LAUSR

Abstract Previous researchers have reported the earthquakes (EQs) induced ionospheric abnormalities, such as electron density depletion and/or enhancement, which they claimed were related to the impending EQs. In this paper, the peak plasma frequency of the ionosonde data from the Islamabad station was studied related to the July 24, 2015 (M5.1), October 26, 2015 (M7.5) and April 25, 2015 (M7.8) EQs in Pakistan and Nepal, respectively. The baseline of the running 30-day median (commonly used in other studies) was deployed on the foF2 data to detect the abnormality in the long term data before the EQs. A detailed analysis of the foF2 variability in a month before all the EQs of this study is then undertaken, where two out of three EQs showed simultaneous deviation in peak plasma frequency relative to the confidence intervals. There is no percentage deviation in foF2 data for the days associated with M7.8, proved no ionospheric signatures in the peak plasma frequency. We believe that the reason is the distance between the epicenter of M7.8 and the ionosonde station, which was approximately 12° longitudinal extension. On the other hand, the negative deviation beyond the lower confidence interval in critical frequency was detected for the mega thrust of M5.1. Similarly, positive foF2 anomalies were recorded within 10 days before the main shock of M7.5, where there was no high intensity geomagnetic storm. These results suggested that in order to detect reliable seismoionospheric precursor, the ionosonde station must operate within the range of seismic breeding zone.