青藏高原东北缘地壳结构的各向异性远震接收函数成像研究

The northeastern Tibetan plateau (NETP) is the key region for studying the growth mechanism of Tibetan plateau. Many studies have shown the northeastern Tibetan plateau exhibits anisotropic characteristic. Its crustal and mantle structure characteristics can provide important information to reveal the role of regional tectonic evolution and dynamics. The teleseismic body wave receiver function multiple-phase H-k stacking imaging is an important method to obtain crustal thickness, Vp/Vs ratio. Comparing to traditional H-K stacking method which is based on the assumption of isotropic crust, the new method has a much better constraint on the estimation of the crustal thickness and Vp/Vs ratio in the strongly anisotropic tectonic domain. We extend the H-k stacking approach to anisotropic media. The synthetic seismograprams method based on ray theroy is used to calculate the amplitude and arrival time of each phase in anisotropic media. Consequently, we develop H-k stacking technique applied in anisotropic media to obtain better crustal thickness, Vp/Vs ratio and Poisson’s ratio structures in the northeastern Tibetan plateau in purpose of providing constraints and evidences on the lateral growth of Tibetan plateau.

青藏高原东北缘是研究青藏高原扩展机制的关键带域。前人研究表明东北缘的地壳结构呈现明显的各向异性特征，其壳幔结构特征可为揭示区域构造演化和动力学作用提供重要信息。远震体波接收函数多震相H-k叠加成像是获得地壳厚度、Vp/Vs波速比和泊松比的重要途径。传统的H-k叠加方法基于地壳各向同性假设，因此在各向异性较强的构造域计算地壳厚度和波速比会存在偏差。本项目拟基于各向异性介质射线理论，计算所有接收函数Ps震相及其多次反射震相的理论到时和振幅，从而发展基于各向异性介质的H-k叠加技术，准确获取青藏高原东北缘各向异性地壳的地壳厚度、波速比及泊松比结构。为高原扩展机制提供深部的约束和证据。

青藏高原东北缘是研究青藏高原扩展机制的关键带域。各向异性及其壳幔结构特征可为揭示区域构造演化和动力学作用提供重要信息。本项目利用国家和区域台网（190个）的宽频带台站记录的3499个远震事件，采用P波接收函数H-K叠加方法获得了研究区的地壳厚度、地壳波速比。研究结果显示,地壳厚度约为30~60km,整体表现为西深东浅南深北浅的变化特点。青藏高原东北缘东部显示较高的波速比（>1.7）自西向东增大。使用布设在甘肃和青海省的固定宽频带地震台记录的远震波形资料，挑选了高质量SKS震相，联合使用最小切向能量方法和旋转互相关方法获得了230对高信噪比分裂参数，同时对接收函数中Pms震相随方位角的变化进行拟合，得到了24个台站的地壳各向异性分裂参数。整个区域SKS分裂快波方向均值为123°，而Pms分裂快波方向均值为132°，且大部分区域SKS、Pms快波方向与地表构造走向相一致，说明青藏高原东北缘以岩石圈垂直连贯变形为主，地壳上地幔相互耦合。Pms、SKS分裂时差均值分别为1.0 s和0.6 s，说明地壳各向异性对于SKS分裂时差有较大贡献。昆仑断裂附近Pms、SKS分裂快波方向与昆仑断裂走向大致平行，说明昆仑断裂可能是岩石圈尺度的深大断裂，而阿尔金断裂东缘二者快波方向的差异性说明阿尔金断裂在东缘可能仅为地壳尺度的断裂。为高原扩展机制提供深部的约束和证据。