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Indonesian Throughflow as a preconditioning mechanism for submarine landslides in the Makassar Strait

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Abstract

The Makassar Strait is an important oceanic gateway, through which the main branch of the Indonesian Throughflow (ITF) transports water from the Pacific to Indian Ocean. This study identifies a number of moderate (>10km3) to giant (up to 650km3) mass transport deposits within the Makassar North Basin Pleistocene-Recent section. The majority of submarine landslides that formed these deposits originated from the Mahakam pro-delta, with the largest skewed to the south. We see clear evidence for ocean current erosion, lateral transport and contourite deposition across the upper slope. This suggests that the ITF is acting as an along-slope conveyor belt, transporting sediment to the south of the delta, where rapid sedimentation rates and slope over-steepening results in recurring submarine landslides. A frequency for the >100 km3 failures is tentatively proposed at 0.5 Ma, with smaller events occurring at least every 160 ka. This area is therefore potentially prone to tsunamis generated from these submarine landslides. We identify a disparity between historic fault rupture triggered tsunamis (located along the Palu-Koro faultzone) and the distribution of mass transport deposits in the subsurface. If these newly-identified mass failures are tsunamigenic, they may represent a previously overlooked hazard in the region.
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Accepted Manuscript
Geological Society, London, Special Publications
Indonesian Throughflow as a preconditioning mechanism for
submarine landslides in the Makassar Strait
R. Brackenridge, U. Nicholson, B. Sapiie, D. Stow & D. R. Tappin
DOI: https://doi.org/10.1144/SP500-2019-171
Received 30 September 2019
Revised 20 November 2019
Accepted 21 November 2019
© 2020 The Author(s). This is an Open Access article distributed under the terms of the Creative
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Indonesian Throughflow as a preconditioning mechanism for submarine landslides in the
Makassar Strait
R. Brackenridge1, U. Nicholson1*, B. Sapiie2, D. Stow1, D. R. Tappin3, 4
1 School of Energy, Geoscience, Infrastructure and Society, Heriot-Watt University, Edinburgh, EH14 4AS,
UK
2 Faculty of Earth Sciences and Technology, Institut Teknologi Bandung, Indonesia
3 British Geological Survey, Keyworth, NG12 5GG, Nottingham, UK
4 Department of Earth Sciences, University College London (UCL), London, UK
*Corresponding author (e-mail: U.Nicholson@hw.ac.uk)
Abstract: The Makassar Strait is an important oceanic gateway, through which the main branch of
the Indonesian Throughflow (ITF) transports water from the Pacific to Indian Ocean. This study
identifies a number of moderate (>10km3) to giant (up to 650km3) mass transport deposits within
the Makassar North Basin Pleistocene-Recent section. The majority of submarine landslides that
formed these deposits originated from the Mahakam pro-delta, with the largest skewed to the
south. We see clear evidence for ocean current erosion, lateral transport and contourite deposition
across the upper slope. This suggests that the ITF is acting as an along-slope conveyor belt,
transporting sediment to the south of the delta, where rapid sedimentation rates and slope over-
steepening results in recurring submarine landslides. A frequency for the >100 km3 failures is
tentatively proposed at 0.5 Ma, with smaller events occurring at least every 160 ka. This area is
therefore potentially prone to tsunamis generated from these submarine landslides. We identify a
disparity between historic fault rupture-triggered tsunamis (located along the Palu-Koro faultzone)
and the distribution of mass transport deposits in the subsurface. If these newly-identified mass
failures are tsunamigenic, they may represent a previously overlooked hazard in the region.
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The Indonesian Archipelago is seismically active due to its location at the intersection of four major
tectonic plates. Earthquakes, volcanic eruptions and tsunamis represent significant geological
hazards across the island nation. Of these natural hazards, tsunamis pose a specific risk to the
sustainability and resilience of coastal communities. There have been a number of devastating
Indonesian tsunamis over the last 15 years, triggered by various mechanisms. The 2004 megathrust
earthquake and tsunami offshore of Sumatra resulted in over 220,000 fatalities across the Indian
Ocean region, 165,000 of these on Sumatra, making it one the worst natural disasters of the last 100
years (National Geophysical Data Center / World Data Service, NGDC/WDS). The mechanism of the
Palu tsunami (Sept 2018) is still uncertain, but is proposed as a result of combination earthquake-
related seafloor rupture and subaerial/submarine landslides. Two surges, with maximum wave
heights of over 10 m (NGDC/WDS) were recorded. Over 4,000 fatalities occurred as a result of
tsunami surges and widespread onshore liquefaction due to seismic shaking (Sangadji 2019). The
Palu event was closely followed in December 2018, by the Anak Krakatau tsunami, where flank
collapse probably triggered by the erupting volcano, resulted in tsunami run-up of 30 m and over
400 fatalities on the local coasts of Java and Sumatra (NGDC/WDS; Grilli et al. 2019). The high
number of fatalities of these two events has been attributed to the limited understanding of tsunami
generation from mechanisms other that fault rupture, and the lack of an effective tsunami warning
systems for non-seismic events (Williams et al. 2019, Yalciner et al. 2018).
The Makassar Strait has the highest frequency of tsunamis in Indonesia (Prasetya et al. 2001).
Historical records show that most are caused by earthquake-generated fault rupture of the seafloor,
with the exception of the September 2018 Palu event, which probably had a landslide component
(Jamelot et al. 2019). However, there are numerous other factors in the strait that could make it
susceptible to submarine landslide-triggered tsunamis, including oversteepening of the continental
slope due to carbonate growth and faulting, or sediment influx from the Mahakam Delta. The Strait
is also the main channel for the Indonesia Throughflow, a strong current that transports 10 - 15
million Sverdrups (where 1 Sv = 1x 106 m3 per second) of water from the Pacific into the Indian
Ocean (Kuhnt et al. 2004). Elsewhere, it has been shown that ocean currents and their associated
deposits can precondition the slope for failure through rapid sediment deposition and erosion
(Laberg and Camerlenghi 2008; Nicholson et al. under revision).
This study evaluates slope stability in the Makassar Strait, specifically the role of the Indonesian
Throughflow in preconditioning the slope for failure. Understanding the distribution of mass
transport deposits (MTDs) that result from submarine landslides will allow us to identify specific
regions of hazard and risk should these landslides be tsunamigenic. This has important implications
for hazard mitigation and early warning systems across coastal regions of the Makassar Strait.
Geographic and Oceanographic Setting
Regional Tectonic Setting
The Makassar Strait separates the Indonesian Islands of Kalimantan (Borneo) and Sulawesi in
Southeast Asia (Fig. 1). It sits within a highly complex and dynamic plate tectonic setting (Daly et al.
1991), located at the intersection between four major lithospheric plates (Fig. 1 A). The Indo-
Australian Plate, located to the south of the Makassar Strait, is colliding with the Eurasian Plate in a
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northwards direction and is subducting below Sumatra and Java at a rate of ~7 cm yr-1 (Bergman et
al. 1996; Koop et al. 2006). To the east, the large Pacific Plate interacts with the NW moving (and
clockwise rotating) Philippine Sea Plate (Hall et al. 1995). As a result of plate interactions, a complex
system of subduction, back arc thrusting, extension, and major transform zones has developed (Hall
1997; Prasetya et al. 2001) (Fig. 1 A). The region is extremely susceptible to seismic activity and
volcanic arcs line the compressional plate boundaries including the Sunda and Banda Arcs to the
south, and the Sangihe Arc to the northeast (Katili 1975).
