## 3 Results  
What happens before large stress drop? In the nucleation dominated regime [10], the mechanism responsible for fracture triggering is analogous to static fatigue and delayed failure. The correlation between earthquake occurrence and tidal phase vanishes and failure is ultimately controlled by stress maxima. This can appear to be contradictory because we underlined that tidal stress modulations are \(\sim\)10\({}^{4}\) smaller than the seismic stress drop. Mechanical triggering must play a key role in statistical seismology, so that a tiny initial perturbation always have a not vanishing probability to become an earthquake. In this view, the problem of triggered instability reduces to a trivial threshold phenomenon in which perturbations are just \"the straws breaking the camel's back\". Analogously to mechanical engineering studies delving into the periodic supervision of facilities to detect signals of progressive weakening or corrosion, it is possible that seismicity could show significant variations in the correlation between seismic activity and stress modulations before a major event. Below we analyze three among the most important seismic sequences recorded in Greece in the last 20 years. Greece is a country prone to elevated seismic risk with both continental and insular territories prone to large earthquakes. Since in the Mediterranean Sea the amplitude of liquid tides does not reach relevant values (<50 cm), and they are even smaller in the investigated region, i.e., Greek Ionian Sea, where M\({}_{2}\) tide is about 5 cm high [36], sea tides can be neglected for our purpose. In our analysis, we focus on three different regions: Northern Thesally Region, Northern Ionian Greek Islands, and Southern Ioanian Greek Islands. Northern Thesally Region was recently hit by the Larissa seismic sequence (mainshock 3 March 2021, M\({}_{w}\) 6.3, 11.5 km depth, USGS), causing widespread damage and one casually. The still ongoing seismicity occurred along normal faulting. The analysis of correlation between the \(\Delta\)CFS and the nucleated seismic energy is performed between 1990 and 1 May 2021 in an area within latitude 39.5-40.2\({}^{\circ}\) N and longitude 21.7-22.4\({}^{\circ}\) E considering only earthquakes with M\({}_{L}\) > 2.0. After an initial period featured by elevated correlation values corresponding to a seismic swarm (M\({}_{w}\leq 5.0,\rho\sim 0.45\)) which occurred between 1993 and 1995, a decennial decrease of correlation followed. The trend switched in 2005-2007 and continued till the correlation turns positive. The progressive increase stopped in 2010 when diffuse swarms were recorded. Then, tidal correlation becomes negative again till 2020, when a peak is rapidly reached at the beginning of 2021 (\(\rho\sim 0.29\)), when Thesaly was shaken by a M\({}_{w}\) 6.3 earthquake. Our results are summarized in Figure 1.  
Northern Ionian Greek Islands are often involved by seismic sequences because they rise along the regional plate boundary between the Africa and Eurasia plates, which converge at a rate of about 9 mm/yr towards the north-north west. Nubian lithosphere subducts beneath the Aegean Sea along the Hellenic Arc. Focal mechanisms clearly denote compressive earthquakes. The largest seismic events during since 1990 occurred on 17 November 2015, M\({}_{w}\) 6.5 [37], and it is usually known as the Lefkada earthquake because it resulted in several fatalities and dozens injuries on the Greek Island of Lefkada. This major seismic crisis was forerun by a two-years-long seismic instability, which also included large-magnitude events such as the Cephalonia earthquake (26 January 2014, M\({}_{w}\) 6.1). Correlation analysis shows an about five years lasting increase of \(\rho\), which reached its maximum in 2013 (\(\rho\sim 0.13\)), then a sharp fall is observed so that tidal correlation becomes compatible with zero. Our results are in Figure 2.  
Figure 1: Correlation between \(\Delta\)CFS and seismicity in Thesaly Greece, between 1990 and 2021, M\({}_{L}\) > 2.0, NOAIG Catalog.