PRE-SEISMIC IONOSPHERIC TOTAL ELECTRON CONTENT ANOMALIES ASSOCIATED WITH THE JUNE 28, 2023, EARTHQUAKE IN THAILAND

Abstract

This study investigated ionospheric Total Electron Content (TEC) anomalies as potential precursors to a moderate earthquake magnitude of 4.5 that occurred in Phai Lom, Bang Krathum District, Phitsanulok Province, Northern Thailand, on June 28, 2023 (Universal Time, UT). High-resolution GPS data from the UTHG GNSS station were analyzed at 1-second intervals over 15 days (June 21–July 5, 2023) using the ±2σ statistical boundary method to detect abnormal variations in TEC. The analysis revealed a pronounced TEC enhancement of approximately 2.61 TECU on June 23, five days before the earthquake, followed by a significant depletion on the day of the event. These variations exceeded the Upper and Lower statistical limits (UB and LB), indicating statistically significant deviations from the expected median trend. A moderate geomagnetic storm (Dst = –57 nT) was recorded on June 25; however, correlation analysis (r = –0.507) indicated only a moderate negative relationship between geomagnetic activity and TEC variations. During the earthquake period, the Dst index remained above –30 nT, signifying geomagnetically quiet conditions. To validate the findings, global TEC data obtained from the GNSS-TEC database displayed consistent temporal trends, confirming that the anomalies were not site-specific. Moreover, analysis of the Rate of TEC Index (ROTI) revealed short-term irregularities on June 23 and June 28 UT, further supporting the presence of ionospheric disturbances related to the seismic event. Overall, these results align with previous studies and emphasize the potential of GNSS-based ionospheric monitoring and correlation analysis as reliable tools for short-term earthquake forecasting, particularly in low-latitude regions such as Thailand.