Due to its active plate tectonic setting, earthquakes are common across the Indonesian archipelago,
with some associated with tsunamis. Along the Sunda Arc, historic tsunamis are generally generated
by megathrust earthquakes. The mechanisms for these events are well understood, and have been
studied extensively following the devastating 2004 Indian Ocean tsunami (e.g. McCloskey et al. 2008;
Okal and Synolakis 2008). Prasetya et al. (2001) show that the high frequency of historic tsunamis
within the Makassar Strait, which is located away from any major subduction zone, is the result of
shallow-depth earthquakes along the Palu-Koro transform fault zone. (Fig. 1 B).
Geological Framework
The Makassar Strait forms a deep seaway that separates Kalimantan from Sulawesi. It varies in width
from 100 to 200 km and is approximately 600 km in length. It formed in the middle Eocene, when
rifting and sea floor spreading was initiated (Hall et al. 2009). The onset of compression in the Late
Miocene to Pliocene resulted in fold-and-thrust belt development along the margins of the Makassar
Strait Basins (Bergman et al. 1996). At present, far-field stresses and resulting faulting generate
topographic features on the seabed, particularly in the east where Miocene-Pliocene compression
has generated the West Sulawesi Fold Belt (Puspita et al. 2005). The Palu-Koro fault zone is a NNW-
SSE trending strike-slip fault, and connects with the North Sulawesi Trench in the Celebes Sea. It
crosses the Makassar Strait at its narrow northernmost opening, separating it from the Celebes Sea
(Prasetya et al. 2001). The NW-SE trending Eocene Paternoster fault zone acts as a topographic
barrier separating the North and South Makassar Basins (Fig. 1 B).
The North Makassar Basin is north-south trending, 340 km long and 100 km wide, with water depths
of 200 to 2000 m (Fig. 1 B). The South Makassar Basin shows a similar range of water depths, is 300
km long, 100 km wide, and NE-SW trending. The margins of the Makassar Strait show contrasting
characteristics. To the west, the Paternoster Platform forms a wide shelf less than 200 m deep. This
shelf has been periodically exposed during glacial conditions to form part of the Sundaland
landmass, connecting Borneo (Kalimantan), Java and Sumatra with the Asian continent (Bird et al.
2005; Hall 2009).
In the east of the Makassar Strait two fold belts are expressed as topographic features on the seabed
(Fig. 1 B). Puspita et al. (2005) define a Northern (NSP) and Southern (SSP) fold belt separated by a
Central Structural Province (CSP). They note the different characteristics of the two fold belts, with
the northern showing a steep western deformation front, and a chaotic internal seismic character,
whereas the Southern front shows well-developed thin-skinned fold-and-thrust deformation. These
differences are attributed to interaction with basement structures and sediment supply, with the
NSP likely consisting of muddier sediment (Puspita et al. 2005).
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Studies of the active sedimentary systems within the Makassar Strait are limited, but reveal a
complex distribution of different depositional systems. Shallow water sedimentation along the
margins of the Makassar South Basin is largely carbonate dominated. The Paternoster Platform in
the southwest of the Strait forms a ca. 40,000 km2 medium- to coarse-grained carbonate sand sheet
made up of bioclasts close to reefal build-ups, and benthic foraminifera in open marine regions
(Burollet et al. 1986). The actively prograding Mahakam Delta is a major source of clastic sediment
to the study area, with an estimated annual sediment discharge rate of 8 x 106 m3 yr-1 of clay, silt and
sand-rich sediments (Roberts and Sydow 2003). Outboard and north of the delta front, carbonate
deposition is found on the shelf edge, where a number of bioherm and shelf-edge build-ups are
identified on bathymetry and seismic data (Roberts and Sydow 2003).
The continental slopes surrounding the Makassar Basins are locally fault-controlled (Guntoro 1999;
Prasetya et al. 2001). Canyons are present on steepest slopes, particularly in areas where sediment
input into the basin is restricted. Mass transport deposits are identified at the foot of the slope, and
generated by sediment failure (Saller et al. 2012). Outboard of the Mahakam Delta, high sediment
influx onto the slope has formed sinuous channel-levee complexes (Saller et al. 2012).
The Makassar basin floor is smooth, with no evidence of tectonic disturbance (Puspita et al. 2005).
Seismic mapping at the base of the slope in front of the Mahakam delta shows a large number of
deep-water depositional features, including turbidite channels, levees and splays, as well as
significant MTDs (Posamentier et al. 2000). Sediment waves are a common feature, with Decker et
al. (2004) attributing those along the Sulawesi Slope Apron to non-channelised hyperpycnal or
turbidite flows triggered by storm events.
Oceanographic Setting
The Indonesian Throughflow (ITF) is made up of numerous branches sourced from the Pacific Ocean.
The main branch, the Makassar Throughflow (MTF), constitutes around 80% of water transfer (Fig. 1
A) (Gordon et al. 2008) making the Makassar Strait the main location for water and heat exchange
between the two Oceans. The MTF exits the North Pacific between the Philippines and New Guinea
and enters the Makassar Strait via the Celebes Sea (Tillinger 2011) where an average of 9.3 ± 2.5 Sv
of water are transferred (Kuhnt et al. 2004) (Fig. 1 C). It then splits into two branches, one of which
flows through the Lombok Strait, between the islands of Bali and Lombok, and the other through the
Timor Sea, between Timor and Australia (Fig. 1 A; C) (Kuhnt et al. 2004). The Makassar Strait is the
only low-latitude ocean gateway in the global, ocean ‘conveyor belt’ of thermohaline circulation and
is, therefore, a critical regulator of global oceanic energy exchange (Gordon and Fine 1996; Kuhnt et
al. 2004; Rahmstorf 2006).
Within the Makassar Strait, moorings (Susanto and Gordon 2005) and modelling (Mayer and Damm
2012) show the MTF to act as a western boundary current along the Kalimantan continental slope.
The flow of the surface 50 m is primarily wind-driven, with average velocities of 0.5 ms-1 (Gordon et
al. 2008). Velocities increase with depth, reaching a maximum of around 1 ms-1 within a subsurface
‘jet’ extending down to 300 m, with a core corresponding to the thermocline, at about 100-150 m
(Mayer and Damm 2012). Below 300 m, velocities reduce to under 30 cm s-1 (Tillinger 2011). The
depth and speed of this high velocity water mass varies seasonally in response to monsoonal forcing.
It shallows and intensifies during the southeast and northwest monsoon seasons (March-April and
August-September) (Gordon et al. 2008). Locally in the Makassar Strait, the MTF exerts a control on
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sedimentation. Seismic mapping of the Mahakam Delta by Roberts and Sydow (2003) shows the
fine-grained prodelta (at ca. 40 m water depth) is deflected to the south due to interaction with the
MTF, which can reach velocities of 80 cms-1 at the delta front (Roberts and Sydow 2003).
Data & Methodology
Bathymetric Data
This study is based on three multibeam echosounder bathymetry surveys, acquired by Gardline
Geosurvey on behalf of TGS, for their Indonesian Frontier Basins programme. The TGS_MakN,
TGS_MakS and TGS_Pat surveys (Fig. 2) were acquired between December 2006 and February 2007
and cover around 60,000 km2 in total. The multibeam echosounder data has been gridded at 25 x 25
m resolution, with features enhanced by a shaded relief map (0° azimuth, 45° angle). Backscatter
data, at 5 m resolution, was also used to inform interpretation. This data covers the shelf edge, at
around 200 m depth, to the basin floor, at over 3000 m water depth.
