GPS和GRACE联合获取藏南垂直形变的关键问题研究

In southern Tibet ongoing vertical motions are caused by the the collision between India and Eurasia, characterized by a complex pattern and varying mechanism. Large numbers of GPS continuous and campaign sites were installed in Himalaya and southern Tibet, we focus on a study of vertical uplift or subsidence in the southern part of the Tibetan Plateau by performing a joint analysis of GPS and GRACE data. Vertical displacments measureed by GPS usually include tectonic deformation as well as non-tectonic time-dependent signals. The origin and nature of the transient processes can't be identified straightforward, which are reasons why any result of vertical motions inferred from GPS measurements are not be simply related to tectonic deformation. Seasonal signals (annual and semi-annual cycles) due to surface mass loading are detected in GPS postion time series. The signals are especially pronounced in vertical components. To quanitify the regional long-term deformation using GPS, seasonal elastic deformation signals assoicated with surface loading must be excluded as a source of observation noises. Surface loadings related to atmosphere and ocean, are currently modelled well. However, the hydrological redistribution on land (soil moisture, groundwater, snow and ice) is poorly constrained for most regions of the world, the deformation induced by these loadings is significant but is not readily removed from the sparse time series of a campaign GPS site according to hydrological models. With the development of the Gravity Recovery and Climate Experiment (GRACE), the reditribution of the land water can be estimated at an unprecedented resolution in terms of time-variable gravity fields. The GRACE-derived time-variable gravity field coefficients can be converted to harmonic coefficients for transient deformation in three components, providing a way to quantitatively compare and correlate GPS and GRACE measurements. Here we are proposing to combine GPS and GRACE measurements to distinguish non-tectonic signals from GPS-derived vertical motions, mainly seasonal variantions and secular trends induced by hydrological processes. Our studies will help to estabilish 3D surface velocity field, which would shed more insights into the kinematics of active deformation and mechanism aspects that control this process in the southern Tibet and Himalaya.

藏南地区垂直运动幅度大、机制复杂、变化剧烈，GPS观测资料基础好，是研究青藏高原垂直隆升的最佳场所。GPS虽能捕捉到地壳垂直形变，但其观测中包含构造形变和非构造形变，后者根源与性质难以准确界定，是GPS分析垂直运动的最大挑战。对研究青藏高原构造演化而言，地表质量载荷引起的非构造形变是需要剔除的噪声, GPS垂直形变中包含陆地水载荷引起的季节性及长期变化，很难通过模型化方法有效剔除，对GPS流动观测而言更为困难。GRACE可准确探测出由于陆地水质量变化引起的地壳垂直形变，有助于分析其他垂向变化信息。本项目拟利用GRACE准确估算陆地水载荷引起的季节性及长期性变化，用以校正GPS观测，特别是藏南地区大量GPS流动观测资料，从而提高垂直构造形变的精度和可靠性。该研究有助于构建区域地壳运动的三维形变场, 为分析构造隆升的分布特征、变形机制，以及建立大震震间变形模型提供更好的观测约束。

由于印度板块向欧亚板块的俯冲作用，藏南地区垂直运动幅度大、机制复杂、变化剧烈，该区域在近二十年间布设有大量的GPS连续站和流动站，观测资料基础好，是研究青藏高原垂直隆升的最佳场所。GPS观测到的信号中，不仅仅含有构造变形信息，同时含有大量的非构造信息。对研究青藏高原构造演化而言，地表质量载荷引起的非构造形变是需要剔除的噪声, GPS 垂直形变中包含陆地水载荷引起的季节性及长期变化，很难通过模型化方法有效剔除，对GPS 流动观测而言更为困难。卫星重力GRACE对陆地水变化敏感，可准确探测出由于陆地水质量变化引起的地壳垂直形变，有助于分析其他垂向变化信息。研究发现，GPS观测到的季节项与GRACE模型化的季节项具有很强的一致性，对季节项建模结果表明，藏南地区最大的季节项振幅出现在每年的七到八月的夏季季风季节，由此判断季节项变化的最大因素为夏季季风及其带来的降雨。本项目利用GRACE 准确估算陆地水载荷引起的季节性及长期性变化，用以校正GPS 观测，特别是藏南地区大量GPS 流动观测资料，通过季节项改正前后中误差的分析发现，通过GRACE建模可有效改正GPS观测资料中的季节项误差，从而提高垂直构造形变的精度和可靠性。同时结合2015年尼泊尔Mw7.8级地震，探讨季节项波动对地震震后变形的影响。该研究有助于构建藏南区域地壳运动的三维形变场, 分析构造隆升的分布特征和变形机制。