## 4 Discussion
### Pockmarks Spatial Distribution and Morphometry
There, as well as in the previously mentioned high-density area, the Holocene sedimentary deposit thickness is low, and the depth occurrence of the ATZs is shallow, suggesting that despite the faulting, the number of pockmarks is also controlled by the sediment thickness (Figures 8 and 11). The density of the pockmarks in Belfast, Blue Hill and Passamaquoddy Bays is related to the thickness of the Holocene sediments, with the highest densities occurring in areas with thinner sediments [29]. The direction of the maximum axis of the ellipsoidal pockmarks and the collinear cluster of pockmarks (\"chains\" or strings) is of particular interest, as it may indicate the directionality of the hydrodynamic and endogenic processes controlling the formationof the pockmarks [1; 85; 86; 83]. The measurements in the ellipticity modes of the Patras pockmarks showed that the orientation of the major axes, in common and composite pockmarks, is not random but follows two main directions: WNW-ESE and WSW-ENE (Figure 4e). The WSW-ENE direction coincides well with the directions of the main local faults, suggesting that the fluids escaping along those faults play an important role in the field formation. This is further supported by the fact that the pockmark collinear clusters are found lined up in the same direction (Figure 3). The linear arrangement and the eccentricity of the pockmarks have commonly been linked to the underlying geological structure, whereby faults provide active conduits for fluids to migrate to the surface [87; 88; 89]. The other prevailing direction (WNW to ESE) shows an offset of about 20\({}^{\circ}\) to the faulting direction. This is probably due to gas escapes through other localised failures or along permeable horizons at the proximity of the faults since the Holocene clays are characterised by low permeability. A similar explanation has been given by other authors to explain the displacement of the pockmark appearance compared to underlying geological structures. [PERSON] and [PERSON] [90] reported that pockmarks may not necessarily be located directly above a fault but can be formed in the peripheral parts of tectonic lineaments due to gas migrating sideways within the uppermost parts of the sedimentary strata. [PERSON] et al. [91] attributed the displaced appearance of the pockmarks in the Oslojord (Norway) relative to the underlying structural elements to the same mechanism. The arrangement of the PGPF seems to not be significantly affected by hydrodynamic processes. Although the occurrence of elliptical pockmarks in several fields has been attributed to the transport of sediments by seafloor currents during pockmark formation [85; 92], the SW-NE direction of ellipticity, which is expected to be related to the direction of the prevailing currents, is extremely weak in all classes of pockmarks. This is in accordance with the weak SW-NE-trending bottom currents in the area [93]. The sidewalls of the PGPF pockmarks have slopes ranging from 2\({}^{\circ}\) to 48\({}^{\circ}\), the steeper of which are observed mainly in the large pockmarks of the southern sector. The mean value of the slope (26\({}^{\circ}\)) is significantly higher than those in the Belfast Bay (8-12\({}^{\circ}\)) [28] and Oslojord (13\({}^{\circ}\)) pockmark fields [91]. The steep sidewalls are unstable and thus often result in slumping [59; 91]. Such gravitational movements have been found by acoustic imaging and direct observation in pockmarks with steep sides in the Gulf of Maine [94]. The PGPF pockmarks slope more than the proposed 20\({}^{\circ}\) angle of ropose for fine-grained sediments [59]. This, together with the fact that the sediments of SS I are soft (clays with interlayers of sandy mud), suggest that the PGPF pockmarks with steep sidewalls are likely to be affected by sidewall slumping. At least two pockmarks from the southern sector exhibit evidence of wall slumping and partial infilling of the pockmarked floor (Figures 12 and 13b).