## 1 Introduction  
### Natural Hazard Signatures Associated with Geodynamic Processes  
Natural hazards (e.g., earthquakes) have significantly threatened humans throughout history [1, 2]. Remote sensing technologies operating at wide frequency ranges, using either sound or electromagnetic emitted waves, have become valuable tools for detecting and measuring signatures presumably associated with such events [3, 4]. In the last three decades, predicting when and where natural hazards were to occur has been challenging in geoscience-related research [5, 6]. The occurrences of earthquakes are correlated with the dynamics of the earth's crust [7, 8], consisting of semi-rigid tectonic plates of various sizes [9]. When these plates collides, they can cause one edge of the plate to slide under the other [10, 11], resulting in stress accumulation within the earth's crust [12]. This leads to mechanical deformations, and results in the rupture of the crust when the deformation exceeds the limit of the mechanical forces [12]. Since most of the governing forces and basic mechanics of earthquake events cannot be cast into physical and numerical models, and since we lack sufficiently detailed and real-time measurements, accurate forecasting of such events is not currently possible [3, 13, 14, 15]. On the other hand, observational and modeling results have confirmed the existence and detectability of earthquake and tsunami signatures in the ionosphere caused by both acoustic and gravity waves, disturbing the electron density in the F-region [16, 17, 18, 19, 20, 21]. Regarding the connection between the F-region and earthquakes--the source of the earthquake generates acoustic and gravity-acoustic waves that propagate laterally and upward, away from the source and through the ionospheric layers. This means that such hazards can create atmospheric and ionospheric perturbations via direct coupling [19]. Such signatures have also been confirmed by anomalous increases in the intensity of the electromagnetic signal, received in the VLF/ULF bands, during the period immediately preceding an earthquake [22; 23; 24], and are caused by ionospheric D-layer electron density disturbances. However, there are still a fair amount of ambiguities regarding the scientific validations of these discoveries as reliable tools that may be used for detecting precursor natural-hazard-generated ionospheric perturbations. For example, after the devastating Tohoku-oki earthquake (Mw 9.1) and tsunami that took place in Japan on 11 March 2011, using Japan's dense GPS satellite network, [PERSON] [14] reported that a clear precursor was detected in the form of a positive anomaly of ionospheric total electron content (TEC). This began about 40 min before the earthquake and reached nearly 10% of the background TEC, and lasted until atmospheric waves arrived at the ionosphere. [PERSON] et al. [20; 21] suggested that these ionospheric density variations could be caused by Earth surface charges (or currents) produced from electric currents associated with the stressed rock. However, when [PERSON] et al. [25] investigated the global ionospheric TEC perturbations just before and after the event, they concluded that the geomagnetic activity indicated storm conditions that were already apparent from processed GEONET data one day prior to and after the event. The same ambiguity can be found with ionospheric D-layer electron density disturbances extracted from VLF measurements. [PERSON] et al. [22] reported on a possible VLF sub-ionospheric precursor for the same earthquake in Japan. They found a remarkable anomaly for the NLK-Chofu great circle path on 5 and 6 March in the form of nighttime average amplitude signal decrees that exceeded \(-4\sigma\). The anomaly was found on the same day along other propagation paths (from NLK to both Kasugai and Kochi), although it was less enhanced.