Co-seismic displacements associated with the 2015 Nepal Mw7.9 earthquake and Mw7.3 aftershock constrained by Global Positioning System Measurements

Abstract

A magnitude of Mw7.9 earthquake occurred in Nepal on 25 April 2015 along the Himalayan seismic zone. Twelve days later, a magnitude of Mw7.3 aftershock ruptured the southeast side of initial rupture region of the Mw7.9 earthquake. In this study, we report in detail the co-seismic displacements, which were caused by the two events, based on continuous observations in Nepal obtained from the permanent Global Positioning System (GPS) stations of the Crustal Movement Observation Network of China (CMONOC) and from campaign GPS measurements in southern Tibet before and after the Mw7.9 earthquake. The largest co-seismic horizontal offset caused by the Mw7.9 earthquake was 1.89, and it was reported from the KKN4 station in Nepal. In southern Tibet, the co-seismic horizontal displacements varied from several millimeters to approximately half a meter within a 100-400 km range of the epicenter. The horizontal offset in the Everest Mountains was about 3 cm. Referring to the Mw7.3 earthquake, co-seismic offsets of up to 2 cm could be detected as far as approximately 200 km away from the epicenter. Near-field GPS stations also recorded significant vertical deformations caused by the great thrust earthquake. The GPS displacements in the InSAR Line of Sight (LOS) direction agreed well with the InSAR LOS values. In addition, high-rate GPS deformation waveforms were recorded by the high-rate GPS stations in Mainland China within a distance of approximately 2000 km from the epicenter, indicating that the amplitude of the seismic waves in the direction of the fault rupture increased. The GPS stations located along the direction of seismic rupture recorded much larger deformation waveform amplitudes than those located in the opposite direction. Finally, forward calculations were performed using a simple elastic half-space dislocation model, in order to investigate the co-seismic displacements and the principle strain changes around the epicenter, especially in the southern Tibet. The principle strain results indicated that the Nepal Mw7.9 earthquake resulted in the tensile stress changes in southern Tibet and probably triggered the Dingri Ms5.9 earthquake.