Geophysical multilayer and multi-instrument investigation of the earthquake preparatory phase: the case studies of 2018 Palu (Indonesia) Mw=7.5, 2019 Ridgecrest (California, USA) Mw=7.1, and 2020 Jamaica Mw=7.7.

In the last decades, many attempts have been made to study earthquake precursors looking at just one single parameter at the time. In most of the cases, the investigated observable is perturbed before the earthquake, but it is not clear why not always and in a different way. Looking at the lithosphere, atmosphere and ionosphere as a unique system (as suggested by the concept of Geosystemics, De Santis et al., Entropy, 2019, https://doi.org/10.3390/e21040412) could overpass the problems of investigating a single layer or even a single parameter. In this work, we applied this approach investigating the behaviour of lithosphere, atmosphere and ionosphereup to about six months before three Mw7+ earthquakes occurred in the recent years. The events here studied happened in Palu (Indonesia) on 28 September 2018 (Mw=7.5), Ridgecrest (California, USA) on 6 July 2019 (Mw=7.1) and Jamaica on 28 January 2020 (Mw = 7.7). All the detected anomalies identified in each layer are tried to be read as a unique phenomenon. For all three investigated case studies, we found that the absolute value of the electron density increased about one month or a little more before each event. The observation is confirmed at least by 2 different instruments or platforms (for Indonesia earthquake it was detected by Swarm and CSES, Marchetti et al., JAES, 2019, https://doi.org/10.1016/j.jseaes.2019.104097; for Ridgecrest earthquake by Point Arguello ionosonde and Swarm Alpha satellite, De Santis et al., Front. Earth Sci., 2020, https://doi.org/10.3389/feart.2020.540398; for Jamaica earthquake by CSES and Swarm satellites). Finally, all three events show a chain of phenomena that, even with slightly different sequences of anomalies, successfully detect the geophysical system alteration possibly due to each earthquake preparation phase. The differences could be due to the focal mechanism, the location (Jamaica is a sea earthquake, Ridgecrest is located on land and Indonesia fault involves both sea and land), and the environmental conditions (geology, mineralogy, surface land coverage, etc.). Apart from these differences, our results strongly support the concept of studying the Earth as a multilayer and unique system, which could be the key to better understand such a complex phenomenon: the earthquake.