Additional bathymetric data, used for regional mapping (Fig. 2), include the Global Multi-Resolution
Topography synthesis (Ryan et al. 2009) and the SRTM30_PLUS Global Bathymetry Data at 30 arc
seconds resolution (Becker et al. 2009).
Slope gradient maps were extracted from bathymetric data, and depth profiles generated, to aid
interpretation of physiographical domains. Structural lineations and depositional bodies were
interpreted using bathymetric, slope, and backscatter data to generate environment of deposition
maps.
Seismic Data
Two 2D seismic surveys from Multiclient Geophysical were interpreted in this study. The Makassar
Regional Super-Tie MCG2D Survey (MCG-1051) provides regional coverage of the Makassar North
Basin (Fig. 2) with 21 2D Pre-Stack Time Migration (PSTM) seismic lines totalling ~5000 km line
length. The data was acquired in 2010 and processed by CGG Veritas in early 2011. The MCG-1051
survey was followed in 2012 by an infill survey (MCG-1251). The Makassar Infill 2D PSTM (Fig. 2)
consists of 27 lines with a total of ~2400 km line length. Both seismic surveys are high resolution in
the upper few seconds below the seabed, which is the main section of interest in this study.
From the seismic data, we map Quaternary sediments deposited in the Makassar North Basin.
Precise age constraints were not possible because of the lack of well control, however, a Top
Pliocene horizon was interpreted based on del Negro et al. (2013). This allows us to date the age of
the youngest sedimentary sequence at <2.6 Ma. Seismic data were interpreted using standard
seismostratigraphic methods, and seismic facies were used to distinguish the different environments
of deposition. Reflection terminations were used to identify key seismic stratigraphic surfaces,
including tops and bases of individual mass transport deposits (MTDs). Due to the spacing of the 2D
seismic data (1740 km) only MTDs larger than ca. 80 km2 and 60 ms (approx. 110 - 130 m thick)
were mapped. Events smaller than 100 km3 are under-represented due to limitations in spatial
coverage of the 2D seismic survey, and the resolution for imaging thinner events. Seismic resolution
deteriorates with depth, therefore accurate mapping of MTDs was limited to the Pleistocene-Recent
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section. The volume of each MTD was calculated, and converted to depth using a velocity of 2000 m
s-1, based on well velocity data gathered from MTDs of similar depositional character and burial
depths in the Falkland Basin (Nicholson et al. under revision). An uncertainty range of 20% has been
added to capture possible MTD velocities between 1800 and 2200 m s-1. The total number of
mapped landslides over the last ca. 2.6 Ma was used to derive an average frequency for events of
the different volumes. As a result of the low seismic imaging resolution and line spacing, estimates
of the frequency of smaller events (<100 km3) are considered to be conservative.
Additional Data
As part of the TGS-NOPEC Indonesian Frontier Basins programme, a number of additional data-sets
were acquired to compliment the multibeam echosounder data used in the study. These data
include gravity and magnetics acquired during the bathymetry survey, as well as regional gravity
data compiled by Sandwell et al. (2014). Also, 234 piston cores were acquired within the region of
the bathymetry survey (Fig. 2) in 2017 by TDI-Brooks International for geochemical analysis. From
the cores over 600 samples were analysed for hydrocarbon indications. Shear strength
measurements and basic sedimentary information were also available for each sample.
Historical earthquake data was compiled from the United States Geological Survey Earthquake
Catalog (USGS). These records extend back to 1900 and list earthquake date, time, magnitude and
epicentre depth. Historical tsunami data was downloaded from the National Geophysical Data
Center / World Data Service Global Historical Tsunami Database (NGDC/WDS), which documents
source location, date, time, event magnitude, maximum water height, total number of deaths,
injuries and damage for each event.
Additional information was compiled from the literature where data coverage was unavailable for
this study. Specifically, modelled routing of the MTF was georeferenced and digitised from Mayer
and Damm (2012). Additional information about the features on the seafloor were gathered from
studies by Fowler et al. (2004); Saller and Dharmasamadhi (2012); and Frederik et al. (2019).
Results & Interpretation
Surface bathymetry interpretation
Physiographic domains. The main physiographic domains of the Northern Makassar Basin were
mapped regionally on the SRTM30_PLUS Global Bathymetry Data (Becker et al. 2009). The
continental shelf-slope break is defined by a change in slope gradient from less than 0.2° across the
shelf, to slope gradients of 3 23° (Fig. 3). The shelf break occurs at approximately 200 m water
depth, and varies significantly with distance from the coastline. To the west of the Basin, a broad
shelf of around 30 50 km extends out in front of the Mahakam Delta. This is significantly wider
than the eastern shelf, which reaches a maximum width of 10 15 km and where a narrow, and
locally very steep (up to 24°) continental slope grades to the basin floor (Fig. 3). The northern and
southern limits of the basin are defined by structural highs (the Mangkalihat High and the
Paternoster Platform respectively) which are bisected by deep, narrow channels. The deep Southern
Makassar Basin is significantly smaller than the Northern Basin at approximately 180 km wide, and
200 km in length (Fig. 3). This is a result of the extensive Paternoster Platform extending out from
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the Sulawesi Margin. Two channels connect the Southern Makassar Basin over the Dewakang Sill, to
the Flores Sea in the South.
Structural features. The resolution of the SRTM30_PLUS Global Bathymetry Data (Becker et al. 2009)
is insufficient to identify structural and sedimentary features in detail. However, the active tectonic
nature of this region is evidenced by a number of features mapped on the high resolution TGS
bathymetric surveys (Fig. 4 - 6). Most notably, structural features include: (1) major strike-slip fault
zones; and (2) fold-and-thrust complexes. The Palu-Koro fault zone (FZ) is clearly imaged in the
bathymetry data (Fig. 4). It extends from Palu Bay, Sulawesi, north-westwards to the northern edge
of the Mangkalihat High. On the high resolution TGS_MakN survey, the details of this fault zone can
be mapped (Fig. 4). It is made up of multiple individual left-lateral strike-slip faults with both
transtensional pull-apart and transpressional pop-up structures evident along the fault zone. The
faults generate topography on the seabed of 250 300 metres. The basin floor is modified in the
East of the Makassar Strait where two fold-and-thrust belts are evident on the bathymetry, where a
series of thrust-cored anticlines are clearly imaged on the seafloor (Fig. 4). Puspita et al. (2005)
define two distinct fold-and-thrust belts named the Northern and Southern Structural Provinces (Fig.
1).
Sedimentary features. There is evidence for multiple downslope systems transporting sediment into
the Makassar North Basin. From the backscatter data we interpret subtle sedimentary features,
distinguishing bedrock or carbonates of high acoustic reflectivity (strongly negative values) from
medium reflectivity sands and low reflectivity mud-dominated deposition (values close to zero) (Fig.
5). Dominant sedimentary features include: (1) slope canyons; (2) basin floor fans; (3) slope failure
scarps; and (4) and deep-water turbidite channels.
The TGS_MakN bathymetric survey, on the eastern margin of the Makassar North Basin, images a
deep water channel in the northern fold belt (Fig. 4). The channel is sourced from the mouth of Palu
Bay, which incises to a depth of 300 m in the upper slope, down to 600 m deep in its distal portion.
