The study of marine geohazards through their imprint in the late Quaternary sedimentary record is of great significance, since it can provide further information on geohazard events recorded in historical records or even extend this record to much earlier times. The identification and recurrence patterns of mass transport deposits (MTDs) resulting from submarine landslides in sedimentary basins and lakes provide valuable information on possibly associated tsunamis as well as their potential trigger (e.g., earthquake). Tsunami hazard is particularly an issue of concern in the Mediterranean Sea, where more than 300 tsunamis have been listed in the historical and sedimentary records (Soloviev, 1990; Salamon et al., 2007; Lorito et al., 2008).

This paper focuses on the Gulf of Corinth, Greece, located in the most seismically active part of the Corinth rift. This area shows one of the largest seismic hazard in Europe (Woessner et al., 2013) and is affected by a tsunami once every 19 years on average, leading to a significant risk (Papadopoulos, 2003; Papathoma and Dominey-Howes, 2003). The gulf's western tip is the most active part of the Corinth rift, characterized by an extension of 15 mm yr-1 (Briole et al., 2000) and by frequent submarine or coastal landslides (e.g., Heezen et al., 1966; Papatheodorou and Ferentinos, 1997; Lykousis et al., 2009). Small to medium failure events (106–107 m3) occur on average every 30–50 years (Lykousis et al., 2007). These landslides trigger tsunamis (Galanopoulos et al., 1964; Stefatos et al., 2006; Tinti et al., 2007) and induce coastal erosion by upslope retrogression (Papatheodorou and Ferentinos, 1997; Hasiotis et al., 2006). Tsunamis reaching an intensity ≥ 4 consequently represent a significant hazard in the western Gulf of Corinth (Beckers et al., 2017) and are documented for the last two millennia from historical sources and on-land geological studies (De Martini et al., 2007; Kontopoulos and Avamidis, 2003; Kortekaas et al., 2011). However, these data sets are incomplete.

A dense grid of high-resolution seismic profiles acquired in this area (Beckers et al., 2015) was used to realize an inventory of the large MTDs that may be interpreted as the result of submarine landslides. Dated from the Late Pleistocene and the Holocene, the mapped MTDs range from 106 to 109 m3. Average recurrence intervals are presented and discussed, as well as pre-conditioning factors that might have played a role in the occurrence of these large submarine landslides. The MTDs' temporal distribution is discussed, as well as the implications of their occurrence on tsunami hazard.

The western Gulf of Corinth is characterized by a relatively flat deep basin dipping gently to the east. Featuring a narrow canyon in the west (the Mornos Canyon), it widens in the east (Delphic Plateau, Fig. 1). It is bordered by steep slopes on all sides (Fig. 1). To the north, it is limited by the Trizonia scarp with slopes ranging from 25∘ to locally more than 35∘ and the associated Trizonia fault (Nomikou et al., 2011); these slopes are mostly devoid of sediments which are trapped in the bay areas to the north (Fig. 1b). To the south, the western gulf is bordered by 400m high Gilbert deltas built by the Erineos, Meganitis and Selinous rivers that lie in front of the active Psathopyrgos, Kamari and Aigion faults running along or near the coastline. Delta fronts have 15 to 35∘ slopes incised by gullies (Lykousis et al., 2007; Nomikou et al., 2011) and consist of a thick pile of fine-grained sediments. The delta-front sediments accumulated over the Holocene and the previous glacial–interglacial period have thicknesses, respectively, larger than 50 and 100 m (Fig. 1b and c; Beckers, 2015; Beckers et al., 2016). At the northwestern end of the gulf lies the largest fan delta of the Mornos River that drains 913 km2 and is by far the largest watershed among the rivers flowing toward the westernmost Gulf of Corinth (Fig. 1a). The delta fronts are highly unstable (Ferentinos et al., 1988; Lykousis et al., 2009), which favors frequent submarine landsliding (Stefatos et al., 2006; Tinti et al., 2007; Fig. 1b). During the last centuries, submarine landslides have been triggered by earthquakes and by sediment overloading on steep slopes (Galanopoulos et al., 1964; Heezen et al., 1966). Numerous debris-flow deposits and MTDs have thus accumulated at the foot of the deltas (Ferentinos et al., 1988; Beckers et al., 2016; Fig. 1b). Alongside these gravity-driven sedimentary processes, contour-parallel bottom currents also influenced sediment transport in this area (Beckers et al., 2016).

