Ionospheric Anomalies Associated with the Mw 6.0 and 6.4 South Iran Twin Earthquakes from GPS and Ionosonde Observations

Ionospheric anomalies associated with the Mw 6.0 and Mw 6.4 south Iran twin earthquakes were investigated through GPS and ground Ionosonde observations. The pre-seismic and the co-seismic ionospheric disturbances (CIDs) were observed in the GPS measured Total Electron Content (TEC) and the Ionosonde measured NmF2 and hmF2. The results show positive enhancement of TEC at near field stations ~2–6 days prior to the twin earthquakes. The estimated NmF2 and hmF2 values also show positive values in the pre-seismic phase compared to their mean values. These anomalies are attributed to seismic activity as the Planetary K index (Kp) and Disturbance Storm-time index (Dst) were quiescent during the entire observation period. Observation of sudden CIDs at near field stations immediately after the earthquakes explains the co-seismic energy propagation mechanism through the Lithosphere Atmosphere Ionosphere (LAI) coupling. Our observation on the differential NmF2 marks a dip in the peak electron density by -2.84 × 1011 e/m3 during the time of earthquakes. The estimated TEC, NmF2, and hmF2 results show a consistent reduction in their values on the day of earthquakes. Moreover, their changes were similar in both pre- and co-seismic phases and hence they are qualified as potential parameters for earthquake precursors. Thus the pre- and co-seismic ionospheric anomalies detected in this study reveal the characteristic signatures of twin earthquake preparation processes and the co-seismic energy propagation.