于田2008年和2014年两次MS7.3地震孕育的应力环境

2008年3月21日新疆于田发生M_S7.3级地震,2014年2月12日于田再次发生M_S7.3级地震,两次地震相距约110 km.但是,前者震源机制为正断层,后者震源机制为左旋走滑断层.为进一步探讨这两次地震的孕育应力环境、发震机制及其动力学成因,本文进行三维有限元数值试验分析,计算了该区域在GPS约束条件下的速度场、应力和应变场变化,并与实际观测资料进行对比.数值计算得到的区域内几条主要大的走滑断层错动性质,与实际地质观察到的断层左旋或右旋性质吻合,验证了计算结果的可靠性.结果表明于田及其临近区域整体上处于北东-南西向挤压和北西-南东向拉张状态.在GPS速度约束条件下,2008年于田地震震中区域最大主张应变率大于最大主压应变率,处于以拉张为主的应力状态,NE走向断层受到北东-南西方向的拉应力作用,从而形成正断层;2014年于田地震处于拉张应变率与压应变率几乎一致的区域,NEE走向断层在NE-SW主压应力和NW-SE主张应力作用下发生左旋走滑.

Analysis of the stress environment of the 2008 and 2014 Yutian MS7.3 earthquakes

An M_S7.3 earthquake occurred on 21 March, 2008 in Yutian, Xinjiang Uygur Autonomous Region. Six years later, on 12 February 2014, another M_S7.3 earthquake happened about 100km apart from the former one in the same area. Focal mechanism shows that the earthquake in 2008 is associated with strike-slip fault, and the one in 2014 is related to normal fault. We calculate the velocity field, stress field and strain field of the research area using 3D Finite Element Method numerical simulation, and compare the simulating result with the actual observational data. The relative movement directions of several main strike slip faults derived from numerical simulation result are consistent with field geological observation, which shows the reliability of our simulation. Based on the numerical result, we further discuss the stress environment, seismogenic mechanics and dynamic factor of two earthquakes. It is suggested that Yutian and its neighborhood area are mainly in northeast-southwest compression and northwest-southeast extension. Under the constraints of GPS velocity, the principal extensional strain rate in northwest-southeast direction is much greater than compression strain rate in 2008 Yutian earthquake epicenter, which led to normal fault in NNE direction. While the principal extensional strain rate is nearly equal to the principal compression strain rate in 2014 Yutian earthquake epicenter, which can easily cause sinistral strike slip fault in NEE direction.