利用GPS数据分析四川“Y”型构造区地壳运动状态

Analysis of crustal movement characteristics in Sichuan“Y”type tectonic area based on GPS data

四川地区地质构造复杂,地壳活动剧烈,为了深入揭示该区“Y”型构造区地壳1999年以来近20年的动态演化规律,基于1999—2017年7期GPS数据,解算各周期网格速度场、应变率场,研究地壳应变场演化过程。结果表明:①2008年以前的3期GPS速度场相对稳定,汶川地震后,速度场变化最大的龙门山断裂带由4.0—5.0 mm/a增至8.0—10.0 mm/a;②汶川震后,“Y”型构造区最大剪应变高值区出现在汶川以东,由2.0×10^(-8)/a增到22.0×10^(-8)/a;龙门山断裂带以SE或SEE向主压应变为主,变化速率约5.0×10^(-8)/a—12.0×10^(-8)/a,鲜水河断裂由震前NS向主拉应变转为震后EW向主压应变,安宁河断裂东侧由震前SE向主压应变6.0×10^(-8)/a减至震后的2.0×10^(-8)/a;面膨胀结果显示,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,且存在以金川至都江堰、北川至青川为条带的2个正负交替过渡区;③汶川地震发生压应力释放后,该区SEE向压性特征又逐渐增强,且持续至2017年,释放了龙门山断裂带地壳内部SEE向压应力多年累积能量,但汶川地震对鲜水河断裂与安宁河断裂的整体运动状态则无明显触发作用。

Sichuan Province is an area with frequent crustal activities in China.In order to reveal the dynamic evolution of the crust in the“Y”type tectonic region of Sichuan Province in the past 20 years,based on seven periods of GPS velocity field data from 1999 to 2017,the grid velocity field and strain rate field of each period in the study area are solved and the evolution process of crustal strain field in the area in recent 20 years is studied.The results show that:①The GPS velocity field in the three periods before 2008 is relatively stable and after the Wenchuan earthquake,the Longmenshan fault zone with the largest change in velocity field increases from 4.0—5.0 mm/a to 8.0—10.0 mm/a;②After the Wenchuan earthquake,the highest shear strain in the“Y”type tectonic zone appears in the east of Wenchuan,changing from 2.0×10^(-8)/a to 22.0×10^(-8)/a;The Longmenshan fault zone is dominated by the ES or EES oriented main compressive strain,the variation rate is about 5.0×10^(-8)/a to 12.0×10^(-8)/a,the main tensile strain of the Xianshuihe fault changed from NS before the earthquake to EW after the earthquake.At the east side of the Anninghe fault,the main compressive strain decreased from 6.0×10^(-8)/a in the ES direction before the earthquake to 2.0×10^(-8)/a after the earthquake.The results of surface expansion show that the low-density gradient zone immediately changed into a high-density change zone parallel to the strike of Longmenshan fault zone,with two alternating positive and negative transition areas in Jinchuan to Dujiangyan and Beichuan to Qingchuan;③After the Wenchuan earthquake,the pressure in this area are significantly enhanced,and this enhancement is maintained until 2017(nearly 10 years).It releases the energy accumulation of the pressure stress in the crust of the Longmenshan fault zone for many years,but the Wenchuan earthquake has no obvious change to the overall movement state of the Xianshuihe fault and Anninghe fault.