The channel profile shows steep, erosive flanks, bordering a flat channel axis. The channel shows
moderate sinuosity (Cs = 1.4), which is modified by thrust-cored anticlines in the outer part of the
fold belt. There is no evidence for an associated depositional fan on the basin floor.
A downslope (turbidite) depositional fan is seen the TGS_MakS bathymetric survey, where an
extensive sediment wave field has developed along the eastern side of the Basin. The Makassar
South backscatter data images a small downslope channel ca. 2.5 km wide, 200 m deep, which feeds
a sediment fan with low backscatter values compared to the surrounding basinal sediment (Fig. 5).
The sediment wave field suggests periodic unconfined flow across the continental margin between
the two fold-and-thrust belts. Wave crests are orientated parallel or slightly oblique to the
continental slope. The sediment waves are sinuous with wave lengths of 0.5 and 1 km, and heights
of 5 - 30 m. Backscatter data shows higher values on the lee slide of waves.
Where sinuous channels and sediment waves dominate the eastern margin of the basin, v-shaped
and straight canyons dominate on the northern, western and southern margins. High resolution
bathymetric data only covers a small portion of the continental slope of the Mangkalihat High, and
the Paternoster Platform. Both these continental margins show canyon systems of very different
characteristics. The Mangkalihat High canyons are tightly spaced, straight, v-shaped, downslope
canyons that range between 40 and 80 m deep (Fig. 4). The TGS_MakS bathymetric data covers a
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number of canyons fed from the Paternoster Carbonate Platform. Here, four large v-shaped canyons
(over 5 km wide, 400 700 m deep) are separated by slope-perpendicular spurs (Fig. 5). This
change in character may be due to clastic vs. carbonate dominated sedimentation and erosive
processes in the north and south respectively. We do not have access to high resolution bathymetric
data over the western continental slope of the Makassar North Basin, however, published studies
suggest that downslope canyons and associated slope apron fans are widespread (e.g. Saller et al.
2008; Fowler et al. 2004).
Small failure scarps (<5 km2) are a common feature of the forelimbs of thrust belt anticlines (Fig. 6)
and in the TGS_MakN bathymetric data to the north of the Paul-Koru Fault zone (Fig. 4). The
backscatter imaging over the TGS_MakS bathymetric survey highlights a region of mass transport
deposition (Fig. 5). These deposits are characterised by low backscatter values, suggesting low
acoustic reflectivity, and therefore high mud content. Kinematic indicators, expressed as lineaments
on the seafloor (Fig. 5 B), suggest a flow direction from the west. Evidence for slope failure is also
seen in the TGS_Pat bathymetric survey (Fig. 6). Large blocks (reaching 20 35 m in diameter) are
distributed at the base of slope along the western margin of the Paternoster Platform in the
Makassar South Basin (Fig. 6B).
The TGS_Pat bathymetric survey data images a number of features that we interpret as contourites.
Immediately south of the Labani Channel, a scalloped, scour-like feature is imaged at 1300 m water
depth (Fig. 6 A). The feature is orientated perpendicular to the slope, with a steep lee and shallow
stoss side; all characteristic of formation by alongslope erosion. To the south, there is an erosional
terrace across the upper slope at 200 m water depth (Fig. 6 B). This terrace is 700 800 m wide, and
is elongated alongslope for approximately 10 km. It is formed along the north side of a large
embayment. On the south side of this embayment, a mounded topographic feature around 500 m
high is seen at the base of the slope. It is elongated alongslope, (Fig. 6 A) and tentatively interpreted
as a contourite mounded drift, however further geophysical data is required to confirm this
interpretation.
Subsurface Seismic Interpretation
Seismic tectonostratigraphy. The Top Pliocene was interpreted across the entire seismic survey and
an isochron thickness map of the seabed to Pliocene sequence generated to show depositional
trends in Pleistocene-Recent sediments of the Makassar North Basin (Fig. 7). The isochron thickness
map reveals a basin depocentre in the north, with a general thinning to the south. An additional
depocentre is identified in the west on the edge of the data in front of the Mahakam delta (Fig. 7).
Seismic facies & environment of deposition. Seismic facies (SF) interpretation identifies eight
dominant facies (I-VIII) (Fig. 8; Fig. 9; Table 1). Each facies is characterised from its internal reflection
character, continuity, amplitude and frequency, in addition to external morphology and/or bounding
relationships (Table 1). The regional extent of each of the facies was mapped (Fig. 8). These
observations were combined with those from the bathymetric mapping to interpret an environment
of deposition for each facies (Table 1).
Seismic Facies I) Two regions of polygonally faulted pelagic sediments (SF-I) are located in the
deepest part of the basin, directly in front of the two fold-and-thrust belts (Fig. 8). Here, high-
amplitude, high-frequency reflections are offset due to densely spaced high angle extensional
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faulting (Fig. 9 I). These faults are characteristic of sediment compaction and fluid expulsion from
fine-grained biosiliceous sediments (e.g. Davies and Ireland 2011). These facies are associated with a
prominent bottom simulating reflector (BSR). Based on the polarity reversal of this BSR, and the
polygonally faulted nature of the associated facies, we interpret this as a diagenetic Opal A/CT
boundary zone within highly siliceous pelagic sediments (Lee et al. 2003). Polygonal faulting and a
strong Opal A/CT transition suggest a high silica content, implying a lack of terrigenous sediments
(turbidites, hemipelagic mud), allowing us to distinguish this facies from hemipelagites (see SF-II).
Seismic Facies II) High-amplitude, high-frequency, laterally continuous reflections with draping
morphology are interpreted as hemi-pelagic facies (Fig. 9 II). SF-II transitions towards the basin
centre into SF-I. Based on the lack of polygonal faulting and BSR, we interpret this facies as a less
biosiliceous, but still very fine grained, hemi-pelagites.
Seismic Facies III) Sediment waves are found locally on the western basin margin and an extensive
sediment wave field dominates the eastern margin, between the two fold-and-thrust belts (Fig. 8).
The waves are characterised by hummocky, subparallel, semi-continuous reflections (SF-III) (Fig. 9
III). The package is 200-400 ms in thickness TWT, and affects the seafloor bathymetry as seen in the
Makassar North Basin (Fig. 5). The base of this seismic package is a high amplitude reflection
showing opposite polarity to the sea floor. This is likely a BSR associated with gas hydrates that could
have promoted gravitational movement (creep) downslope. The seabed topography generated by
such a process appears to have promoted the growth of upslope-migrating sediment waves that
have likely been deposited by unconfined downslope (turbiditic) processes.
Seismic Facies IV) Dominating the Pleistocene basin fill are numerous MTDs, clearly identifiable from
their semi-transparent and chaotic seismic signature (SF-IV). The MTDs are characteristically
deformed, chaotic, discontinuous and low amplitude (Fig. 9 IV). Rare internal reflections show
extension in their proximal portions, and imbrication and compression in their distal parts (Fig. 10).
Individual MTDs are mappable because they form lenticular bodies with erosional bases of negative
acoustic impedance (AI), and a strong positive AI upper surface onto which younger stratigraphy
onlaps (Fig. 10). MTDs thin, and pinch out, upslope and laterally. Their downslope extent is either
thinning (e.g. Fig. 10), or shows an abrupt, stepped change in facies. Seismic facies IV is
predominantly distributed along the western side of the basin, with the largest events clustered in
the SW (Fig. 8).