The shallow sedimentary infill of the Gulf of Corinth consists of a distinct alternation between seismic–stratigraphic units with parallel, continuous high-amplitude reflections and units with parallel, continuous low-amplitude reflections to acoustically transparent seismic facies (e.g., Bell et al., 2008; Taylor et al., 2011). Generally, the semi-transparent units are thicker than the highly reflective units (e.g., Taylor et al., 2011). These alternating seismic–stratigraphic units have been observed throughout the Gulf of Corinth and have been interpreted as depositional sequences linked to glacio-eustatic cycles (Bell et al., 2008; Taylor et al., 2011). Because of the presence of the 62 m deep Rion sill at the entrance of the gulf, the Gulf of Corinth was disconnected from the World Ocean during Quaternary lowstands and was thus a non-marine sedimentary environment. The marine and non-marine environments are associated with different climatic regimes (e.g., Leeder et al., 1998). During glacial stages, the sparse vegetation cover was more favorable to erosion than during interglacials, so high quantities of sediments were routed towards the gulf (Collier et al., 2000). These lowstand deposits appear as thick, low-reflective units. The thin, high-reflective units are interpreted to represent the marine highstand deposits. The last lacustrine–marine transition has been sampled in different sedimentary cores (Collier et al., 2000; Moretti et al., 2004; Van Welden, 2007; Campos et al., 2013).

Two seismic reflection surveys were carried out in 2011 and 2014 with the aim of imaging the subsurface below the westernmost Gulf of Corinth floor. The data were acquired by the Renard Center of Marine Geology of the University of Ghent along a grid of 600 km high-resolution seismic profiles with a Centipede sparker seismic source combined with a single-channel high-resolution streamer as receiver (see details in Beckers et al., 2015; the seismic grid is shown in Fig. 2). The expected vertical resolution at depth is ∼ 1 m. In the deep basin (Mornos Canyon and Delphic Plateau areas, Fig. 1), the maximum penetration depth below the sea floor is about 360 ms two-way travel time to the east and about 100 ms two-way travel time to the west, i.e., 270–360 and 75–100 m, respectively.

The inferred stratigraphic framework (Beckers et al., 2015) permits us to identify two temporal horizons. Reflector 1 has been mapped in the whole study area except in a basin west of the Trizonia Island (Fig. 1). This reflector corresponds to the beginning of the last post-glacial transgression, at 10.5–12.5 ka (Cotterill, 2006; Beckers et al., 2016). The second temporal horizon, reflector 2, has been mapped in the Delphic Plateau area only. It corresponds to the marine isotopic stage 6 to 5 transgression, which occurred at ca. 130 ka.

Mass transport deposits have been identified on high-resolution seismic profiles based on their typical seismic facies made of discontinuous to chaotic reflections. The shape of each deposit in map view has been interpolated manually based on the seismic profiles that intersect the MTD. Thicknesses were derived using a seismic velocity of 1600 m s-1 (Bell et al., 2009). For the largest MTDs, an inverse distance weighted interpolation between thickness data points was used to derive isopach maps of the deposits and estimate their total volume. However, this interpolation method cannot be used for smaller MTDs crossed only by a few seismic lines. In this case, the volume was estimated by multiplying the MTD surface by an average thickness value. The derived volumes of small MTDs (surface area < ∼ 2 km2) are thus rough estimates, especially for MTDs crossed by only two or three seismic profiles. By contrast, volume estimates of large MTDs (surface area > ∼ 5 km2) are more accurate with volume uncertainties probably < 20 %.

Landslide head scarps have been mapped using three different data sources, namely (1) the grid of high-resolution seismic profiles acquired for this study, (2) an analysis of three submarine landslides in the study area by Tinti et al. (2007), and (3) a 3-D bathymetric view of the area between the Erineos and the Selinous fan deltas from Lykousis et al. (2009). In the absence of multi-beam bathymetry over the whole study area, the mapping of late Quaternary submarine landslides head scarps presented here is certainly not exhaustive. The location of potential head scarps associated with the largest MTDs mapped in the following are also discussed considering the location of the thickest deposits and the nearest upslope delta-front sediments.

Thirty-two MTDs have been imaged in the study area, from which 67 % are located in the large E–W-trending basin located below the flat deep basin (Mornos Canyon and Delphic Plateau, Fig. 2). Eight MTDs have been identified in the northern margin of the gulf, and two in the Nafpaktos Bay to the west of the Gulf of Corinth (Fig. 2). The age of 24 MTDs has been estimated based on the stratigraphic framework developed previously (Beckers et al., 2015): 19 of them occurred during the Holocene and 5 during the period between ∼ 130 and ∼ 11.5 ka. A finer stratigraphy could be established in the flat deep basin, thanks to the relative continuity of the reflectors over this 20 km wide area. Consequently, this work focuses on the 22 MTDs located in this area.

