Ionospheric disturbances triggered by the 25 April, 2015 M7.8 Gorkha earthquake, Nepal: Constraints from GPS TEC measurements

The ionosphere response to the April 11, 2015 (Mw 7.8) Gorkha earthquake, occurring in the Himalayan arc, is analysed using GPS Total Electron Content (TEC) measurements, from GPS sites in Nepal and India, situated both close to and far from the epicentre. In the near field, the Coseismic Ionospheric disturbance (CID) following the earthquake rupture propagation arrive east of the epicentre, within 5-7 min with a propagation velocity of 980 m/s, equal to the speed of the shock acoustic waves at the ionospheric heights, and on to the west with a reduced speed of 650 m/s, within 8-11 min, after the earthquake occurrence. The phenomenon of CID splitting into two modes, east and west of the epicentre is observed. In the far-field region, up to epicentral distances of 2200 km, Rayleigh wave induced ionospheric disturbance are recorded with a propagation speed of 2.6 km/s. Higher TEC amplitude of 0.2-1.5 TECU is observed east of the epicentre compared to the west with 0.1-0.3 TECU. The characteristics of this dip-slip earthquake are well projected in the TEC waveforms. The ambient magnetic field in the mid-latitudes prohibited the propagation of ionospheric disturbance in the northward direction. In the present study the observed primary CID is essentially in congruence with the rupture propagation of the earthquake in E-SE direction.