Mass failures are common occurrences along slopes of various continental margins. The occurrences are generally associated with sediment mobility (Jenkins and Keene, 1992; Sultan et al., 2004, 2007; von Huene et al., 2004; Jackson et al., 2004; Vanneste et al., 2006; Locat et al., 2009; Loncke et al., 2009a, b). Through the inducement of gravity, movements of the sediment generally lead to failures of slopes (Mulder and Cochonat, 1996; Masson et al., 2002; Silva et al., 2004; Seeber et al., 2007; Levchenko et al., 2008; Locat et al., 2009; Bellec et al., 2010). The failure events may involve a variety of sediment mobilities ranging from sediment creep to submarine landslide (Mulder and Cochonat, 1996; Moscardelli et al., 2006; Bull et al., 2009; Sun et al., 2018).

Increasing economic activities along coastal areas have increased the need for better understanding submarine slope failures events that are a potential significant cause of natural hazards to the coastlines (Terry et al., 2017; Sun and Leslie, 2020; Pan et al., 2022). Unfortunately, direct observation and measurement of the processes are generally impossible due to the irregular nature of the failure events. However, morphometric analysis makes indirect observation of submarine slope failures possible. The analysis involves characterization of near-surface seafloor sediment movements which can be mostly distinguished by a high resolution bathymetric chart (Hampton et al., 1996; Mulder and Cochonat, 1996; Brune et al., 2010a, b, c) and related to the corresponding subbottom profiler records (Damuth and Hayes, 1977; Damuth, 1975, 1978, 1980; Pratson and Laine, 1989; Chough et al., 1997; Reddy and Rao, 1997; Lee and Baraza, 1999; Lee et al., 2002; Mosher et al., 2008).

On the outer shelf of the southwestern of Mergui Ridge in the Andaman Sea within the Thai exclusive economic zone (EEZ), generally no hydroacoustic data have been available for geotectonic study before the 2004 tsunamis. Recently, an area of about 3 000 km² covering the upper slope of the outer shelf was investigated using subsurface hydroacoustic surveys under the framework of the project “Morphodynamics and Slope Stability of the Andaman Sea Shelf Break” (MASS). In this paper, we determine the morphology of the western slope of Mergui Ridge and identify scarps, headwalls, escarpments, debris toes and other manifestations of slope failures and downslope mass transport, and current state of the slope area derived from high resolution bathymetry and present the recent submarine slope failure features and morphodynamic submarine landslides. These prerequisites are needed for a better understanding of the current status of the slope and supporting better information for tsunami hazard assessments in the Andaman Sea.

An area on the seafloor of the East Andaman Sea is characterized as an active backarc basin between the Andaman-Nicobar-Islands, Sumatera and the west coast of the Peninsular Malaysia (Curray, 2005). From a tectonic point of view, the east basin could be related to several stages of reconstruction and formation faults. Various strike-slip faults generally lying on a N–S direction are currently considered as the active faults zone, however, their positions are current poorly known (Polachan and Racey, 1993; Curray, 2005; Tingay et al., 2010). In addition, a pull-apart extension of the fault in the central basin could affect the east basin. The fault is generally oriented on NE–SW direction and probably started approximately 4 Ma ago with initial rate of about 1.6 cm/a and about 3.8 cm/a at present (Raju et al., 2004; Khan and Chakraborty, 2005). Moving of the extensional fault could make the east basin compress and also could possibly generate pathways for fluid escape from shallow sub-seafloor, especially the Mergui Ridge.

The surface of the Mergui Terrace is covered largely by muddy sand and small shell-fragments. These compositions render the sediment poorly sorted. The Irrawaddy Rivers discharge could be a source of sediments supply to the terrace (Rodolfo, 1969) at a rate of over 360 × 10⁶ t (Rao et al., 2005). Approximately 70% of the sediment settles at the rivers delta, while the rest could be delivered through Mataban Canyon to the deeper part and accumulated on the seafloor far from the source (Ramaswamy et al., 2004). The sediment transportation through the deeper areas might be accomplished by some types of gravity-controlled sediment phenomena, and forms the basin (Rodolfo, 1969; Rao et al., 2005). The graded beds are up to few meters thick and comprise surface-layer sediment and associated structures of the shelf. It is suggested that the sediment deposition was primarily by turbidity currents with a sedimentation rate below 10 mm/1 000 a (Rodolfo, 1969). A pelagic component of interbedded sediment such as foraminiferal ooze suggests that these sediments accumulated continuously and slowly (Saidova, 2008).