Seismic Facies V) Upslope of the MTDs and basin floor, SF-V is characterised by subparallel and semi-
continuous reflections (Fig. 9 V). Locally, this facies shows a mounded external morphology, and
thinning upslope. Reflections dip basinward, but are at a lower angle than the seabed forming the
upper continental slope. This facies is interpreted as a turbidite-dominated slope apron consisting of
sediment fans and channel-levees.
Seismic Facies VI) Seismic on the upper slope shows varying reflection amplitudes. Internally
reflections are laterally semi-continuous, but commonly are truncated by downcutting, erosive
canyons (Fig. 9 VI). Basinward dipping reflections are seen in addition to mounded morphologies.
Rare BSR reflections are also identified within SF-VI.
Seismic Facies VII): On the upper slope in the southwest of the Makassar North Basin (Fig. 8), the
seismic facies is characterised by basinward-dipping, parallel, semi-continuous internal reflections
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(Fig. 9 VII), interpreted as contourites. The external morphology is distinctly convex up, with
evidence of alongslope elongation (despite limited 2D data coverage). The upslope progradation of
internal seismic reflections, combined with their external morphology, is typical of contourite-
plastered drifts (Rebesco et al. (2014).
Seismic Facies VIII) Characterised by horizontal to basinward dipping, semi-parallel, discontinuous
reflections, SF-VIII is confined to the continental shelf (Fig. 9 VIII). Internal reflections are low
amplitude, compared to the seabed reflection horizon which locally is high amplitude, particularly in
the north of the study area. Locally on the shelf or at the shelf break, isolated build-ups (Fig. 11 A; B)
are interpreted as biohermal carbonates (Roberts and Sydow 2003). These suggest that the locally
high amplitude seabed seismic reflections are due to carbonate deposition.
Combining the observations and interpretations from the bathymetric and seismic mapping, an
environment of deposition map for the Quaternary has been compiled (Fig. 12).
Mass transport deposits. In total, nineteen individual MTD deposits were mapped in detail (Table 2;
Fig. 12), with volumes ranging from 5 km3 to over 600 km3 (Table 2). Five MTDs are over 100 km3 in
volume, with the three largest located at the base of the continental slope in the southwest of the
Makassar North Basin, south of the Mahakam Delta (Fig. 13). Mapping of internal and top-surface
directional indicators (imbrications and lineations) reveals that the largest MTDs are sourced from
the south-western margin of the basin. Smaller (<50 km3) events are sourced from the Mangkalihat
High in the north, and the Paternoster Platform in the south. No significant volume MTD events are
identified in the fold-and-thrust belt although numerous smaller events (<5 km3) associated with
thrust-cored anticlines are evident in the bathymetric data (e.g. Fig. 6 B).
Based on their internal and external characteristics, the MTDs mapped in this study, are interpreted
as deposited from translational submarine landslides. The isochron thickness map identifies a thin
Pleistocene Recent sequence in the upper slope which could be a possible failure zone (Fig. 7 C).
This suggests that the submarine landslides were initiated in the upper- to middle continental slope
between water depths of 500 and 1250 m.
Contourite features. On the upper slope, erosional and depositional contourite features are
interpreted. Along the western edge of the Northern Basin, outboard of the shelf break there is a
steep (1520°) erosional escarpment that correlates well with the maximum velocity core of the
Makassar Throughflow at 100 150 m water depth (Gordon et al. 2008). Locally, the base of this
steep scarp is associated with a low gradient upper slope (1.5°) before a second shelf-slope break at
450 ms TWT (approximately 300 m water depth). This is seen clearly on seismic line MCG_1051 L639
(Fig. 11 B) and across the TGS_Pat bathymetric data (Fig. 11). This ‘step’ in the upper slope
corresponds to the average water depth of the base MTF at 300 m (Mayer and Damm 2012). It is
therefore interpreted as a contourite terrace, formed by MTF erosion. The mechanisms for
contourite terrace formation are poorly understood (Hernandez-Molina et al. 2008), however, the
association between terrace depth and the base of the MTF suggests the possibility of enhanced
current energy or internal wave-related erosion at the thermocline or pycnocline. The presence of
an erosional scarp upslope of the terrace shows that this western boundary current is highly erosive
in nature along much of the western margin of the Makassar North Basin. Downslope of the
erosional features, SF-VII shows (Fig. 9) a distinctly convex upwards slope morphology, alongslope
elongation, and upslope progradation of internal reflections all features diagnostic of contourites
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(Nielsen et al. 2008). We interpret these depositional features as plastered contourite drifts. In the
Makassar South Basin, the Paternoster Bathymetric Data (Fig. 6) shows a number of additional
features that could represent contourites including: (1) an erosional scour orientated perpendicular
to the Paternoster margin (Fig. 6 A insert). These scours are characterised by steep lee and shallow
stoss sides, and are interpreted as formed by bottom current turbulence within the MTF on exiting
the Labani Channel, similar to features described by Nicholson and Stow (2019); (2) a contourite
terrace similar to that seen in the Makassar North Basin (Fig. 6 B); and (3) a potential mounded drift
within an embayment (Fig. 6 B). However additional seismic data is required to corroborate this
interpretation.
Discussion
Drivers for slope instability
There are a number of factors that contribute to instability of continental slopes: (1) seismic activity,
including seismic loading and earthquake-related shaking; (2) slope oversteepening, due to erosion,
carbonate build up, faulting or diapirism; (3) loading, from water, sediments or ice. This can occur
over various timescales; for example short-lived cyclic loading during storm events, or longer-term
sea-level changes; (4) rapid accumulation and underconsolidation, that can result in elevated pore
pressures; and (5) gas hydrate disassociation (Locat and Lee 2002). All these processes act to either
reduce the shear strength of continental slope sediments, or increase the stress acting upon them
(Hampton et al. 1996). In addition, settings that result in steep seabed morphology such as fjords,
volcanoes and carbonate build-ups are particularly susceptible to failure (Hampton et al. 1996). Here
we discuss the possible drivers for slope failure in the Makassar Strait.
Carbonate build-up. Carbonate slopes are capable of reaching gradients that far exceed that of their
siliciclastic counterparts (Schlager and Camber 1986). This ability to reach gradients of up to 29°,
combined with their capacity to form a concave upward slope morphology makes them susceptible
to mass failure (Hampton et al. 1996). Carbonates are present across the western margin of the
Makassar Strait, both as isolated biohermal build-ups along the northwest continental shelf, and
more extensively across the Paternoster Platform in the southwest. Based on kinematic indicators
and gross morphology, a number of moderate size (< 50 km3) MTDs are sourced from the
Paternoster Platform (Fig. 13). However, the largest MTDs are sourced from the western continental
margin (Fig. 12) where clastic sedimentation from the Mahakam Delta dominates, rather than the
carbonate platform margin. It is therefore unlikely that oversteepening of carbonate slopes is a
driver for large-scale submarine landslides in the Makassar North Basin.
Seismic activity. There is historical evidence for earthquake rupture of the seabed causing tsunamis
(NGDC/WDS) and seismic activity is frequent across the Makassar Strait region (Fig. 1) (USGS),
particularly in Sulawesi where earthquakes of up to Mw 8 are associated with the left lateral Palu-
Koro Fault zone (Fig. 1 B). There are historical earthquakes along the eastern margin of the
Makassar Strait, however our mapping shows that the MTDs are sourced from the western margin.
It is therefore unlikely that seismic activity and related seafloor fault rupture is the primary driver for
these. Nevertheless, with the narrow Makassar Strait ca. 300 km in width, it is possible that far-field
seismic shaking is the triggering mechanism for submarine landslides of the Kalimantan continental
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margin. The distribution of the largest MTDs in the southwest of the Makassar North Basin, away
from historic earthquake epicentres, suggests that there is an additional preconditioning mechanism
in this region for the generation of large-volume MTDs.
