基于GPS速度场变化结果研究汶川地震前后南北地震带地壳运动动态特征

本文针对制约多期GPS速度场动态演化特征分析的关键问题,提出速度场稳定基准选取中顾及基准偏移的影响,给出了改进拟准检定(QUAD)初选指标的方法,有效解决多期速度场的基准偏移对粗差剔除的影响.并在此基础上通过对1999—2013年南北地震带地区多期GPS速度场做以华南地块为统一参考基准的处理、最小二乘配置统一协方差参数估算,获取了南北带地区多期GPS速度场的网格化结果.结果显示: (1) 汶川地震影响在震后迅速衰减;震前2004—2007年及2007—2009年相对于1999—2004年南北地震带地区中北部地区存在大范围的北东向运动增强.(2) 2007—2009年相对于2004—2007年的速度场差值变化图像显示,这期间汶川地震的调整影响范围较大,鲜水河左旋剪切有所减弱,龙门山南段应变积累增强.(3) 川滇块体南东向的挤出滑移在2011—2013年相对2009—2011年有所增强.

Dynamic characteristics of crustal movement in north-south seismic belt from GPS velocity field before and after the Wenchuan Earthquake

With the implementation of the projects "Crustal Movement Observation Network of China" and "Chinese continental tectonic environment monitoring network", a large number of GPS time series and velocity fields were produced. Based on above GPS data, many scholars have made a number of important progresses in crustal movement and deformation. Because of the intuitive feature of the GPS velocity fields, they are widely used in crustal movement and deformation analysis. In this paper, we focus on the dynamic characteristics of the crustal deformation based on the multi-periods of GPS velocity fields.GAMIT/GLOBK and QOCA software were used in GPS data processing. First, GAMIT software was used to calculate the station coordinates and single-day relaxation solution of satellite orbits. Then, QOCA software was used to combine all the single-day relaxation solutions and the velocity on the reference of ITRF2005 was calculated after deducting the coseismic movements. Because the error distribution and points' location may be different in multi-periods of velocity fields, the unified reference must be established before extracting the dynamic deformation characteristics from multi-periods GPS velocity fields. In order to establish the unified benchmarks, an improved method developed from Quasi-Accurate Detection of gross errors (QUAD) was proposed. Firstly the least squares collocation with unified covariance parameters was used to form unified model of GPS velocity fields and calculate the comparable grid velocity fields. Then, improved QUAD method is used to obtain continuous distribution of the velocity field after selection and testing. Thus, the differences of grid values between two periods can be calculated, because the grid velocity fields are on one unified benchmark after above two steps.Using above methods, the differences in grid velocity fields between 2004—2007/2007—2009/2009—2011/2011—2013 and 1999—2004, between 2007—2009 and 2004—2007, and between 2011—2013 and 2009—2011 were obtained. The results were as follows: 1) the differences between 2004—2007/2007—2009/2009—2011/2011—2013 and 1999—2004 show that the influence of the Wenchuan earthquake is gradually reduced. 2) The differences of grid velocity fields between 2004—2007/2007—2009 and 1999—2004 show that the NE movements in a wide region except for the South China block and Sichuan-Yunnan were enhanced with the rate about 3~5 mm·a^-1. 3) The difference of grid velocity fields between 2007—2009 and 2004—2007 show that the impact of the Wenchuan earthquake is still obvious in its northern adjustment region. 4) The difference of grid velocity fields between 2011—2013 and 2009—2011 show that the southeast movement of Sichuan-Yunnan block has been enhanced.The results showed that 1) The difference of grid velocity fields between 2004—2007/2007—2009 and 1999—2004 in the middle-northern segment of the North-South Seismic Zone show a wide range NE movement enhancement (3~5 mm·a^-1), which was similar with NE enhanced movement of Qinghai-Tibet plateau after Sumatra earthquake occurred in 2004. 2) The difference of grid velocity fields between 2007—2009 and 2004—2007 reflected that dynamics load enhanced on southern segment of the Longmenshan fault zone after the Wenchuan earthquake. 3) After the Wenchuan earthquake, the SE sliding of Sichuan-Yunnan block was not enhanced until 2011, and then it accelerated to some extent up to 2013.