Analysis of the  mode interference  of Very Low-Frequency Electromagnetic Waves during Seismic Events

Abstract

We present the characterization of the  mode interference  of Very Low-Frequency (VLF) electromagnetic waves at dawn/dusk based on amplitude and phase data received by LAVNet (Latin America Very low frequency Network) from signals emitted by Omega-Network stations. LAVNet is a collaborative project consisting of four receiving stations located at different points in the Americas (Argentina:-65.0W,-30.0S; Chile:-71.99W,-36.64S; Colombia:-76.5W,3.93N and Mexico:-99.15W, 19.47N). Its purpose is to detect and characterize the signatures left by various types of geophysical events when VLF electromagnetic waves, propagating through the Great Circle Path, are captured by these stations. To identify the signatures of a geophysical events in the VLF signal, where a geophysical event is defined as any event that disturbs the VLF signal from 'above' (outer space) or from 'below' (lithosphere), it is necessary to understand the typical signal's behavior  during all seasons of the year. In this case, we are interested in the possibility of detecting disturbances in the signal due to activity preceding an earthquake. To achieve this goal first,  it is necessary to characterize the signals over a long period of time (several years) to establish the typical behavior of each path, especially during dawn/dusk when shifts of the mode interference minima  may take place. These shifts would be characteristic of lithospheric activity. In particular, here we  present the characterization of VLF signals received by the station located in Mexico, specifically the typical minima times during dawn/dusk and its time-shifts along  the year.  Also, we present a preliminary analysis of the mode shifts related to seismic events observed  in the 2012-2017 time period.