Deltaic deposition and sediment loading. A prominent sedimentary feature of the Kalimantan
margin is the Mahakam Delta. With an estimated annual sediment discharge of 8 x 106 m3yr-1
(Roberts and Sydow 2003), the Mahakam River provides significant volumes of sediment to this
actively prograding delta. Delta fronts studied elsewhere (Hampton et al. 1996) are known to be
vulnerable to collapse due to the interplay of various factors. For example, in the case of the
Mississippi Delta, high sedimentation rates result in under-consolidated sediment accumulations and
frequent failure (Coleman et al. 1993). Furthermore, these deltaic sediments are generally fine-
grained with high organic content and gas charge, which acts to reduce sediment shear strength. As
a result, such sediments are preconditioned for failure.
In the case of the Mahakam Delta, similar processes to those on the Mississippi Delta may promote
mass failure of the continental slope. The Makassar Strait is a major oil and gas province, and
sediments supplied to the delta are high in organic content (Saller et al., 2006), and therefore are
likely to be low in shear strength. In addition, microbial consumption of organic matter and
thermogenic maturation of kerogens is likely to result in methane production, which could reduce
stability. Detailed interpretation of the seismic, bathymetry and gravity cores offshore the Mahakam
Delta (Roberts and Sydow 2012) reveals that during Pleistocene lowstand conditions, delta growth
occurred along the shelf-slope transition. This would have provided sediment directly to the upper
slope, causing oversteepening and sediment loading, resulting in the slope being highly susceptible
to failure. This situation explains why mass transport deposits are distributed only along the western
slope of the basin, unlike the eastern margin, where there is no comparable sediment influx.
However, this observation does not explain the distribution of the largest mapped MTDs clustered to
the southwest of the Mahakam Delta (Fig. 13).
Slope failure preconditioning by the Indonesian Throughflow. Mapping the present day route of
the MTF (Fig. 1 C) shows that this western boundary current is interacting with the Kalimantan
margin and the Mahakam delta. Detailed mapping of the delta front shows the fine-grained prodelta
(at ca. 40 m water depth) is deflected to the south due to interaction with the MTF (Roberts and
Sydow 2003). Regional gravity and depth to basement mapping (Cloke et al. 1999; Moss and
Chambers 1999; Sandwell et al. 2014) demonstrate that the Mahakam depocentre has a marked
asymmetry from north to south. Oceanographic measurements show the MTF currents reaches 1 m
s-1 within a subsurface ‘jet’ extending down to 300 m below the sea surface with a high velocity core
at 100 150 m (Mayer and Damm 2012). Currents of this strength are sufficient to erode and
transport sediments (Stow et al. 2008), as evidenced by the distribution of contourite erosional and
depositional features across the upper slope of the Kalimantan margin, herein named the Makassar
Contourite Depositional System (CDS) (Fig. 11).
Previous authors have highlighted that contourite deposition is often associated to slope instability
along continental margins (Laberg and Camerlenghi 2008; Stow et al. 2012; Nicholson et al. under
revision). Along the Northwest European margin, the giant (~3,500 km3) Storegga Slide is, in part, a
result of the reduced shear strength within the basal failure plane comprising clay-rich contourites
(Bryn et al. 2005). In the South Falkland Basin, an over steepened and basinward-dipping contourite
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drift on the Burdwood Bank has regularly failed due to upper-slope deposition and lower-slope
erosion within a contourite moat (Nicholson et al. under revision). Along the east Canadian margin,
MTDs are associated with shallow water contourites (Rashid et al. 2017). A geotechnical review of
these sediments concludes that their high fluid content, together with the highly cyclical nature of
the sediments resulting in failure surfaces throughout the succession, may promote liquefaction
from seismic shaking (Rashid et al. 2017). In these three regions, the cause of sediment failure is
attributed in large part to the nature and morphology of contourite sediments.
Thus, three key factors make contourites susceptible to mass transport failure. Firstly, the
anomalously high sedimentation rates of contourites drifts reaching 60 cm ka-1 (Howe et al. 1994)
can lead to high fluid content, and elevated pore pressures upon burial. Secondly, the fine muddy
nature of contourite sediments makes then likely to have low internal shear strength (Laberg and
Camerlenghi 2008). Finally, their convex-upwards external morphology can result in locally high
gradients on the continental slope making failure more likely Stow et al. (2012). Laberg and
Camerlenghi (2008) note that there is often an association between gas hydrate accumulation and
contourite deposition due to the elevated pore space in these rapidly deposited accumulations and
may be an additional factor making contourites sediments susceptible to failure.
In the case of the Makassar CDS, the high velocity MTF jet extends correlates well with the formation
of a steep (20°) erosional scarp and relatively flat (1.5°) contourite terrace on the upper slope
directly to the north and east of the Mahakam Delta (Fig. 11). This is evidence for the erosion and
removal of sediments from this area, which are then transported and deposited downslope and
downcurrent. This winnowing of sediments from the north results in enhanced sedimentation on the
slope to the southeast of the delta, directly above the region where the largest Quaternary MTDs
have been mapped (Fig. 12). Contourite plastered drifts are identified along the margin, generally
directly below the base of the Makassar Throughflow at 300 m water depth (Fig. 11). These regions
of enhanced sedimentation are vulnerable to failure due to high fluid content, the high organic
content of the Mahakam Delta sediments, and their convex upwards morphology which results in an
oversteepened, basinward dipping slope, of around 15°. We propose these regions as the main
source of mass transport sediments to the deep basin. It is noted that the plastered contourite drifts
identified in this study are significantly smaller than examples seen along other passive continental
margins (Hernández-Molina et al. 2008). We propose this to be a result of regular removal of
material from the drifts during submarine landslide events.
The MTF is likely to have varied significantly in response to Quaternary climatic fluctuations, which
may further play a role in controlling slope instability. The resolution of the data in our study, and
the lack of well control, does not allow detailed evaluation of the timescales and the control of sea-
level or climate on triggering these events, but it is expected that climate exerts a strong control on
slope stability by controlling the depth, velocity and nature of the MTF. The evidence from previous
studies, however, is conflicting. During glacial lowstands, much of the Paternoster Platform and the
continental shelf was exposed (Hall, 2009; Roberts and Sydow, 2012), and the width and cross-
sectional area of the Makassar Strait was considerably reduced. As a result, the MTF may have
initially been accelerated due to flow constriction (Hall, 2009). However, ocean current velocity
proxies suggest that the MTF was significantly reduced, or even switched off during glacial
conditions (Gingele et al. 2002; Holbourn et al. 2011), due to the reduced exchange between the
Pacific and South China Seas (Godfrey, 1996) and reduced Atlantic meridional overturning
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(Hendrizan et al. 2017). Modelling of the effect of sea-level on ocean current in SE Asia is further
complicated by the effect of monsoonal conditions on the region. Although the interplay between
sea-level induced restriction of the MTF and changes in velocity profile are not well understood, it is
probable that sea-level fluctuations resulting from Pleistocene glacial-interglacial cyclicity affected
slope instability. Further work is required to constrain variations in the timing and frequency of slope
failure events to reveal the drivers.
