## 1 Introduction  
Pockmarks are depressions in the seabed that form due to the focused expulsion of fluids from subsurface sedimentary layers through the seafloor [1]. Fluids can be aqueous liquids, like porewater [2; 3; 4; 5] and/or seeping hydrocarbons, especially methane, which can be of microbial origin [6], thermogenic origin [7; 8; 9] or a mixture of them [10; 11; 12; 13]. Pockmarks associated with the venting of gas fluids have become widely observed since their first discovery offshore Nova Scotia [1; 14]. Pockmarks are usually formed as a consequence of gas and/or porewater eruption, most often followed by a period of unknown duration of recurring fluid escape. They often occur along continental margins commonly linked to subsurface structural and/or depositional features, including faults and salt diapirism in several settings [15; 16; 17], which reinforce fluid escape. Pockmarks have also been identifiedin shallow waters, including semi-enclosed shallow embayments and even in lakes [18,19] and lagoons, as well as in deep water environments [20,21]. The distribution of pockmarks has commonly been linked to faults that constitute very important active conduits for fluids to migrate to the surface [22, 23, 24, 25, 26]. In many regions, single pockmarks, composite pockmarks and pockmark strings have been observed above faults and weakness zones in soft sediments, providing vertical pathways for migrating fluids. In the Black Sea, gas venting is abundant along canyon walls and is strikingly linked to the fault outcrops [25]. The pockmark field in the Gulf of Cork, Ireland, is formed by enhanced gas flux from an underlying fault system [4]. In Arctic fjords, faults and igneous sills play an important role in channelling fluid flow and forming pockmarks with high density [9]. In the Barents Sea, most of the pockmarks can be associated with leakage pathways through a shallow fault system or along inclined bedding planes [27]. Pockmark size is also related to sediment thickness. In the Belfast Bay field, small and shallow pockmarks occur in thin sedimentary sequences, while large ones occur in thicker sequences [28]. The Holocene sediment thickness is positively correlated with the pockmark sizes of Passamaquoddy Bay [29]. The normal activity of the pockmarks may be interrupted by occasional gas-escape events [30]. Many studies have reported a relationship between pockmark activity and earthquakes [30]. Any large ground-shaking event, such as an earthquake or large landslide, could cause interstitial gas bubbles to be suddenly expelled into the water column [31]. Seabed fluid flow may occur in advance of earthquakes or be triggered by earthquakes. Earthquakes trigger gas venting from the seafloor in areas with high seismicity, such as in Northern California [32], Sea of Marmara, Turkey [33, 34, 35] and the Arabian Gulf [36]. In areas with low seismicity (Vancouver Island), long-term seepage monitoring showed no correlation between fluid escapes and seismic events [37]. The non-systematic correlation between seepage and seismicity may be attributed to either small magnitude earthquakes being unable to cause fluid mobilisation [38] or dispersed seismic energy in the area [37]. The pockmark field of Patras Gulf is an example, where pockmark activity has been reported as a precursory earthquake phenomenon from the Patras earthquake in 1993 [39]. The same field exhibited strong evidence of activation at least twice--during two major earthquakes of magnitude 5.4 and 6.4 R on July 14 th, 1993 and June 8 th, 2008 [39,40]. For this reason, [PERSON] and [PERSON], in their book _\"Seafloor Fluid Flow\"_, mentioned that: _\"The pockmarks off Patras, northern Peloponness, Greece, represent some of the most spectacular and best-documented events in active pockmarks\"_[30] (pages 230-231). The Patras Gulf pockmark field (PGFP) is located in the southeastern part of the Patras Gulf in the vicinity of the city of Patras (Figure 1a). The site has been under investigation since 1996 [39, 40, 41, 42].