**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 ∼10<sup>4</sup> 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 M2 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 Thessaly Region, Northern Ionian Greek Islands, and Southern Ioanian Greek Islands.

Northern Thessaly 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 casualty. The still ongoing seismicity occurred along normal faulting. The analysis of correlation between the ΔCFS and the nucleated seismic energy is performed between 1990 and 1 May 2021 in an area within latitude 39.5–40.2° N and longitude 21.7–22.4° 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* ≤ 5.0, *ρ* ∼ 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 (*ρ* ∼ 0.29), when Thessaly was shaken by a M*w* 6.3 earthquake. Our results are summarized in Figure 1.

**Figure 1.** Correlation between ΔCFS and seismicity in Thessaly Greece, between 1990 and 2021, M*L* > 2.0, NOAIG Catalog.

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 *ρ*, which reached its maximum in 2013 (*ρ* ∼ 0.13), then a sharp fall is observed so that tidal correlation becomes compatible with zero. Our results are in Figure 2.

**Figure 2.** (**Top**) correlation between ΔCFS and seismicity close to the Greek Ionian coasts (38.0– 39.0° N, 20.0–21.0° E, M*L* > 2.0, NOAIG Catalog.) between 1990 and 2021. Main event was located in southwestern region of Lefkada Island between the villages Athani and Agios Petros. The 2014–2016 seismic sequence involved faults within different tectonic settings. (**Bottom**) ΔCFS as a function of time. Before the 2014–2016 seismic sequence, significant changes in the Coulomb stress distribution was observed, partially due to contribution of deeper (>20 km) earthquakes.

The tectonic setting of the Southern Ioanian Greek Islands (37.0–38.0° N, 20.0–21.0° E) is similar to the previous one.

Great part of seismicity is still featured by a compressive focal mechanism even if a fraction of crustal events with significant oblique component are recordered.

In this territory, two main quakes happened in the last thirty years.

The largest hit Lithakia on 25 October 2018 with M*w* 6.8, depth 15 km, it also produced a small tsunami with about ∼20 cm high anomalous waves.

The other was a M*w* 6.6 earthquake occurred in the same area on 18 November 1997.

The Lithakia earthquake was preceded by an about 7–9 years long period of progressively increase of the tidal correlation between ΔCFS and seismic energy. The peak of the correlation was measured between 2018 and 2019 with *ρ* ∼ 0.11, then a fast decrease is observed.

At the same time, an anomaly in the localization of seismicity is noticed, which is reported in the lower part of Figure 3. Two seismic shadow zones are located at a depth of 0–10 km (2012–2017) and below 19 km (2015–2018).

**Figure 3.** Correlation between ΔCFS and seismicity close to the Greek Ionian coasts (37.0–38.0° N, 20.0–21.0° E) between 1985 and 2021. A nine-years-long preseismic phase is highlighted both by an increasing value of correlation in the upper part of the picture, and seismic quiescence, represented by two red shadow zones, at a depth of 0–10 km (2012–2017) and >20 km (2015–2018). The yellow star represents the M*w* 6.8, 25 October 2018 Lithakia mainshock.

The seismic quiescence [38] in the forementioned layers is statistically significant especially in the upper one, where the reduction of the seismic rate reached 60% with respect to the preceding 2007–2012 period.

Observations sugges<sup>t</sup> that *ρ* tends to increase during the preparatory phase of significant crustal earthquakes in Greece. The values of correlation are higher in extensional tectonic settings rather than compressive ones of about 100–200%, normal fault quakes are found to correlate stronger with tidal stress modulations also in other regions. The energy conditions are instead stricter for events occurring in a compressive tectonic setting, so it is reasonable to expect that they correlate very little with the intensity of tidal perturbations, especially in the case of deep hypocenters. Therefore, even though correlations are weak, their modulations can provide precious information about the condition of instability of local crustal volumes, especially if jointly analyzed with other seismological and geodetic recordings. We also analyze the spatial density of the Coulomb stress variation induced by the action of tidal perturbations. For each seismic event, ΔCFS value is calculated, then a map is created that shows the fraction of ΔCFS generated in each location of the selected region. Therefore, the areas with elevated ΔCFS density are those in which seismicity has statistically occurred at more elevated Coulomb stress values or characterized by higher seismicity rate with respect to the surrounding areas.

Since the seismic rate is ultimately controlled by the maximum nucleated magnitude, for each case study, we also plot the areas hit by earthquakes with large magnitude, as written in the caption according to the relative intensity of the regional seismicity. The green contours are drawn according to the finite fault maps of the USGS catalog. Whenever earthquake sequences occur at significantly high ΔCFS values, the brightest

spots (according to the vertical colorbar on the left in Figure 4) are located within the green profiles, which sugges<sup>t</sup> self-triggering. On the contrary, shiny stains just outside the main shock areas point out zones featured by elevated stress transfer.

**Figure 4.** ΔCFS density map for seismicity in Greece, NOAIG Catalog, 1990–2021, M*L* ≥ 2.0. Highest density areas are located on the Greek Ionian Islands along Hellenic Trench and along normal fault system of Gulf of Corinth.

The ΔCFS density map for Greece shows diffuse signal sometimes due to strong motion recordings (e.g., Gulf of Corinth seismic sequence, 1995, and Ionian Arc seismicity in 1997); nevertheless, no correlation is found between the intensity of the signal and large magnitudes, as proven in the cases of the Methoni M*w* 6.9 earthquake and the Aegean Sea M*w* 6.9 event.

The brightest patches are located where the 2014–2015 Ionian and the 2018 Lithakia seismic sequences occurred. This means that seismic activity in the Aegean region is usually self-triggered at large spatial scales. Compare with Figure 4 and Figure 5.

**Figure 5.** Map of seismicity in Greece between 1990 and 1/8/2021, M*L* > 2.0, NOAIG Catalogue.
