基于GPS速度场的四川地区1999~2018年应变场特征演化分析

The characteristic evolution analysis of scale strain field in Sichuan area from 2001~2018 based on GPS velocity field

基于1999~2018年GPS水平运动速度场数据,解算并分析了四川“Y”形构造区各周期网格速度场、地壳应变率场,并讨论了近20年尺度的地壳应变场演化过程。研究表明:1)2008年汶川地震前1999~2007期GPS速度场相对稳定,整体“Y”型构造区地壳运动变化不大,但汶川地震后龙门山断裂带发生较大变化,由4.0 mm/a增至10.0 mm/a。2)1999~2007年,整个四川“Y”型构造区应变场演化特征微弱,而汶川地震之后的两个周期,最大剪应变自龙门山山前断裂向西到汶川一带,形成了由高到低、平行于龙门山断裂带走向的高密度梯度带。龙门山断裂带以ES或EES向的主压应变为主,其量值变化范围为5.0×10^-8/a^12.0×10^-8/a;鲜水河断裂由震前主拉应变,改为震后近EW向的主压应变特征。面膨胀结果则显示龙门山断裂带由震前低密度梯度带瞬间变为平行于龙门山断裂带走向的高密度变化区。3)2008年汶川地震和2013年芦山地震是最为重要的时刻分割点。近20年的应变率场变化,更似一个“时间—地壳构造运动”的大轮回,目前四川“Y”型构造区整体处于2008年汶川地震前较为稳定的活动周期。龙门山断裂带仍值得我们做出更为深入的研究。

The grid velocity field and the crustal strain rate field of“Y”tectonic region of Sichuan and Yunnan in each period are calculated and analyzed,and the evolution process of crustal strain field in the past 20 years is discussed based on the data on the GPS horizontal velocity field of three periods in 2001~2018.The research indicates that:1)The GPS velocity field in three periods is relatively stable,and the crustal movement in the whole“Y”tectonic region changes a little before Wenchuan earthquake in 2008,but the Longmenshan fault zone changes from 4.0 mm/a to 10.0 mm/a after Wenchuan earthquake.2)The whole“Y”tectonic region of Sichuan Province shows weak evolution characteristics of strain field from 2001 to 2007.From Longmenshan piedmont fault to Wenchuan,the maximum shearing strain forms a high-density gradient zone,which is from high to low and parallel to Longmenshan fault in two periods after Wenchuan earthquake.The principal compressive strain in the direction of ES or EES is the principal strain in the Longmenshan fault zone,and its magnitude range is 5.0×10^-8/a^12.0×10^-8/a;the principal tensile strain of Xianshuihe fault before the earthquake is changed to the principal compressive strain near the EW direction after the earthquake;The surface expansion results show that the Longmenshan fault zone changes from a low-density gradient zone before the earthquake to a high-density zone parallel to the strike of the Longmenshan fault zone for a second.3)Wenchuan earthquake in 2008 and Lushan earthquake in 2013 are regarded as the most important time division point.The change of the strain rate field in recent 20 years is more like a large circle of“time-crust tectonic movement”.At present,the“Y”tectonic region in Sichuan is in a relatively stable activity cycle before Wenchuan earthquake in 2008.Longmenshan fault is still worthy of further study.