###### Abstract
The onshore borders and coastlines of the five sub-areas analyzed in the current study are drawn with colored outlines and labeled with the numbers 1–5. Fault lines on the main map are from the NOfaults v4.0 database [6]. Years of occurrence of significant earthquakes of the historical and instrumental era, mentioned in the text, are marked with labels next to their corresponding epicenter. effect. Numerous research groups have studied this mainshock and its rich aftershock sequence (e.g., [9; 10; 11]). Nearly 20 years later, on 19 July 2019, a M\({}_{\rm w}\) = 5.1 event struck the same area [8], at the western part of Parnitha Mt. [PERSON] et al. [12] relocated both sequences, concluding that the latter event occurred on the same fault, whose up-dip extension towards the east is associated with the surface expression of the Fili fault on Parnitha Mt. Another significant earthquake sequence that affected Attica's mainland was the one consisting of three major events that occurred on 24 and 25 February and 4 March 1981 at the Eastern Gulf of Corinth [13; 14; 15]. These earthquakes caused extensive collapses near the epicentral area, and damage was also reported in Attica. In total, 109 earthquakes with M \(\geq\) 4.0 were recorded in 1981, whereas only 27 occurred in 1982. The area close to the Eastern Gulf of Corinth recently exhibited increased seismic activity during a swarm that evolved between March and September 2020 at the Perachora peninsula, with magnitudes M \(\leq\) 3.7 [16; 17; 18]. A notable swarm occurred near Villa in June 2013, including six events of 3.0 \(\leq\) M\({}_{\rm L}\)\(\leq\) 3.5, whereas an isolated M = 4.4 event also struck further west on 20 September 2013 but was not followed by significant aftershocks [19]. Just north of Attica, another area that is known to host significant earthquakes is Thiva. The recent seismicity in this region includes a M\({}_{\rm w}\) = 4.6 event on 2 December 2020 [20] and the major events (3.4 \(\leq\) M \(\leq\) 4.2) of a long-lasting swarm that began in July 2021 [21]. These events were also felt in Athens; however, due to their small magnitudes, they did not cause any damage. In the Marathon area, Eastern Attica, a small sequence with a maximum magnitude of M = 4.2 occurred in 2018 [22]. All of the above-mentioned earthquakes of the instrumental era involved mainly normal faulting in a roughly E-W direction, in accordance with the regional stress field, which indicates extension in an approximately N-S direction [23]. Concerning the islands of the Saronic and Argolic Gulfs, as well as the areas of Trizinia and Methana, the most significant historical earthquakes include the 480 BCE, M = 6.3 one that was mainly felt at Salamina Island; the 1457 CE, M = 6.3 one at Hydra, with estimated intensity of I = VII; the 20 March 1837 CE, M = 6.0 one at Trizinia with I = VII; and the 25 July 1873 CE, M = 5.9 one at Epidaurus, with I = VII [2]. The Saronic Gulf and Trizinia hosted several significant earthquakes during the instrumental era. In 2016, a series of earthquakes occurred east of Poros Island, which is considered a rare instance for that area [24]. The largest concentration of activity is located at the westernmost end of the Saronic Gulf. Within the latter, and towards the Aegean Sea, seismicity is less frequent and occurs at greater depths. In the immediate area of Hydra and Spetses Islands, no concentration of significant seismicity is apparent, which is consistent with the absence of major mapped tectonic structures. On the contrary, some earthquakes of significant magnitude in the broader area have occurred at intermediate depths, associated with the subducting slab, such as an M\({}_{\rm w}\) = 6.0 one in 1948 at a focal depth of 88 km with the epicenter near Dokos Island, and an M\({}_{\rm w}\) = 6.4 one in 1962 at a depth of 94 km below Corinth. More recently, on 6 January 2008, an M\({}_{\rm w}\) = 6.3 event occurred at a depth of 84 km in the epicentral area of Leonidio [25; 26]. This event was also felt on Attica's mainland.