Co-seismic and Post-seismic Changes in ZTD and TEC of the 2015 Nepal Earthquake

We studied changes in the zenith total delay (ZTD) and total electron content (TEC) associated with the large 2015 Nepal earthquake (Mw 7.8). We analyzed the ZTD derived from GPS data received at HYDE, LCK3 International GNSS Service (IGS) stations and MSUN operated by National Geophysical Research Institute (NGRI). The ZTD value, which is sensitive to tropospheric weather conditions, showed a sharp drop during and following the mainshock as well as the largest aftershock of the Nepal earthquake sequence. The changes in ZTD most likely represent changes in the atmospheric pressures, although their changes due to seasonal changes in hydrology (surface loading of water storage) cannot be completely ruled out. Effects of earthquake processes such as large stress drop, low radiation efficiency of the 2015 event, and conversion of elastic energy to shear heating (anelastic loss) during propagation of surface waves from large earthquakes over teleseismic distances contributes to the real ground deformation, as inferred from changes in GPS-derived ZTD. The coincident decrease in ZTD and TEC suggests hydration in the atmosphere by the joining of ions in the atmosphere during earthquakes. Almost the same results regarding the changes in ZTD and TEC were obtained for another large event (the 1999 Chamoli earthquake of Mw 6.8) in the Himalayan region, although they differ slightly in intensity depending on the observation distance.