将地壳Lg波和上地幔剪切波Q模型与其它地球物理资料相结合，从青藏高原北部到东部，再到沿四川盆地西部边缘的东南部，发现了一个下地壳流动通道。