Abstract
Global Navigation Satellite System (GNSS) measured Total Electron Content (TEC) is now widely used to study the near and far-field coseismic ionospheric perturbations (CIP). The generation of near field (~500–600 km surrounding an epicenter) CIP is mainly attributed to the coseismic crustal deformation. The azimuthal distribution of near field CIP may contain information on the seismic/tectonic source characteristics of rupture propagation direction and thrust orientations. However, numerous studies cautioned that before deriving the listed source characteristics based on coseismic TEC signatures, the contribution of non-tectonic forcing mechanisms needs to be examined. These mechanisms which are operative at ionospheric altitudes are classified as the i) orientation between the geomagnetic field and tectonically induced atmospheric wave perturbations ii) orientation between the GNSS satellite line of sight (LOS) geometry and coseismic atmospheric wave perturbations and iii) ambient electron density gradients. So far, the combined effects of these mechanisms have not been quantified. We propose a 3D geometrical model, based on acoustic ray tracing in space and time to estimate the combined effects of non-tectonic forcing mechanisms on the manifestations of GNSS measured near field CIP. Further, this model is tested on earthquakes occurring at different latitudes with a view to quickly quantify the collective effects of these mechanisms. We presume that this simple and direct 3D model would induce and enhance a proper perception among the researchers about the tectonic source characteristics derived based on the corresponding ionospheric manifestations.
Highlights
Imprints of seismic forcing on the ionosphere during earthquake events are extensively studied using Global Positioning System (GPS) – Total Electron Content (TEC) measurement technique[1,2,3,4,5,6,7]
It could be stated that Global Navigation Satellite System (GNSS) measured near field coseismic ionospheric perturbations (CIP) azimuthal distribution offers a reasonable alternative opportunity to determine the seismic source characteristics from the ionosphere, provided non-tectonic forcing mechanisms favor the evolution of CIP
A 3D model to map the combined effects of the non-tectonic forcing mechanisms of geomagnetic field, GNSS satellite geometry, and ambient electron density gradient is proposed for the first time
Summary
Imprints of seismic forcing on the ionosphere during earthquake events are extensively studied using Global Positioning System (GPS) – Total Electron Content (TEC) measurement technique[1,2,3,4,5,6,7]. Efforts are on to identify the tectonic source characteristics, such as rupture propagation direction, crustal deformation pattern and thrust orientations, based on the azimuthal distribution of near field CIP as recorded by GPS2,5–7,11,12. Bagiya et al.[6,7] further investigated the CIP efficiency in identifying the tectonic source characteristics, during the May 2015 Nepal and November 2016 Kaikoura earthquakes, by introducing an elementary satellite geometry factor which estimates the wave phase cancellation effects on CIP amplitudes arising from the varying satellite geometry Based on these studies, it could be stated that GNSS measured near field CIP azimuthal distribution offers a reasonable alternative opportunity to determine the seismic source characteristics from the ionosphere, provided non-tectonic forcing mechanisms favor the evolution of CIP. The effects of ambient electron density variations are estimated as Electron Density Factor (EDF)
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