球坐标解析定位技术在龙门山近震层析中的应用

通过对球坐标梯度速度模型的角距离和走时公式的推导,获取了射线参数驱动的可解析计算的全球多震相时距及其对于模型,震中和地表速度的导数表达式.与TAUP软件进行对比发现除了散射震相外,所有震相的时距曲线误差均可忽略,该方法考虑了高程,在性能和应用方面也可以完全取代TAUP相应的模块.在此基础上升级了耦合一维模型的定位系统,并应用到龙门山地区的近震重新定位,通过对远震的局部化处理,增加了近震层析的射线覆盖范围和密度,使下地壳速度模型在中上地壳模型的约束下有所呈现.通过近震层析处理发现松潘地区浅部存在大量的高低速相间分布,四川盆地深部为扬子板块高速物质控制,浅部则以低速为主.龙门山断裂带浅部呈现高速为主的特征,而中地壳广泛存在低速物质的分布,龙门山断裂控制的彭灌杂岩体存在根部,收束在20 km深度中地壳滑脱带上.

The spherical analytic relocation technique and its application to local seismic tomography in Longmenshan area

By deducing the formulae of arc distance and travel time according to the gradient velocity model in spherical coordinates, driving by ray parameters,this work obtained the analytical expressions of time-distance and their derivatives to the model, hypocenter, and surface velocity. Comparing to TAUP software package, except for diffraction phases, the time-distance curve error could be neglected for all phases. Moreover, this method surpasses TAUP in the efficiency and application since it also considers the elevation of stations. On the basis of these formulae, coupled with 1-D model optimization, an earthquake relocation system is upgraded and used in the earthquake relocation for the Longmenshan area. Through localization for teleseismic phases, both ray coverage and ray density are improved for local seismic tomography, so that the velocity model of lower crust is illustrated with the constraint of the upper crustal model. By this tomography, we found that in the shallow subsurface of the Songpan area exists abundant alternating distribution of high and low velocities. The deep part of the Sichuan basin is dominated by high velocity material from the Yangtze plate while its shallow part is dominated by low velocity. The shallow subsurface of the Longmenshan area is featured by high velocity, while its middle crust has large amount of low velocity materials. The Pengguan complex which is bounded by the Longmenshan fault is rooted and convergent to a detachment zone at 20 km depth.