Possible seismo-ionospheric anomalies of Mw 6.0 and 6.4 south Iran twin earthquakes on 14 November 2021 from GPS and ionosonde observations

Abstract

The possible seismo-ionospheric anomalies of the recent Mw 6.0 and Mw 6.4 twin earthquakes of 14 November 2021 in southern Iran were studied through Global Positioning System (GPS) and ground ionosonde observations. GPS measured total electron content observations, and ionosonde measured NmF2 and hmF2, showing ionospheric anomalies in the pre-seismic and co-seismic phases of the earthquakes. A positive increase in ΔTEC was observed at near-field stations ~ 2–7 days before the twin earthquakes. Our observations also show an increase in the estimated values of NmF2 and hmF2 in the pre-seismic phase compared to their mean values. As the “Kp” and “Dst” indices were quiet during the observation period, these anomalies are apparently generated by seismic activity. The observation of sudden co-seismic ionospheric disturbances at near-field stations immediately after the earthquakes explains the mechanism of co-seismic energy propagation through the lithosphere-atmosphere–ionosphere coupling. The estimated differential NmF2 values show a dip in the peak electron density of − 2.84 × 10¹¹ e/m³ during the time of earthquakes. Our results also show anomalous pre-seismic enhancements in all the three parameters used in this study—TEC, NmF2, and hmF2—an indication of precursory signatures of the twin events. In addition, these parameters also show similar changes in their co-seismic phases. Thus, the observed pre- and co-seismic ionospheric anomalies reveal the characteristic signatures of the preparation processes and the co-seismic energy propagation of the twin earthquakes.