2016年青海门源Ms6.4地震前的区域地壳形变特征

摘要

A Ms6.4 earthquake occurred at Menyuan,Qinghai province,on January 21,2016.In 1986,a Ms6.4 earthquake happened in the same area with the same focal mechanism.This paper analyzes the regional crustal deformation by using data from the 10 continuous GPS stations and 74 campaign GPS stations in a 200 km range of these events.(1)Based on the velocity field of over ten year's GPS data,the results of crustal strain rates and seismic moment accumulation rates of the Qilian-Haiyuan fault are significantly higher than other regions.In a 20 km× 20 km range of the seismogenic area,the maximum and minimum principal strain rates are 21.5 nanostrain/a (in NW-SE direction,tension) and-46.6 nanostrain/a (in NE-SW direction,extrusion),the seismic moment accumulation rate is 17.4 N · m/a.The principal strain extrusion direction is consistent with the focal mechanism.(2) Based on the GPS continuous stations coordinate time series of 6 years before the event,the result shows the surface expansion at the seismogenic zone is in constant "non-linear" reduced trend since 2010.This means the seismogenic zone is in a state of shrinking.However,2～3 months before the event,the surface expansion and maximum shear strain occurred with significant inverse trends.These abnormal changes of crustal deformation may reflect the nonlinear adjustment of the stressstrain accumulation when it is close to the critical seismogenic zone.%选取2016-01-21青海门源Ms6.4强震发震区域周围200 km范围的10个连续GPS观测站和74个流动GPS观测站资料,分析研究2016年门源Ms6.4地震之前的区域地壳形变特征:1)基于10余年GPS资料的速度场计算结果表明,发震区域所处的祁连-海原断裂系具有显著高于周边区域的地壳应变率和地震矩累积率.在发震区域20 km×20 km范围内,最大和最小主应变率分别为21.5 nanostrain/a(方向NW SE,拉张)和-46.6 nanostrain/a(方向NE-SW,挤压),地震矩累积率达17.4×1015N·m/a.主应变挤压的主轴方向与本次地震的震源机制相一致.2)基于震前6 a连续GPS观测站坐标变化时间序列的计算结果表明,自2010年以来,发震区域的面膨胀值随时间呈“非线性”不断变小的趋势,反映出发震区域一直处于应变的挤压缩减状态,但在震前的2～3个月,面膨胀与最大剪切应变均发生了一次明显的反向趋势变化.这些震前的地壳形变异常变化,或许反映了发震区应力-应变积累在接近临界破裂状态时的非线性调整.