采用SS前驱波研究青藏高原地区地球内部结构

There are several seismic velocity discontinuities in the upper mantle. These discontinuities provide important constrains on thermal and chemical state of the mantle and associated dynamics. To study the discontinuities beneath regions where few seismic stations are deployed, the most ideal phase is the SS precursors, which reflect off the underside of discontiuities and arrive prior to the surface reflection SS. The deep interior of the Earth beneath the Tibetan plateau is complicated, where the continental collision caused significant orogeny. In this proposal, we examine SS precursors to study deep structure of the Earth beneath the Tibetan and adjacent regions. We primarily use European waveform recordings of earthquakes in west Pacific. We will estimate discontinuity properties, including their depth, topography, and velocity and density jumps based on traveltimes and amplitudes of SS precursors. We quantitatively examine the accuracy of traveltime and amplitude estimates with synthetics which are calculated using the spectral-element method. In order to improve the accuracy of waveform analysis, we use the finite-frequency theory to calculate the traveltime correction due to the velocity heterogeneity in the mantle. The main goal of this proposal is to study mantle discontinuities beneath the Tibetan region based on a new SS precursor data set to determine whether the continental collision and plateau uplift affect the deeper parts of the upper mantle.

上地幔内存在很多速度间断面，这些间断面为地球内部温度、物质成分及其动力学过程的研究提供重要的理论依据。对于缺少台站覆盖地区的间断面研究，目前最理想的震相是SS前驱波，即到达于地表反射SS波之前的间断面底部反射波。青藏高原地区地球内部结构复杂，大陆碰撞造成剧烈的造山运动。该项目利用SS前驱波研究青藏高原及其周边地区地球内部结构。观测数据主要来源于欧洲地震台网记录到的地震位于西太平洋地区的波形。通过拾取SS前驱波走时和振幅异常，获得间断面深度、起伏和间断面两侧速度与密度对比等信息。利用谱元法计算理论地震波形，对走时和振幅计算误差进行定量评估。将有限频理论引入到速度校正过程，提高求解精度。该项目的核心目标是，基于新的SS前驱波波形数据，探测青藏高原地区地幔间断面特征，为研究大陆碰撞与高原隆升对上地幔深部结构的影响提供新的理论依据。

本研究从实际观测与理论地震波形两个方面入手，通过收集全球地震台网记录到的波形数据，采用谱元法和多种三维地壳和地幔模型计算理论地震波形，提取SS波在410公里和660公里间断面底部反射波的走时和振幅，获得间断面深度、起伏和间断面两侧速度与密度对比等信息，并对走时和振幅的研究精度进行定量性分析。研究发现，地幔过渡带厚度横向变化高达50公里，其空间分布特征直接反映板块俯冲等地球动力学过程。基于射线追踪方法对路径上三维速度结构进行校正，可以很好地去除结构对走时的大尺度影响，但是可能无法正确识别5公里以下的间断面起伏。尽管很多研究认为SS波与前驱波的传播路径大体相似，但是三维速度结构造成的波形聚焦作用在振幅观测中不可忽视。660公里间断面的观测振幅远远小于PREM模型预测的理论振幅，表明PREM模型660公里间断面的参数需要加以改进。