GPS观测结果反映的尼泊尔M_w7.8地震孕震特征

针对2015年4月25日尼泊尔Mw7.8地震的孕震特征,本文首先对覆盖尼泊尔及周边地区的5套GPS水平速度场结果进行了融合,得到了近似统一参考框架下的速度场结果;在此基础上通过对此次地震震源区及周边地区的速度场、应变率场、基线时间序列分析,识别了震前变形特征.GPS应变率场结果显示,喜马拉雅主边界断裂存在大范围挤压应变积累,震源区处于近南北向应变积累高值过渡区.跨喜马拉雅构造带的GPS基线时间序列结果表现为持续缩短现象,表明印度板块与欧亚板块之间的持续挤压变形特征,2012年以来的缩短增强现象反映了印度板块对青藏块体的推挤增强作用明显.距离震中较近的西藏南部GPS同震位移结果以南向运动为主且指向震中,反映了青藏高原存在逆冲应变释放现象.综合此次尼泊尔地震前变形和同震应变释放特征,认为此次地震的孕震区域和同震应变释放区域均较大,将会对青藏高原的地壳变形与强震孕育产生深远影响.

Characteristics of the seismogenic model for the 2015 Nepal Mw7.8 earthquake derived from GPS data

On April 25, 2015, an M_w7.8 earthquake hit Nepal, causing serious casualties. In order to recognize the seismogenic process of this event, this work analyzed the pre-seismic and co-seismic crustal deformation in Nepal and adjacent regions. The parameters involved include the GPS horizontal velocity field, strain rate field, GPS baseline time series and the coseismic displacements in the Tibet plateau.Firstly, we collected 5 sets of the GPS horizontal velocity field covering Nepal and adjacent areas, and then transformed them to an approximately unified reference frame. Secondly, we calculated GPS strain rate fields in the seismic source and its vicinity using the least square collocation method on a sphere surface, and analyzed their characteristics. Thirdly, we analyzed the dynamic changes of long baselines of IISC-LHAS, IISC-DLHA, which cross the Himalayan. Finally, the coseismic displacements were obtained through processing the GPS data of 22 April to 24 April and 26 April to 28 April, respectively.The GPS strain rate field shows that regional compressive strain accumulated on a large scale around the Main Boundary Thrust of the Himalayan tectonic zone, and the huge earthquake occurred in the region between two high-value areas of the N-S directed strain. GPS geodesic time-series indicates the baselines across the Himalaya shortened continuously, implying continuous extrusional deformation between the Indian and the Eurasian plates. The phenomena that the shortening enhanced after the year 2012 reflects the extruding force from the Indian plate has been strengthened. The co-seismic displacements recorded by the permanent GPS stations in Tibet neighboring Nepal are dominantly directed to south, just towards the epicenter, likely resulting from the release of thrust strain in southern Tibet.From the feature of pre-seismic and coseismic crustal deformation of the Nepal great shock, we suggest that both its seismogenic and co-seismic strain release areas are relatively large, which would generate long-term impact on the Tibetan Plateau, such as dynamic adjustment in the crust or even to trigger unlocking of the Himalayan thrusts to spawn big quakes.