In the Delphic Plateau basin (eastern part of the deep flat basin), most MTDs are imaged as lenticular bodies of low-amplitude, incoherent reflections (Figs. 3 and 4). They generally have a flat upper surface and pinch out on their margins. Their thickness ranges between a few meters, which is the minimal thickness for a MTD to be imaged with the seismic system used, and 53 m. The geometry and seismic facies indicate subaquatic mass-flow deposits (e.g., Moernaut et al., 2011; Strasser et al., 2013). The seismic facies of many MTDs also suggests a fine-grained lithology. However, this statement must be viewed cautiously considering the uncertainties on the interpretation of seismic facies in terms of grain size, especially for reworked sediments. For instance, failure of coarse-grained deltaic deposits commonly results in their total disaggregation and transformation into grain flows and turbidity currents, whereas finer-grained deposits evolve to landslides and cohesive debris flows (Tripsanas et al., 2008).

In the Mornos Canyon basin (western part of the deep flat basin), the MTDs present the same general characteristics but the reflector pattern is more variable (Fig. 5). Some high-amplitude reflections and coherent layering are observed in some MTDs, suggesting coarser-grained sediments and locally preserved stratigraphy.

Finally, some of the 22 MTDs show sediment and/or fluid escape features at their top (Figs. 3 and 5). Such features might have been produced by the combination of under-compaction (excess porewater pressure) and shaking, thus possibly pointing to paleoearthquakes (e.g., Moernaut et al., 2007, 2009). The volume of sediments in individual MTDs ranges from 7.7 × 105 to 8.6 × 108 m3 (Fig. 6).

Landslide head scarps have been identified in different parts of the study area (Fig. 2a). They are particularly numerous on the slopes of the large Gilbert fan deltas of the Erineos, Meganitis and Selinous at the southeast and Mornos at the northwest. In the latter area, one up to 50 m high head scarp is imaged in the seismic data. The absence of undisturbed sediments on the erosional slope, downslope of the head scarp, suggests a recent age. In the Erineos, Meganitis and Selinous fan delta slopes, head scarps have been identified in the seismic data and on the 3-D view from Lykousis et al. (2009). Most of these head scarps are relatively small, lunate-shaped features linked to gullies (see also the bathymetric map in Fig. 1). Two large head scarps are localized on the northern slope as well (Fig. 2a). Linking a head scarp to a particular MTD is often delicate for two reasons. First, the age of the head scarps is difficult to estimate because these erosional forms often affect steep slopes in coarse-grained deposits, making it impossible to define a seismic stratigraphy in such areas. Second, at the foot of these erosional slopes, a high number of MTDs are stacked (e.g., Fig. 3). Exceptions, detailed hereafter, concern three recent submarine landslides and the largest observed MTD (MTD 19 in sliding event F).

The stratigraphic position of MTDs in Mornos Canyon and in the Delphic Plateau basins is not random. Most of them are clustered and are defining multi-MTDs temporal “events”, based on common un-deformed underlying or overlying reflections that can be followed across the basin. Such correlations suggest that six events of large clustered submarine mass wasting occurred over the last 130 ka. Two sliding events (SE) are represented by clustered MTDs located between reflectors 2 and 1 (SE E and F). The four others occurred during the Holocene: SE D comprises MTDs deposited just on top of the reflector 1, SE C is located in the middle of the Holocene sequence, SE B somewhat higher, and finally SE A includes MTDs at or near the sea floor responsible for its present-day hummocky topography. The spatial distribution and the total volume of the MTDs associated with each of these events are represented in Fig. 2.

In some zones (Fig. 2), the existence or the geometry of MTDs is difficult to evaluate because of seismic blanking and strong chaotic reflections affecting some stratigraphic intervals. Above reflector 1, the stratigraphy is clear except regarding the southern extension of MTD 14 in SE D. The low-amplitude, almost transparent reflections characterizing the MTD deposit extends to a more chaotic and thicker deposit associated with surface mounds (Fig. 4). We could not decipher whether the chaotic reflections that disturb the seismic stratigraphy were associated with MTD 14 in SE D or in relation with sediment remobilization from the underlying SE F (Figs. 4 and 5). So the mapped extension of MTD 14 in Fig. 2e is conservative and considered as a minimum. Below reflector 1, the amplitude of the reflectivity sharply decreases, which is a characteristic of lowstand deposits in the gulf (Bell et al., 2008), and blanking occurs in two areas. In the Mornos Canyon area, a wide blanking area exists at a depth of about 50 to 70 m below the sea floor, a few meters below reflector 1, in direct continuity with the delta of the Mornos River. Blanking is thus a low-stand-related feature and might correspond to coarse-grained, organic-rich sediments of the Mornos River. Consequently, the stratigraphy of MTDs between reflectors 2 and 1 is well established only below the Delphic Plateau. The other area associated with blanking and strongly disturbed sediments forming mounds occurs at the junction between the Mornos Canyon and the Delphic plateau at the foot of the Erineos foreset beds, at a depth similar to SE F. Its origin is unknown, but it might be related to an MTD deposit in relation with MTD 19.

