## 1 Introduction
### Regional Setting  
Patras Gulf is a semi-closed gulf which is open to the Ionian Sea on its western side and is linked through the Rio-Antirio strait to the Gulf of Corinth on its eastern side. Patras Gulf is the western part of the Patras-Corinth rift zone. The bathymetry of the gulf is controlled by linear structures (extensive active faults) trending WNN-ESE, which are the result of the  
Figure 1: (**a**) Simplified geological map of Patras Gulf showing the major faults in the area and the location of the pockmark field; index map of Greece showing study area. (**b**) Tectonic map of Western Greece, (Geological data from [44]; Fault data from [45]) (ATF: Agia Triada Fault; RPTF: Rio-Patras Transfer Fault; PG.: Patras Gulf; A.G.: Amvrakikos Gulf; T.L.: Trichonida Lake; A.R.: Alfios River). (**c**) Seismicity map of Patras Gulf (Earthquake location data were retrieved from NOA-IG [46]. The color scale depicts depth distribution of the epicentre of the events).  
geological setting that characterises the gulf area (Figure 1a) [47; 48; 49; 50]. The southern flank of it is controlled by the major faults and the northern by antithetic and synthetic faults, forming an asymmetric graben with the north-dipping major lixtic fault [50]. The WNW-trending faults are presently active, as revealed by the recent seismicity in the area, and exert a strong geomorphological control on the landscape, including river courses (e.g., Alfios River), lakes (e.g., Trihonis Lake) and the shallow gulfs of Patras and Amvrakikos (Figure 1b) [45].  
The graben seems to terminate against the ENE-WSW normal faults controlling the Plio-Pleistocene sequences outcropping onshore in the SE part of the Gulf. A NE-SW trending fault system called the Rion--Patras fault zone also controls the Patras rift zone [48]. The pockmark field is located at the intersection of WNW-ESE, ENE-WSW and the NE-SW fault zone. The evolution of the Gulf of Patras during the Late Quaternary is the consequence of the interaction between active tectonic subsidence (3-10 mm/yr), high sedimentation rate (2-3 mm/yr) and global sea level rise [50; 51]. High-resolution profiles showed the existence of two layered sequences through the Gulf of Patras. The upper, attributed to the Holocene [50; 51], is almost transparent and overlies the lower sequence that contains strong parallel to sub-parallel reflectors. The highly reflective surface on the top of the lower sequence is attributed to the basal unconformity of the Gulf, which is associated with the Last Glacial Maximum (\(\sim\)21 ka) and the early part of the following transgression (Late Pleistocene) [50; 51] when the low sea level (\(\sim\)120 m below present level) most probably disconnected the gulf from the Ionian sea and the gulf became subaerial exposed. Surficial sediments mainly consist of silt and clay with low amounts of sand--except in the areas near the Rio-Antirio straits and the deltas at the northern part of the Gulf [51].  
Western Greece is one of the most seismically active areas in the Mediterranean [52]. The Patras Gulf generally represents lower seismicity compared with the high seismicity of the Rio-Antirio strait, while the central and western parts of the gulf show a seismicity gap [49]. The spatial distribution of the earthquake epicentres for the time interval 2011-2022 shows medium seismicity in the eastern part of Patras Gulf, low in the central and western part and high in the Rio-Antirio strait (Figure 1c).