Exploring the difficulties in forecasting earthquake location with inhomogeneous ionospheric perturbations

Abstract Earlier studies have recorded seismic effects in sub-ionospheric Very Low Frequency (VLF) signal, predominantly examining a single VLF propagation path associated with a specific earthquake. This research expands on these findings by taking a more advanced setup. Using Japan's VLF network with eight stations nationwide and a single transmitter signal from JJI (22.2 kHz), our study examines four earthquakes, each with a magnitude over 6, happening near VLF network locations. Chosen earthquakes are Iwaki on July 11, 2014, Aomori prefecture on August 10, 2014, Ogasawara on May 30, 2015, and Makurazaki on November 13, 2015. Three are shallow and close to land, but deliberately included are one deep oceanic quake (Ogasawara) at 664 km depth and one occurring under pre-geomagnetic storm conditions six days before (Makurazaki). We examine nighttime VLF signal distortion during the earthquake events and investigate the presence of atmospheric gravity waves (AGW). This study supports the notion that deep oceanic earthquakes can perturb the ionosphere at considerable distances from their epicenter. Furthermore, we investigate the simultaneous ionospheric impact due to geomagnetic storms and the earthquake. Through the examination of seismic effects preceding the storm occurrence, we finds the likelihood of seismogenic effects that are not contaminated by other factors. The response of the initial seismogenic perturbation varies at different distances from the epicenter. Surprisingly, nearby receivers do not experience early disturbances. This indicates how challenging it is to forecast the location of earthquake.