A Network-Based Framework for Characterizing Pre-Seismic Ionospheric Disturbances Using the TEC Anomaly Significance Index

This study investigates pre-seismic ionospheric Total Electron Content (TEC) disturbances preceding the Mw 6.9 Northern Aegean Sea earthquake of 24 May 2014 using observations from 13 Global Navigation Satellite System (GNSS) stations. A pronounced negative TEC disturbance was identified on 22 May 2014, approximately two days before the earthquake, under comparatively quiet solar and geomagnetic conditions. Station-wise Z-score analysis, which expresses the TEC departure from the reference mean in units of standard deviation, revealed significant negative deviations across the network, while inter-station correlations indicated a temporally coherent but spatially heterogeneous ionospheric response. To characterize the disturbance beyond peak-based measures, the TEC Anomaly Significance Index (TASI) was developed by integrating the mean absolute Z-score, coefficient of variation, and Shannon entropy. TASI showed strong agreement with the maximum absolute Z-score ranking (Spearman’s ρ=0.89, p<0.001) while providing greater sensitivity to cumulative and persistent anomaly behaviour. It exhibited stronger associations than maximum ∣Z∣ for six of the seven evaluated temporal descriptors, particularly those representing anomaly duration and consecutive persistence. Among the analyzed GNSS stations, KASI recorded the highest TASI despite not being the nearest station to the epicenter, indicating that anomaly significance was not governed solely by epicentral distance. The proposed framework provides a multidimensional, network-based approach for characterizing potential pre-seismic ionospheric disturbances and establishes a basis for future multi-event and control-period validation.