On Mergui Basin, sediments have been studied by three wells drilling. The upper thin layer of sediment on the seafloor of about 180–240 m thickness comprises silty shales and fine-grained sandstones, whereas the lower layer of sediment of about 700–1 000 m thickness comprises coral/algal limestones. The sediment in between both layers reveals a thin layer of sediment turbidites (Polachan and Racey, 1993). In addition, the sediment core of the Sumatera shelf basin reveals a very thin volcanic clayey sand layer below the core top (Rodolfo, 1969). This interpretation may draw sediment processes on the area which could be the turbidity current deposit. The interpretation also suggests another source of terrigenous sediments which could supply the basin from the Sumatera−Malacca stream southward.

As the Andaman Sea locates under the effects of the Southeast Asia monsoon systems, generally, winds of the SW monsoon (summer winds) blow steadily at a rate more than twice of the NE monsoon (winter winds). Arrival of strong summer winds dramatically drive the westerly current towards the prevailing south-easterly current, resulting in the transportation of riverine sediment moving eastward along the inner shelf (Rodolfo, 1969; Ramaswamy et al., 2004; Rao et al., 2005). In contrast, the winter winds cause south-easterly currents to flow towards westerly currents. The north-westward current in the Malacca strait is generally restricted to the northern part of Sumatera. The sediments are forced and transported by Sumatera streams from the strait into the south Andaman Sea throughout the year as the density flows (Rodolfo, 1969).

Little is known about the seafloor and the tectonic structure of the Thai EEZ of the Andaman Sea. According to global bathymetric data GEBCO 1-min (IOC et al., 2003), the western rim of the Mergui Ridge forms an escarpment-like structure with steep slopes up to a maximum value of about 4.5° may originate submarine landslides on the surface slope (Hampton et al., 1996) and their significant potential tsunamis. Major causes of the landslides are the characters of the sediment and its properties. Especially, fine-grained sediment deposits with high water content can be extremely under consolidated and lead to slope failure events. Another cause is slope steepness. Increasing of slope angle might lead to over steepening and eventually slope failures (Mulder and Cochonat, 1996; Hampton et al., 1996; Leynaud et al., 2009; Brune et al., 2010c; Sun et al., 2018). Nevertheless, the resolution of these datasets is very coarse and only based on satellite information or very sparse single beam soundings. Therefore, locally there well may exist much steeper slopes. To date no high-resolution bathymetric data by a high-resolution multibeam bathymetric survey from the Mergui Ridge is available for research and scientific communities and is critical in need to image the morphology of Mergui Ridge and slope.

The surveys on the upper slope of the Andaman Sea shelf break within Thai EEZ (Fig. 1) were conducted on board of Thai R/V Chakratong Tongyai of the Department of Marine and Coastal Resources, Ministry of Natural Resources and Environment, Thailand during two cruises in November−December 2006 and October−November 2007. The survey cruises covered the southwestern corner of the Thai EEZ, which forms the southern end of the Mergui Ridge by a topographic rise that stretches through the central Andaman Sea in north−south direction. Water depths in the survey area varied from about 500 m on the top of the Mergui Ridge to more than 1 400 m at the southwestern edge of the area. The bathymetric data was collected using the Seabeam 1050 Multibeam Echosounder and the parametric data were collected using the INNOMAR SES 2000 medium Subbottom Profiler. During the surveys, some transducer arrays were damaged resulting in some continuous bathymetric data being unavailable near the southern part of the investigated area. In addition, the vibration of the pole limited the range of the bathymetric data to generally less than 1 500 m water depth. The penetration of the parametric signals into the sub-seafloor was generally less than 10 m with maximum values of 15 m. The bathymetric grid was processed by using the MB-System (Caress and Chayes, 2004) and displayed by using the Generic Mapping Tool-GMT (Wessel and Smith, 1998). The subbottom profiler records were processed using the traditional SES software and subsequently imported into the KINGDOM suite in order to analyze echo-characters. The multibeam echosounding data have been processed by various visualization techniques to image for the morphology of the area in great detail to determine possible submarine landslide features. The theoretical tsunami wave length and maximum amplitude directly over the slope failure generated by disintegrative submarine landslides can be calculated by the semi empirical equations proposed by Watts et al. (2003) and McAdoo and Watts (2004).

