Merging Vlf/Lf data in the epidemic-type aftershock sequence (etas) model: a newperspective operational tool to provide seismic precursors

At the end of January 2020 an intense seismic crisis occurred on Dodecanese islands. The main earthquakes (Mw= 5.6 and Mw= 5.7) happened on January 30. This seismic activity was studied using the receivers of the INFREP network. Pre-seismic anomalies on the three VLF radio signals (19.58, 20.27, 23.40 kHz) collected by the Cyprus receiver and crossing the zone of the seismic activity were identified. The analysis of daily day/night trend of these signals pointed out a clear anomaly during the night of 29 January 2020, one day before the occurrence of the main shocks of the seismic crisis.

In this work we present a methodology to include the anomalies detected in the VLF radio signals into the epidemic-type aftershock sequence (ETAS) model to establish seismic correlations. The ETAS model is characterized as a self-exciting point process where each seismic event has the potential to trigger subsequent offspring events. Recognized as a benchmark method for Operational Earthquake Forecasting (OEF), it utilizes earthquake catalogues to compute the conditional intensity function, λ(x,y,t), representing the total seismicity rate. To incorporate electromagnetic precursors, our methodology modifies λ(x,y,t) by applying a multiplicative weighting factor, w(x,y,t). This factor reflects the occurrence time and location of VLF anomalies, where the spatial extent of influence is constrained by the Dobrovolsky region.

The proposed methodology builds on previous studies regarding VLF anomalies and the ETAS framework, and could provide a step forward in the operational use of VLF anomalies as seismic precursors. Therefore, we present the first results of the proposed methodlogy to model and simulate earthquake occurrence for the Dodecanese earthquake sequence.