Tsunamigenic risk
MTDs of the scale and volume mapped in this study in the Quaternary section of the Makassar North
Basin, by analogy with other locations, would be expected to generate hazardous tsunamis (e.g.,
Harbitz et al. 2006; Tappin et al. 2008, Tappin et al. 2014) (Table 3). Many of the large volume
landslide tsunamis, such as Storegga, are prehistoric, and are recognised and mapped from their
palaeo-record preserved in sediments (e.g., Dawson et al. 1988; Bondevik et al. 1997). There is no
historic evidence for tsunamis originating in the Makassar Strait, and no one has yet looked for
evidence of prehistoric (Pleistocene-Recent) events. This leads us to conclude that either: (1) the
submarine landslides that generated these mass transport deposits were not tsunamigenic; or (2)
tsunamis generated by submarine landslides are of much lower frequency than those generated by
fault movements and have not occurred since historic records began. This needs to be tested by
investigating evidence for palaeo-tsunamis in the surrounding coastal regions, although evidence is
only likely to be preserved for Holocene-Recent events…
From our tentative chronology of the MTDs sourced from the Mahakam Delta, they have occurred
frequently throughout the Quaternary. A conservative estimate of the frequency of submarine
landslides >100 km3 is approximately every 0.5 Ma. Events larger than 10 km3 are conservatively
estimated to have occurred every 160 ka; detailed mapping of high-resolution 3D seismic surveys
would likely reveal multiple smaller events and a significantly higher frequency. It is however, likely
that the last large-scale failure event occurred prior to historic records. If the landslides were
tsunamigenic, they would impact the local Sulawesi and Kalimantan coastlines which previously
were not considered at risk of tsunamis. There are a number of towns along the Sulawesi coast,
approximately 180 km east, and directly in front of the largest MTD deposits that could be affected
by any forward-propagating wave. The distance is comparable to that travelled by previous tsunamis
in Papua New Guinea (Tappin et al. 2008) and Japan (Tappin et al. 2014) (Table 3). The high-energy
point source release of tsunamis from submarine landslides also generates a significant reverse
wave, travelling in the opposite direction to slope failure. This reverse wave would travel only 100
km before reaching the low-lying cities of Balikpapan (population >850,000) and Samarinda
(population >840,000). The destructive nature of such a reverse wave was demonstrated in the 1929
Grand Banks event, when a 13 m wave struck the Newfoundland Coast following a slope failure
some 340 km offshore (Fine et al. 2005). With the exception of distance to shoreline, the
characteristics of the Grand Banks slope failure (MTD volume and water depth) are similar to the
MTDs mapped in the Makassar Strait. Therefore, if the Makassar Strait submarine landslides were
capable of generating tsunami waves, it is expected that the back-wave could also form a tsunami
risk, with local morphological features such as channels and estuaries, including Balikpapan Bay
further amplifying the wave height.
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Tsunamis generated by submarine landslides differ in character from those sourced from fault
rupture of the seabed (Okal and Synolakis 2003). Unlike the linear wave formed by fault rupture,
landslides generate a tsunami wave from a point source that radially spreads out in the direction of
the submarine failure, with a negative reverse wave spreading in the opposite direction (Harbitz et
al. 2006). The magnitude of the wave is much larger than those formed by fault rupture, however
rapid radial dampening limits the destructive distance of the tsunami (Okal and Synolakis 2003). The
magnitude of a tsunami generated by a submarine landslide are strongly controlled by landslide
volume, initial acceleration, and the water depth of initial failure (Harbitz et al. 2006). Tsunami wave
modelling is urgently required to test slope failure scenarios and identify any coastal regions at risk
in order to make accurate recommendations for mitigation measures to be put into effect, and
evacuation plans be implemented in high risk regions.
Conclusions
This study identifies, for the first time, multiple large-volume MTDs generated by submarine
landslides within the Quaternary section of the Makassar North Basin. The largest MTDs are
clustered in the southwest of the basin, and are over 600 km3 in volume. Kinematic indicators
demonstrate that the MTDs are sourced from the western margin of the Makassar Strait, south of
the Mahakam Delta. Mapping along this western margin also reveals a contourite depositional
system along the upper slope: the Makassar CDS.
We propose that the largest and most frequent submarine landslides in the Makassar North Basin
are genetically related to these contourites. Sediments transported into the basin from the
Mahakam Delta are redistributed by the MTF, and deposited on the upper slope to the south of the
delta, resulting in high sedimentation rates to the south of the delta. In combination with fine grain
size, rapid burial promotes fluid retention and overpressure generation, promoting failure. The fine,
muddy nature of the drifts makes them likely to have low shear strength, and their convex-upwards
morphology results in locally high gradients on the upper continental slope. All of these factors make
these deposits inherently unstable and prone to failure.
Based on analogue studies of mass transport deposits elsewhere, it is probable that the submarine
landslides that generated the mapped deposits were tsunamigenic. The wave generated from the
submarine landslide mapped in this study could impact the Sulawesi and Kalimantan coastlines in
regions not previously affected by historical events. It is therefore critical to understand the
triggering mechanisms of these events and their hazard. Future work will aim to constrain the
frequency, and the role of climate and sea level on submarine landslide triggering. Tsunami wave
modelling is required to test slope failure scenarios and identify coastal regions at highest risk.
Acknowledgements
This work was completed as part of a Scottish Funding Council (SFC) Global Challenges Research
Fund (GCRF) pump-priming project at Heriot Watt University (PI Nicholson). We would like to thank
our partners at Bandung Institute of Technology, and the members of the Indonesian Marine
Geological Institute and Geological Survey of Indonesia for their discussion and contribution to this
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research. We thank TGS and Multiclient Geophysical for permission to publish seismic and
multibeam data respectively. D.R. Tappin publishes with the permission of the Executive Director of
the BGS (United Kingdom Research and Innovation).
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Figure Captions
Figure 1: A) Regional tectonic and oceanographic setting. Study area indicated in red. MTF =
Makassar Throughflow. B) Structural features of the Makassar Strait from Puspita et al. (2005) (NSP,
CSP, SSP refer to the Northern, Central and Southern structural provenances); Cloke et al. (1999).
SRTM30_PLUS Global Bathymetry Data from Becker et al. (2009). Historical earthquakes form the
United States Geological Survey Earthquake Catalog (USGS). Historical tsunami data from the
National Geophysical Data Center / World Data Service (NGDC/WDS) Global Historical Tsunami
Database (NGDC/WDS). C) Simulated daily average of Sv (Sv = 1 x 106 m3 s-1) for the main Makassar
Throughflow jet between 52 and 420 m (Mayer and Damm 2012). Sv values from Kuhnt et al. (2004).
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Figure 2: Data used in the study. SRTM30_PLUS Global Bathymetry Data from Becker et al. (2009).
Figure 3: Slope map extracted form of the regional bathymetry RTM30_PLUS Global Bathymetry
Data (Becker et al. 2009). Bathymetry contours at 250 m spacing). A) F) Depth profiles across the
Strait showing the main basin and channel features. Depth of Makassar Throughflow (MTF) indicated
in blue. Velocity-depth profile shows average velocity profile 2004-2009 (Susanto et al. 2012).
Figure 4: Bathymetry data interpretation from the Makassar North TGS survey. Bathymetry data
overlaying a shade map with interpretation of structural and sedimentary features. Survey Location
indicated on Figure 2.
Figure 5: Bathymetry data interpretation from the Makassar South TGS survey. A) Backscatter
overlaying a shade map. Highly negative values (indicated in blues) represent bed rock, carbonates or
sands. Greens indicate mud-dominated deposition. Insert shows zoom-in of sediment wave crests. B)
Interpretation of sedimentary and structural features with seabed facies from piston cores. Survey
Location indicated on Figure 2.
