Relating Far-Field Coseismic Ionospheric Disturbances To Geological Structures

Clear near-epicentral coseismic ionospheric disturbances (CIDs) are observed following the Gorkha and May 2015 Nepal earthquakes and only the Gorkha earthquake excited observable far-field CIDs up to 3,000 km from the epicenter. The near-epicentral CIDs exhibit concentric fields to the south and north of the epicenter. A north-south asymmetry of the CIDs is clearly observed in the near-epicentral area of both earthquakes. The velocity and period of the main part of the near-epicentral CIDs are similar to 1,000 m/s and similar to 6 min, respectively. The wavefronts of the far-field CIDs related to the Gorkha earthquake are mostly straight lines that completely differ from the near-epicentral CIDs. The horizontal trace of the far-field CIDs reaches velocities of similar to 2.5 km/s based on their distance from the epicenter and the CID arrival time. After identifying the propagation direction of the far-field CIDs on a series of two-dimensional (2D) total electron content (TEC) maps, we obtain the real horizontal velocity of far-field CIDs, which is similar to 1.0 km/s, and the period is similar to 3 min, which corresponds to that of acoustic waves in the ionosphere. We find a discontinuity of the far-field CIDs around the Sichuan Basin, which suggests that deep sedimentation can reduce seismic energy coupling into the atmosphere and then the ionosphere. In addition, by introducing a simple acoustic source into the SAMI2 simulation, we show that the large electron density of the background ionosphere and steep inclination of the geomagnetic field are favorable conditions for the generation of detectable CIDs.