Association of Ionospheric Signatures to Various Tectonic Parameters During Moderate to Large Magnitude Earthquakes: Case Study. Issue 3 (8th March 2021) - Sunil, A. S.
Abstract: The sudden ground movement associated with Mw > 6.5 earthquakes is considered a potential source of ionospheric electron density perturbations over the fault region. Coseismic ground displacement is a function of various seismic source parameters such as moment magnitude, focal depth, and focal mechanism etc. We study here the distinct effects of vertical ground displacement, moment magnitude and focal depth on coseismic ionospheric perturbation (CIP) amplitudes during moderate‐to‐large earthquakes. We analyze GPS‐total electron content variations during 59 dip‐slip earthquakes that occurred in the last 20 years. Our study reveals that though CIP amplitudes are primarily controlled by moment magnitude, they are also sensitive to the earthquake focal depth. To understand the influence of focal depth on the displacement field and therefore on CIP amplitudes, we present a simple synthetic test, for a depth range of 0–200 km, highlighting that the maximum vertical ground displacement decreases logarithmically with increasing focal depth while the volume (i.e., integrated vertical ground displacement) of uplifted/subsided material varies very marginally. We conclude that CIP is sensitive to the wavelength of co‐seismic vertical displacement field and that seismic energy propagation to the overlying atmosphere during deep earthquakes is not adequate to generate detectable CIP. Plain Language Summary: A small part of energy released during earthquakes transfer to the atmosphere in terms of mechanical waves. For large magnitude earthquakes (generally Mw > 6.5), the atmospheric amplification of such seismically induced waves can generate disturbances in ionospheric electron density termed as Coseismic Ionospheric Perturbations (CIP). The moment magnitude of earthquakes mainly controls the amplitude of ionospheric perturbations. In this study, the effect of focal depth on CIP amplitudes are investigated. The analysis of 59 dip‐slip earthquakes shows that deep earthquakes generate smaller CIP amplitudes. The sudden ground movement during earthquakes is responsible for the transfer of seismic energy to the atmosphere. Our study shows that CIP amplitudes are not only sensitive to earthquake magnitude but also to their focal depth. Though the integrated displacement field is insensitive to the earthquake depth, its distribution is very different, concentrated in a narrow area for shallow earthquakes, spread over a wide region for deep earthquakes. CIP amplitudes appear to scale with the maximum values of vertical surface displacement rather than their average. Thus, the energy transfer is more efficient during shallow earthquakes. Key Points: Distinct effects of various seismic source parameters such as vertical surface displacement, magnitude and focal depth on coseismic ionospheric perturbation (CIP) amplitudes Amplitudes of coseismic ionospheric perturbations strongly depend on maximum vertical surface displacement Deep earthquakes are less efficient in generating detectable CIPs