Low Ionosphere Density Above the Earthquake Epicentre Region of Mw 7.2, El Mayor–Cucapah Earthquake Evident from Dense CORS Data

In this study, a potential precursor related to the Mw 7.2 Mexico earthquake on April 4, 2010, was investigated by analysing ionosphere total electron content (TEC) fluctuations derived from Global Positioning System data collected from 200 Continuously Operating Reference Stations (CORS) in Mexico and the western United States. Abnormal TEC variations were statistically identified within a 30-day time frame prior to the earthquake event. The two nearest stations at distances of 45 km and 53 km from the epicentre (IID2 and P500) were employed as benchmarks for the detection of anomalous days and time in TEC variations. Notably, a distinctive anomaly was observed on April 2, 2010, a couple of days before the earthquake, featuring a TEC unit deviation of 3–4 (TECU) from the baseline (15-day average value). Maximum TEC deviations (the time of anomaly) were recorded at 14.75 UTC on April 2, 2010. The analysis indicated a decrease in TEC concentration at a rate of 0.0017 TECU per kilometre towards the epicentre, supported by data collected from 200 CORS stations in the region. Spatial interpolation of TEC data from these stations further highlighted a distinct zone of low TEC density in the ionosphere above the epicentre at 14.75 UTC. This low TEC density zone was concentrated in areas with higher density points of geological structures (faults). The study suggests that the low TEC zone may be detected before the earthquake within proximity of the earthquake preparation zone above the epicentre.