Pre-seismic ionospheric disturbances (PIDs) associated With 2021 Mw 7.5 Northern Peru earthquake: GNSS and ground uplift observations

Predicting natural disasters such as earthquakes remains a major challenge in geosciences, with critical implications for early warning systems and disaster risk reduction. Among various precursory signals, ionospheric anomalies have gained increasing attention as potential indicators of impending seismic events. In this study, we examine pre-seismic ionospheric disturbances (PIDs) associated with the Mw 7.5 Northern Peru earthquake on November 28, 2021, using Global Navigation Satellite System (GNSS) Total Electron Content (TEC) and ground vertical velocity data. Significant sequential negative TEC anomalies were observed up to two hours prior to the mainshock, with multiple disturbances recorded by several GNSS receivers. The amplitudes of these disturbances increased as the earthquake approached, suggesting a progressive ionospheric response to the buildup of tectonic stress. Spectral analysis using the Short-Time Fourier Transform revealed center frequencies between 3.63 mHz and 4.80 mHz — within the acoustic/infrasonic range — indicating that such waves, possibly generated by foreshocks, may be responsible for these PIDs. To rule out other sources of TEC anomalies, we examined the TEC data for the previous day along the same trajectories and found no similar disturbances. Furthermore, geomagnetic conditions were quiet during the period, as indicated by Kp and Dst indexes two days before and after the earthquake. These findings contribute to the understanding of seismo-ionospheric coupling and highlight the potential role of ionospheric monitoring as a complementary approach to conventional seismic methods in earthquake early warning systems.