When using the depth accuracy tests, it revealed that 95% of the processed grids obtained values equal or better than 1% of water depth. This is acceptable for the generation of a final grid (Beyer et al., 2003, 2005; de Alteriis et al., 2003) when grid resolution has been set to 50 m (Fig. 2). The depth accuracy of the instrument can be linked to the standard deviation, thus the values should vary between 5 m and 16 m height. This test also reveals that 95% of the grid cells obtained standard deviations better than 16 m height. The new detailed bathymetric data and subbottom profiler record covering the upper slope of the Andaman Sea shelf break within Thai EEZ of approximately 3 000 km² allows the characterization of general morphology of the slope and eventually reveals previously unknown anomalous features on the seafloor (Figs 2 and 3). The area on top of the slope is generally rather smooth and slightly inclined southward, while the area on the outer shelf break steepens westward with gradients varying from 1° to 4.8°, and is cut by gullies. Gradient on the N–S profile, including Profiles NS1-1 and NS1-2 at a water depth between 500 m and 600 m, shows an inclination angle of less than 0.08°. Several profiles between NS2-1 and NS2-4 reveal ruggedness and unevenness of slope morphology on the slope below the smooth area. Interestingly, Profiles NS3-1, NS3-2, and NS3-3 show that most of the areas on down slope at water depth below 1 200 m are strongly dissected by gullies (Figs 3a, b). In contrast, the different inclination angles on the E–W profiles allow dividing of the area on the slope into two subtle zones including the upper slope and the middle slope. The upper slope covers the area on the northern part of the shelf, where gradients range from 1.5° to 2.2° at water depth 600–900 m, and the southern end of the investigated area, where the gradient is approximately 1° at 700–1 200 m water depth. The middle slope occupies the area where gradients vary from 3° along the southern part at 900–1 300 m water depth to 4.8° further north (Figs 3a, c). In addition, there are few small indications of possible submarine landslides on the northern (Fig. 4a) and the middle part (Fig. 4b) of the slope. Plateaus or possible low oceanic guyots (Figs 5a, b), and several sizes of gullies (Fig. 6a) are inferred from the bathymetry.

Morphometric analysis of the sounding data acquired from the Andaman outer shelf and upper slope within the Thai EEZ reveals some indications of recent mass wasting that led to the possible slope failure events in a relatively steep slope in different domains. The presence of many anomalous features on the seafloor are prerequisites for the generation of large submarine landslides (Hampton et al., 1996; Mulder and Cochonat, 1996). In addition, the sounding allows the characterization of a few previously unknown submarine landslides on the slope, which confirm the occurrence of slope failures. These features include large sediment transports from the Irrawaddy and the Salween rivers to the deep basin and eventually rapid deposition, and the occurrence of large earthquakes at close distance. It seems that events leading to slope failures have occurred in the different geological times.

Close analysis of a new detailed bathymetry and subbottom profiler record presented in this paper, shows two distinct submarine landslides. The first landslide is located at 8°29′N and 95°53.65′E in the northernmost part of the studied area (Fig. 4a). Another small landslide is situated in the middle part of the slope at position 8°12.85′N and 95°48.75′E (Fig. 4b). On the basis of the morphology of the failed mass, the difference between its depth (H) and the length (L) are likely key characteristic of the slides, and this suggests the need for classification of the submarine landslides using the Skempton ratio (Mulder and Cochonat, 1996). The ratio for first slide (the depth H = 77 m, and the length L = 1 600 m), which is estimated from bathymetric data, is H/L = 0.05. For the other slide, the ratio (the depth H = 85 m, and the length L = 2 238 m) estimated from subbottom profiler records is H/L = 0.04. These valuable estimations suggest that both slides correspond to translational submarine landslides (Mulder and Cochonat, 1996).