The definition of sliding events reflects a clustering of submarine landslides in a relatively short period of time. It does not necessarily imply a synchronous occurrence of all submarine landslides included in one event. Indeed, the accuracy of the correlation between separated MTDs that are interpreted to belong to the same sliding event is in the order of one or two reflections in the seismic data. Deciphering the exact MTD chronology within a sliding event was not possible because of the discontinuous character of many reflections and the relatively large distance that separates some MTDs (up to 8.5 km). This “stratigraphical” uncertainty corresponds to ∼ 1–2 m of sediment or, based on sedimentation rate estimates, sliding events represent a set of MTDs that occurs over a period of 300 to 1000 years (Lykousis et al., 2007).

Individual sliding events are characterized as follows (Fig. 2b to g):

Sliding event A. Eight MTDs at or near the sea floor have been identified. Their spatial distribution indicates that three of them result from slope failures in the Mornos delta and five from failures at different locations along the southern margin (Fig. 2). The volumes of these MTDs range between ∼ 4.7 × 106 and ∼ 6.2 × 107 m3, and the total volume of the eight MTDs is about ∼ 2.2 × 108 m3.

Some of these MTDs correspond to submarine landslides described in the literature (Galanopoulos, 1964; Papatheodorou and Ferentinos, 1997; Tinti et al., 2007). The MTD located northeast of the Erineos delta results from a coastal landslide on this fan delta in 1963, which triggered a large tsunami on both sides of the gulf (Galanopoulos et al., 1964; Stefatos et al., 2006). The MTD located at the foot of the Meganitis fan delta likely corresponds to a coastal landslide triggered by the 1995 Aigion earthquake on this delta (Papatheodorou and Ferentinos, 1997; Tinti et al., 2007). The volumes of sediments involved in these two landslides have been estimated at ∼ 4.6 × 107 m3 from the data presented by Stefatos et al. (2006) and about ∼ 2.8 × 107 m3 by Tinti et al. (2007), respectively. The corresponding volumes estimated from the present study are ∼ 6.1 × 107 and ∼ 2.2 × 107 m3, which are on the same order of magnitude. Another well-preserved but undated landslide head scarp has been identified by Tinti et al. (2007) on the eastern side of the Mornos fan delta (Fig. 2). These authors estimated the volume of the sliding mass at ∼ 9 × 106 m3. Our data show a MTD located about 1 km downslope of the scarp, with an estimated volume of ∼ 9.9 × 106 m3 that fits remarkably well with the volume derived from the geometry of the scarp.

Sliding event B. The SE B comprises three MTDs located at the western tip of the canyon. They are located between 12 and 16 m below the sea floor and are relatively thin (∼ 2 to 5 m thick) (Fig. 5). Location and geometry of the MTDs indicate that they result from slope failures in the Mornos fan delta and in the Psathopyrgos scarp. The total volume of these MTDs is about ∼ 1.7 × 107 m3.

Sliding event C. The SE C includes one large MTD extending over a wide area below the Mornos Canyon and a part of the Delphic Plateau (MTD 10), as well as smaller deposits located at the foot of the southern slopes, in the Psathopyrgos scarp and Erineos fan delta areas. The thickness of MTD 10 is shown in Fig. 7. Five local maxima are connected by a 2–5 m thick sheet of low-amplitude incoherent reflections. The thickest sediment accumulation (30 m) is located at the foot of the Erineos fan delta. The other maxima are 5 to 10 m thick. Two are located at the western tip of the MTD and suggest sediment inputs from the Mornos fan delta area and from the Psathopyrgos scarp (Fig. 5). The last two maxima are located in the southeastern part of the deposit, with a possible source in the Erineos fan delta (Fig. 7). The total volume that failed during SE C is about ∼ 2.0 × 108 m3, including ∼ 1.4 × 108 m3 for MTD 10.

The geometry of MTD 10 suggests that slope failures occurred simultaneously in different parts of the westernmost gulf during SE C. The main source of sediment was the Erineos fan delta, as attested by the location of the thickest sediment accumulation in the MTD 10 and by the presence of other MTDs at the same stratigraphic level between MTD 10 and the Erineos fan delta (Fig. 2d).

