**6. Conclusions**

In this study, we present the efficiency of our clustering method, MAP-DBSCAN, on a simulated earthquake catalog where the structure of the clusters is known a priori and its competitiveness against well-known clustering algorithms, as in most cases, shows better results. The main seismic clusters in the Corinth Gulf, Central Ionian Islands and North Aegean Sea during 2012–2019 are detected by our method and their clustering properties are investigated. The results show the existence of regional variability in aftershock productivity and background rates. In particular, the Corinth Gulf is characterized by low productivity values and high background rates related to the dominance of earthquake swarms, whereas seismicity in the Central Ionian Islands is comprised by main shock–aftershock sequences with high productivity. Sequence-specific parameters verify the dependence between low productivity values and high background rates with pore-pressure due to fluids migration. We believe that future studies on Operational Earthquake Forecasting should incorporate localized parameters into the models to improve the forecasting accuracy.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/app12041908/s1, Figure S1a–c Residuals (purple triangles) as a function of minimum cutoff magnitude, *Mc*, for the D1, D2 and D3 datasets, respectively. Blue and cyan dotted horizontal lines indicate the 10% and 5% residual thresholds, respectively. *Mc* (red triangle) is found as the first magnitude cutoff at which the confidence 95% is reached. (d–f) Incremental (red triangles) and logarithmic cumulative frequency (blue triangles) as a function of magnitude. The black line is the GR law fit according to the GFT method with *Mc* = 1.5, 2.2, 2.1 for datasets D1, D2 and D3, respectively. Figure S2 (a) Epicentral map of the main seismic clusters during the first semester of 2012. Three major clusters, *C*1, *C*2 and *C*3, and eight smaller clusters with *N* ≥ 10 events occurred. (b) Space-time evolution of seismicity. Colors correspond to different clusters and the size of circles is proportional to the earthquakes' magnitude, Figure S3 (a) Epicentral map of the 2013 Aigion swarm and subsequent sequences in the area with *N* ≥ 10 events. (b) Space-time evolution of seismicity. Colours correspond to different clusters and the size of circles is proportional to the earthquakes' magnitude, Figure S4 (a)

Epicentral map of the seismic activity between November, 2013 and June, 2014. Twelve clusters with *N* ≥ 10 occurred, including the *C*11, *C*12 and *C*14 clusters. (b) Space-time evolution of seismicity. Colors correspond to different clusters and the size of circles is proportional to the earthquakes' magnitude, Figure S5 (a) Epicentral map of the intense seismic activity during the second half of 2014. Five major clusters occurred, the *C*15, *C*16, *C*18, *C*19 and *C*20, and four smaller clusters with *N* ≥ 10 events. (b) Space-time evolution of seismicity. Colors correspond to different clusters and the size of circles is proportional to the earthquakes' magnitude, Figure S6 (a) Epicentral map of the seismic sequence Offsh. Perichora. One major cluster, *C*4, including two sub sequences, the first initiated on 22 September and the second on 30 September, 2012. b) Space-time evolution of seismicity. Colours correspond to different clusters and the size of circles is proportional to the earthquakes' magnitude, Figure S7 (a) Epicentral map of the seismic activity near Itea Gulf during 2014. Two major clusters are occurred, the *C*13, *C*17 and four smaller ones with *N* ≥ 10 events. (b) Space-time evolution of seismicity. Colors correspond to different clusters and the size of circles is proportional to the earthquakes' magnitude, Figure S8 (a) Epicentral map of the 2014 Kefalonia earthquake sequence, *I*1, and a sub-cluster, *I*2, that occurred offshore the southern part of Kefalonia Island. (b) Space-time evolution of seismicity. Colors correspond to different clusters and the size of circles is proportional to the earthquakes' magnitude, Figure S9 (a) Epicentral map of four main clusters, *I*3, *I*4, *I*5 and *I*6 with *N* ≥ 30 between November, 2014 and April, 2015. (b) Space-time evolution of seismicity. Colors correspond to different clusters and the size of circles is proportional to the earthquakes' magnitude, Figure S10 (a) Epicentral map of the 2017 Lefkada sequence, *I*7, along with two sub-clusters in the southwestern part of Kefalonia Island. (b) Space-time evolution of seismicity. Colors correspond to different clusters and the size of circles is proportional to the earthquakes' magnitude, Figure S11 (a) Epicentral map of cluster *I*9 located in the area between Lefkada and Kefalonia. Right: Space-time evolution of seismicity. Colors correspond to different clusters and the size of circles is proportional to the earthquakes' magnitude, Figure S12 (a) Epicentral map of cluster *N*1 comprised by two subsequences. (b) Space-time evolution of seismicity. Colors correspond to different clusters and the size of circles is proportional to the earthquakes' magnitude, Figure S13 (a) Epicentral map of the 2013 North Aegean sequence, denoted *N*3. (b) Space-time evolution of seismicity. Colors correspond to different clusters and the size of circles is proportional to the earthquakes' magnitude, Figure S14 (a) Epicentral map of the 2014, Samothraki sequence confined into three major clusters, *N*4, *N*5 and *N*6. (b) Space-time evolution of seismicity. Colors correspond to different clusters and the size of circles is proportional to the earthquakes' magnitude, Figure S15 (a) Epicentral map of the seismic activity near the Aegean coast of NW Turkey during January–October 2017 confined into three clusters, *N*10, *N*11 and *N*12. (b) Space-time evolution of seismicity. Colors correspond to different clusters and the size of circles is proportional to the earthquakes' magnitude, Figure S16 (a) Epicentral map of the 2017 sequence ( *N*12) that occurred offshore, south of the SE coast of Lesvos Island along with its intense aftershock activity. Two major secondary bursts of activity occurred concurrently on the west (*N*17) and east ( *N*16) side of the sequence. (b) Space-time evolution of seismicity. Colors correspond to different clusters and the size of circles is proportional to the earthquakes' magnitude.

**Author Contributions:** Conceptualization, P.B. and E.P.; methodology, P.B.; software, P.B.; validation, P.B.; formal analysis, P.B. and G.T.; data curation, P.B. and E.P.; writing—original draft preparation, P.B.; writing—review and editing, E.P. and G.T.; visualization, P.B.; supervision, E.P. and G.T.; funding acquisition, P.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research is co-financed by Greece and the European Union (European Social Fund-ESF) through the Operational Programme Human Resources Development, Education and Lifelong Learning in the context of the project "Strengthening Human Resources Research Potential via Doctorate Research" (MIS-5000432), implemented by the State Scholarships Foundation (IKY).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are openly available at https://doi. org/10.7914/SN/HT (accessed on 15 January 2021).

**Acknowledgments:** The editorial assistance and the constructive comments from two anonymous reviewers are greatly appreciated. We are also grateful to I. Zaliapin for providing the code for nearest-neighbor analysis. The software Generic Mapping Tools was used to plot the map of the study area [72]. Geophysics Department Contribution 959.

**Conflicts of Interest:** The authors declare no conflict of interest.