The theoretical tsunami wave length and maximum amplitude directly over the slope failure generated by disintegrative submarine landslides can be calculated by the semi empirical equations proposed by Watts et al. (2003) and McAdoo and Watts (2004). Following the equations, the calculations show that the northernmost landslide would have generated a tsunami wave of about 3 877 m wave length and with a maximum wave height of about 0.19 m, while the middle slide would have generated of about 4 454 m wave length and maximum approximately 0.14 m wave height. These initial wave characteristics could be input criteria for tsunami propagation models (Synolakis et al., 2002; Brune et al., 2010a, b, c) in order to calculate wave propagation and potential tsunami run-up onshore. However, the small initial wave height indicates that the observed submarine landslides are not the potential for generating a devastating coastal tsunami. It is consequently unlikely that submarine sliding has contributed to tsunami hazards along the Andaman Sea coast in the recent geological past. Only small landslides offshore of the Indonesian Sunda Arc have been found by Brune et al. (2010b). The reason for the absence of large tsunami-generating landslides might be found in the nature of the sediments at the Andaman Sea outer shelf and upper slope. Both subbottom profiler records (Fig. 4) and published data on sediment textures and sedimentation rate from Rodolfo (1969), Polachan and Racey (1993), and Saidova (2008), suggest the presence of sandy deposits that would be well drained, precluding the build-up of overpressure.

The surveyed area on southwestern part of slope of the Mergui Ridge is generally rather smooth, while the west rim of the middle and the north part is steepens with gradients up to 4.8° to the north and is cut by gullies. Regarding sediment types and sedimentation rates, sediments mainly consist of foraminiferal oozes on top of the ridge, while silty clays prevail in the adjacent area. Rodolfo (1969) described relict foraminifera and relict corals in modern sediments of the Mergui area, implying that sedimentation rates on the Mergui Ridge are generally low and/or mainly related to biogenic input. This sequence described to exhibit mass flow deposits might be particularly effective potential tsunami generations without large displacements. However, more information from seismic and bathymetric data for calculation the volume of mass-wasting events is an important factor for their tsunami potential to quantify individual mass-wasting events and tsunamis wave propagation model in the near future.

Slope instabilities are revealed along the study area. It is surprising that this slope is rather poorly sediment accumulated. However, this confirms the importance of structural control in the shaping and evolution of submarine domains. The various segments of the slope exhibit contracting slope instabilities.

The areas on the northern and the middle parts of the slope are strongly affected by mass wasting processes such as erosion, debris flow and slumping. These events seem recurrent and define blocks. The driving mechanism seems to be the tilting of the slope and consequent gravity instabilities. The tilting allows the sediment on the upper part to slide downslope. Various pieces of evidence and parallel lineaments of slope failures suggest that the sliding occurred at difference times, but is recently occurring. On the southern part of the study area, eleven slum locations are studied based on subbottom profiler records.

Two locations indicated possible submarine landslides are studied on the slope, maximum approximated volumes of both displaced masses are 4.8 × 10⁷ m³ and 2.2 × 10⁷ m³.

There is no evidence that the sliding has generated tsunamis, but slides exhibit occurring slope failures and mass movements, leading to slope instability assessment.

We acknowledge the Leibniz Institute of Marine Science (IFM-GEOMAR) and Southeast Asia START Regional Center for facilitating cooperative research between Thai and German scientists. We acknowledge the help of: Wilhelm Weinrebe, Ingo Klaucke, Sebastian Krastel, Ernst Flueh, Warner Brueckmann, Christian Hensen, and Suratta Bunsomboonsakul. We are grateful to Captain. Chonwat Singnu and his crew who made the cruises successful. The authors would like to especially thank Katja Lindhorst who gave a nice introduction to software KINGDOM Suite, Colin Tosh for English proofreading and Chantima Piyapong for editing the revised manuscript.