Investigation of ionospheric and atmospheric anomalies associated with three Mw >6.5 EQs in New Zealand

The dual-frequency Global Positioning System (GPS) provides insights to detect the ionospheric perturbations with Total Electron Content (TEC) variations induced by the Earthquakes (EQs) or geomagnetic storm effects. In this paper, we investigate abnormal ionospheric signatures before three large magnitudes (M_w > 6.5) and shallow hypocentral depth < 20 km EQs in New Zealand from daily GPS TEC time series retrieved from International GNSS Services (IGS) stations around the epicenter. Furthermore, geomagnetic activities are completely quiet within 5–15 days before two EQs (M_w 7.8 and M_w 6.8) and one EQ of M_w 6.6 occurred in active storm days (hereafter, geomagnetic storm-EQ event). The anomalies are detected on the basis of a statistical analysis of running median and Inter Quartile Range (IQR) to signify the abnormality in daily TEC time series for 30 days before and 10 days after each EQ. We observe less intensity TEC anomalies of < 2 TECU during UT = 15–24 h (for New Zealand, UT = LT + 13) within 5–15 days before two EQs of M_w 7.8 and M_w 6.8. Whereas, a chain of positive and negative anomalies is generated by geomagnetic storm-EQ event (M_w 6.6) far beyond 5–15 days before and after the main shock due to active storm conditions (Kp> 3; Dst < −50 nT). Furthermore, geomagnetic storm anomalies are dominant on M_w 6.6 EQ day to active storm conditions. To validate the ionospheric anomalies as possible seismic precursors, the atmospheric anomalies in relative humidity and OLR (outgoing longwave radiations) are also examined to exhibit the evolution of ionospheric anomalies from the lithosphere to ionosphere via the atmosphere. Moreover, EQ induced regular and collocated atmospheric anomalies over the epicenters of M_w 7.8 and M_w 6.8 and irregular atmospheric anomalies exist over the seismogenic zone of M_w 6.6 due to geomagnetic storm-EQ nature. The observed EQ anomalies will aid in the improvement of lithosphere atmosphere ionosphere coupling hypothesis during the seismic preparation phase over the epicenter of future EQ.