Sliding event D. Two MTDs are located just on top of reflector 1 and define the SE D. Both are between ∼ 2 and 10 m thick and spread over several square kilometers in front of the Erineos and Meganitis fan deltas. The southern limit of the deposits is unclear, because the stratigraphy in the area between the two MTDs and the Erineos pro-delta is poorly constrained (hatching on Fig. 2e and question marks in Fig. 4). In this area, it is not clear whether the incoherent reflections located south of the SE D MTD at a similar depth represent the same MTD or the underlying, older (SE F) MTD or escape features from the latter, as suggested by the escape features observed at the sea floor (Fig. 4).

The isopach map of the largest deposit (MTD 14) is shown in Fig. 7 and suggests that it was fed by slope failure(s) mostly south of the Delphic Plateau probably from the Erineos Delta Fan. The volume of MTD 14 is estimated at ∼ 8.7 × 107 m3, and the total volume of SE D MTDs is about ∼ 1.0 × 108 m3. Considering uncertainties on the geometry of these MTDs' southern edges, these values are minimum estimates.

Sliding event E. Two MTDs define this sliding event. The largest one is located in the Delphic Plateau basin, just south of the Trizonia Island, and has a volume of ∼ 6.6 × 106 m3. The second is much smaller (∼ 1.3 × 106 m3) and is located in the canyon basin. Stratigraphically, both are located a few meters below reflector 1. However, they are horizontally 8.5 km apart, making the correlation uncertain. The total volume of the two MTDs in SE E is ∼ 7.9 × 106 m3.

Sliding event F. The SE F is defined by one single large complex MTD (MTD 19) (Fig. 2). This deposit is located in the Delphic Plateau basin. Stratigraphically, it belongs to the upper part of the unit between reflectors 2 and 1, suggesting that this event occurred during the last glacial period. With a volume of ∼ 8.6 × 108 m3, this deposit is the largest MTD of the present inventory. It covers an area of 41 km2, i.e., almost the whole Delphic Plateau. The isopach map reveals a main up to 50 m thick sediment accumulation in the southwestern part of the deposit (Fig. 4) and another ∼ 30 m thick depocenter in the northeastern part (Fig. 7). The MTD is imaged as low-amplitude, almost transparent chaotic reflections except in the thickest part, where high-amplitude reflections could indicate coarser-grained sediments and locally preserved layering (Fig. 4). No sedimentological structure has been observed in the seismic profiles between the two maxima in thickness.

The geometry of the deposit and the absence of clear structure between the two depocenters support the idea of at least two simultaneous slope failures having generated this large MTD. The largest failure occurred south of the MTD, on the Meganitis or the Erineos fan delta slopes. Considering the large volume of sediments in the southwestern part of the MTD, we expected a major scar across the southern slopes, which we could not retrieve from either the seismic data or published bathymetries (Lykousis et al., 2009; Nomikou et al., 2011; see our Fig. 1). Indeed, dozens of small head scarps and gullies dissect the slopes of the offshore Erineos and Meganitis deltas, making the identification of large features difficult. The second depocenter occurs near the northeastern edge of the Delphic Plateau basin, and upslope two submarine landslide head scarps located 2 km from each other were evidenced in seismic profiles (bold lines in Fig. 7). Cut through stratified hemipelagites, they are 11 and 15 m high and are located at 300 and 195 m below the sea level, respectively (Fig. 8). Although it is not possible to reconstruct the 3-D geometry of a single large head scarp from the seismic data, this would be a good candidate source of the thick sediment accumulation in the northeastern part of MTD 19.

We documented the existence of large mass wasting events during the Holocene and the Late Pleistocene in the westernmost Gulf of Corinth. Mass wasting events consist in submarine or coastal landslides that occurred during short periods of time. Six large mass wasting events are listed, their associated deposits locally representing 30 % of the sedimentation since 130 ka in the Delphic Plateau basin. In the case of large MTDs (up to almost 1 km3 for the largest), a simultaneous triggering of separate slope failures is proposed, suggesting a seismic origin. However, it is suggested that the temporal distribution of sliding events is primarily controlled by the evolution of pre-conditioning factors. Two main pre-conditioning factors are identified, namely (1) the time needed to slope reloading after an event, which varied in relation with temporally varying sedimentation rates, and (2) dramatic changes in water depth and water circulation that occurred 10–12 ka ago during the last post-glacial transgression. Finally, it is likely that these sliding events have triggered large tsunami waves in the whole Gulf of Corinth, in some cases (much?) larger than those reported in historical sources.