Seismo ionospheric anomalies possibly associated with the 2018 Mw 8.2 Fiji earthquake detected with GNSS TEC

The detection of earthquake (EQ) related ionospheric anomalies using Global Navigation Satellite System (GNSS) has emerged as a convincing approach and new field in the search for seismic precursors. Total Electron Content (TEC) from International GNSS Services (IGS) network may help to distinguish seismo ionospheric anomalies from geomagnetic storm induced anomalies. In this paper, spatial and temporal TEC anomalies are investigated in 1 s resolution data from GNSS stations around the epicenter of the August 19, 2018, Fiji EQ (M-w 8.2). Moreover, median and standard deviation based anomaly detection techniques are implemented on TEC and the variations are quantified on the basis of z-test at alpha = 0.05 significance level. We find evidence of significant ionospheric anomalies of low intensity within 5-10 days before and 5 days after the main shock by estimating TEC from four GNSS stations within the EQ preparation period. On the other hand, a significant ionospheric anomaly of high intensity arises after the main shock beyond 5 days (+7 day and onward) after the EQ, which is associated with an intense geomagnetic storm (Dst <-150 nT and Kp >= 7). More analysis reveals that EQ induced TEC anomalies are significant during Local Time (LT) = 08:00-16:00 (for Fiji, LT= UT+12) and similarly the storm (Dst =-150 nT and Kp >= 7) induced TEC anomaly prevails in all GNSS stations during LT= 08:00-18:00. The TEC intensity of the storm anomaly is more substantial in all four GNSS stations compared to the EQ induced TEC anomaly. All these anomalies before and after the main shock during the EQ preparation period are advantageous towards differentiating seismic versus geomagnetic storm anomalies. Furthermore, this analysis aids in the developing hypothesis of lithosphere ionosphere coupling through TEC monitoring.