Comparative study of the possible anomalies in D-region electron density profile as computed from unusual terminator shifts in sub-ionospheric Very Low Frequency (VLF) signal during Honshu, 2011 and Nepal, 2015 earthquakes

Abstract

We present the perturbations and anomalies in the propagation characteristics of Very Low Frequency (VLF) signals received at Ionospheric & Earthquake Research Centre (IERC) (Lat. 22.50 N, Long. 87.48 E) during and prior to the two devastating earthquakes in Honshu on 11th March 2011 at 11:16:24 a.m. local time (05:46:24 UTC) with magnitude of M=9 and depth 29 km at the Pacific coast of Honsu, Japan and another in Nepal on 12 May 2015 at 12:35:19 pm local time (07:05:19 UTC) with magnitude of M=7.3 and depth 18 km at south-east of Kodari. The VLF signal emitted from JJI/22.2KHz in Japan (Lat. 32.05 N, Long. 131.51 E) shows strong shift in VLF-sunrise terminator times towards nighttime starting from a few days prior to the earthquake. We chose these two earthquakes to check the effectiveness of terminator time shift method on this particular propagation path as these two earthquakes have taken place near the VLF transmitter-end (JJI) and near the VLF receiver-end (IERC) respectively. In this work, we have utilized the situation and simulated the VLF sunrise terminator shifts using the RANGE model and EXPONENTIAL sub-program of Long Wavelength Propagation Capability (LWPC) code. To represent the D-region ionospheric variabilities clearly, we assumed a mean dynamic perturbation over the path and presented them with a set of effective Wait's parameters (_eff, h_eff). We have also reproduced the temporal trend of the normalized VLF signal amplitude at VLF sunrise terminators for a few days around both the earthquakes. Then, we used Wait's two-component exponential ionospheric model for estimating the altitude profile of D-region electron density (Ne(h)) at VLF sunrise terminator times on all those days around both the earthquakes. Hence, we have studied quantitative changes of those Ne(h) profiles during and prior to the seismic events.