## 2 Seismological Data and Results
### Seismicity Results
#### 2.1.2 The 2015 Lefkada Sequence  
A few months after the Cephalonia events, on 17 November 2015, an Mw6.4 earthquake struck Lefkada Island, with its epicenter located near the mid-southern part of the western coast. The seismicity during the period from 17 November to 3 December 2015, relocated with the double-difference method [65], revealed a very different distribution than that of the 2014 Cephalonia sequence. The aftershocks were divided in distinct clusters, distributed at focal depths between 5 and 15 km. The aftershock zone extended to \(\sim\)60 km in a N16\({}^{\circ}\)E direction, mainly onshore of the island. Very few events occurred south of the mainshock, suggesting that a major fault patch was ruptured, leaving only few small unbroken asperities. The largest aftershock was an M5.0 event that occurred on the same day as the mainshock and close to its vicinity. The densest cluster was located just north of the mainshock, in the same region that was activated during an M5.1 event that had occurred on 29 November 1994 [71], and also triggered after an Mw6.3 earthquake that struck the northern part of the island on 14 August 2003 [30; 31]. Notably, the mainshock of 2015 at Lefkada Island, which mainly involved group #3, also triggered seismicity in group #2, between Cephalonia and Lefkada, and even inside the Mytros Gulf. Furthermore, these offshore southern clusters presented similar characteristics as those in the gulf of Mytros, i.e., east-west alignment, likely related to antithetic sinistral faulting, transverse to the CLTFZ. The earthquake rate of group #3 that increased since the occurrence of the 17 November 2015 earthquake, remained higher than the background level (0.1 events/day; for \(M\geq M_{\rm c}=2.0\)) as late as September-October 2017. To exclude possible biases due to the increase of event detectability, setting a higher magnitude threshold of 2.3 (Figure 5) indicated a return of the seismicity rate to its background level around October 2016, dropping to very low levels in May-July 2017. Figure 4: Temporal evolution of the seismic sequence on Cephalonia Island (group #1), for events with \(M\geq M_{\rm c}=2.0\). The upper panel shows the cumulative number of events, with the occurrence of \(M\geq 4.0\) events depicted by stars, while the lower panel describes the seismicity rate on a semi-logarithmic scale, measured for sliding windows of various lengths (5 days to 100 days). 2.1.3. The 2018-2019 Zakynthos Sequence  
The most recent significant seismic occurrence in the region of the Ionian Islands during the study period is the 2018-2019 Zakynthos mainshock-aftershock sequence [29; 49; 72]. An Mw6.7 offshore earthquake occurred on 25 October 2018, 45 km SW of Zakynthos Island, at an estimated depth of \(\sim\)12 km. The focal mechanism of the mainshock indicates dextral strike-slip on a low-angle (25\({}^{\circ}\)), east-dipping plane. The relocated catalogue of over 4000 aftershocks in the period between 25 October 2018 and 31 March 2019 reveals an extensive aftershock zone, \(\sim\)80-km-long in a N110\({}^{\circ}\) E direction and \(\sim\)55 km wide [29]. The largest aftershock was an Mw5.5 event that occurred on 30 October 2018, 15 km NNW of the mainshock, with a similar focal mechanism, albeit with a steeper dip angle. The distribution of epicenters permits the distinction of some sub-clusters within the seismicity cloud. A large group in the vicinity of the mainshock presented a smoothly diminishing seismicity rate. On the other hand, other clusters, particularly at the northern part of the aftershock zone, but also at the southern end, presented outbreaks, some related with the occurrence of significant aftershocks. The most persistent activity was observed at the northern subclusters of group #4, with intense activity that lasted until October 2019. A last burst of seismic activity occurred between March and July 2020 and was triggered at the southern end of the 2018 aftershock zone, after an M4.3 event on 26 March 2020, followed by a few more \(M>4\) events.