2015年尼泊尔Mw7.9地震震前断层闭锁与同震位移特征分析

对尼泊尔震前的GPS水平速度场进行融合处理,获得跨喜马拉雅中东段沿N15°E方向的GPS水平速度场剖面。采用弹性半空间矩形位错模拟反演,结果表明,尼泊尔地震前喜马拉雅主前缘断裂带存在浅部闭锁,深部无震蠕滑,闭锁深度为22.5km,蠕滑段滑动速率为19.0mm/a,断层倾角为10°。GPS观测的同震形变场揭示了尼泊尔地震引起的地壳形变特征,最大同震位移位于尼泊尔境内的KKN4,该站向南移动1.89m;地震还在我国藏南地区造成最大0.54m的永久形变。距离震中400km以外GPS观测到的同震形变微弱,在误差范围之内。依据GPS同震水平位移在N15°E方向的位移剖面,采用矩形位错模型简单模拟了尼泊尔地震的同震破裂。结果显示,GPS同震位移剖面可以用喜马拉雅冲断带前缘主断裂(MFT)以北的基底低角度逆断层引起的弹性位错模拟,浅部的主前缘逆冲断裂、主边界逆冲断裂和主中央逆冲断裂等分支并没有破裂。

Interseismic Coupling and Coseismic Displacements of the 2015 Mw7.9 Nepal Earthquake

Velocity results from published papers, which are in different reference frame, are transferred into a self-consistent frame based on common GPS velocities of each result. We investigate the interseismic coupling of the rupture fault before the Nepal earthquake based on interseismic GPS velocities along a profile in N15°E direction, which runs across the rupture region of the 2015 Mw7.9 Nepal earthquake. We model the creeping fault as a planar semi-space dislocation embedded in elastic half-space, and invert the horizontal velocities to infer the preferred locking depth and slip rate. The preferred model indicates that interseismic deformation is induced by a fault dipping at 10°, buried at a depth of 22.5 km, with a slip rate of 19.0 mm/a. Then, near field coseismic displacements measured from GPS observations are presented. The largest horizontal displacement of 1.89 m was recorded at KKN4 in Nepal. Coseismic offset of 54 cm was measured in southern Tibetan plateau, which is the largest value measured in mainland China. The displacements decrease rapidly to less than 5.0 mm at GPS stations 400 km away from the epicenter. GPS derived coseismic displacements along the N15°E profile show that the offset near the fault trace is not the largest offset measured by GPS, indicating that the shallow part of the fault did not rupture during the Nepal earthquake. The horizontal coseismic offset profile can be modeled well using a buried rectangular dislocation model. Therefore, we infer the rupture fault of the Nepal earthquake may be the main Himalaya thrust (MHT), and the shallow parts did not rupture during the shake.