Figure 6: Bathymetry data interpretation from the Paternoster TGS survey. A) Bathymetry overlaying
a shade map. MTF (insert) indicates the direction of the Makassar Throughflow over an erosional
scour. B) Interpretation of sedimentary and structural features. Pop outs show slope values. Survey
Location indicated on Figure 2.
Figure 7: Seabed to Pliocene mapping and example regional seismic line from the MCG-1051 survey.
Figure 8: Mapped seismic facies. Locations of Figure 9.
Figure 9: Seismic facies of the Makassar Strait. Locations indicated on Figure 8.
Figure 10: Seismic character of mass transport deposits (MTDs) in dip (A) and strike (B). Location of
section indicated in Fig. 12. Red triangles indicate intersection. Flow direction in (B) towards the
reader.
Figure 11: Seismic character of the contourites depositional system. Temperature and salinity data
from Ocean Data View ODV section indicated) (Schlitzer 2016). Slope gradients annotated in sections
B and C. Line locations indicated on Fig. 12. Velocity-depth profile shows average velocity profile
2004-2009 (Susanto et al. 2012).
Figure 12: Environment of Deposition map of the Makassar North Basin. TGS bathymetry data extent
indicates areas of high-resolution data. Additional information gathered from Fowler et al. (2004);
Saller et al. (2012); and Frederik et al. (2019). CDS = Contourite Depositional System.
Figure 13: Risk map showing the Makassar Throughflow. MTDs color-coded with respect to their
volume. Coastlines at risk of infrequent and catastrophic submarine landslide-induced tsunamis are
indicated in red.
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Tables
Table 1: Pliocene-recent seismic facies (SF) across the Makassar Strait MCG surveys.
SF
Configuration & Continuity
Amplitude & Frequency
Bounding Relationship
Depositional Environment
I
Horizontal, parallel,
discontinuous
High amplitude, high
frequency
Fault-plane truncations. Laterally,
merging with SFB
Pelagic with polygonal
faulting
II
Horizontal, parallel, laterally
continuous
High amplitude, high
frequency
Laterally continuous, draping
morphology
Hemipelagic
III
Hummocky, subparallel, semi-
continuous
High amplitude, low
frequency
Laterally continuous, onlapping
upslope
Sediment waves
IV
Deformed chaotic,
discontinuous
Low amplitude, low
frequency
Lenticular, basal erosional
truncations and steps
Mass transport deposits
V
Dipping, subparallel, semi-
continuous
Varied amplitude, mod
frequency
Localised mounded morphology.
Thinning towards slope
Slope apron / turbidite
VI
Dipping, subparallel, semi-
continuous
Varied amplitude, low
frequency
Canyon downcutting results in
erosional truncation
Continental Slope with
canyons
VII
Dipping, parallel, semi-
continuous
High amplitude, high
frequency
Convex upward lenticular.
Elongate alongslope
Contourite
VIII
Horizontal, semi-parallel,
discontinuous
Low amplitude, low
frequency
High amplitude top reflection
with localised build-ups
Continental Shelf
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MTD
Area_km2
km3 @ 2000 m s-1
Uncertainty km3 range ±
A
3410
616.47
123.29
B
938
53.88
10.78
C
3891
422.99
84.60
D
1482
110.60
22.12
E
1313
137.06
27.41
F
444
26.32
5.26
G
3959
212.19
42.44
H
230
41.87
8.37
I
193
5.85
1.17
J
882
50.57
10.11
K
249
12.71
2.54
L
660
58.64
11.73
M
523
36.81
7.36
N
865
57.56
11.51
O
963
75.64
15.13
P
659
42.48
8.50
Q
680
44.48
8.90
R
86
4.91
0.98
S
130
8.14
1.63
Table 2: Measurements form mapped MTDs and their calculated volumes. 20 % uncertainty range
applied to allow for acoustic velocities between 1800 and 2200 m s-1
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Tsunami
MTD
Volume
(km3)
Water
Depth (m)
Initial
Acceleration
(m s-1)
Distance to
Shore (km)
Run up (m)
Reference
PNG
4
1200
0.47
25
10 - 15
Tappin et al. 2008
Japan
500
4000 -
5500
0.37
150
40
Tappin et al. 2014
Falklands
100
860
0.37
180
40
Nicholson et al. under
revision
Grand Banks*
200
500
unknown
340
13
Fine et al. 2005
Makassar
Strait
100-600
km3
500 - 1000
unknown
E=180; W=100
unknown
n/a
*Grand Banks reverse wave documented.
Table 3: Key characteristics of previous tsunamigenic submarine landslides.
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... The morphological characteristics of seabed landslides are very important for determining their potential. Tsunamis can be generated from underwater landslides with evidence of ocean current erosion, lateral transport, and contourite deposition on the slope [2], [3], [4], and the importance of underwater geomorphology characteristics for disaster mitigation [5], [6], [7], [8]. Underwater landslides on the continental edge of the South China Sea consist of complex, slump landslides, and creeping landslides [9]. ...
... Quantitative analysis of the measurement of the more 16 0 has the potential for underwater spills and is supported by soft sediment surface sediment materials such as leach oil and leach sand. To determine the potential for a spill is, it is necessary to calculate the volume of debris material on the cliffs and the ocean currents [2]. Surface sediment was deposited on a slope and then in the eastern part a landslide or "mass movement" occurred, causing the right block to fall and form a scarp in a northwest-southeast direction. ...
... To determine the magnitude of the potential for landslides, the volume of boulder material on the canyon and sea currents must be calculated. Previous research has been conducted on triggering tsunamis and the distribution of mass transport deposits beneath the surface due to Indonesian Throughflow as a preconditioning mechanism for underwater landslides in the Makassar Strait [2]. ...
Article
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Krui Waters, Lampung, Indonesia, is an area that has the potential for a tsunami disaster. Local tsunamis can be caused by underwater landslides owing to ground movement and gravitational forces. Imaging underwater conditions using sonar and shallow seismic systems is an interesting research topic for geological disaster researchers. Underwater landslides are very important for disaster mitigation, especially for tsunami prediction. This research aims to identify the characteristics of underwater landslides around Krui Waters, Lampung, Indonesia, using imaging analysis methods from sonar data and a sub-bottom profiler (SBP). The underwater landslide phenomenon in this area is still not known in detail, and therefore requires further research. All data were obtained from the results of hydrographic, geological, and geophysical surveys of RV Baruna Jaya IV in 2020. The sonar data shows surface characteristics such as the presence of Furrows (Gravel Waves), Cobble (Boulder), as well as Canyons and landslides or mass movements rocks. SBP data show the characteristics of the seabed in the form of a canyon with a slope greater than 16 ⁰ . Finally, the morphological characteristics and features obtained could possibly be a potential source of local tsunamis owing to the high slope.
... Among the eastern area, the highest frequency of tsunami events for Indonesian archipelago takes place in Makassar Strait. Collaborative research has evaluated slope stability in the Makassar Strait as a result of a huge amount of sediment being transported from the Mahakam Delta [24]. The strait, in which is being known as the pathway for transferring water mass from Pacific to Indian Ocean can trigger slope failure due to massive sedimentation or erosion. ...
... It's located at the intersection of four major plates, such as the Indo-Australian Plate in the southern part, Eurasian Plate lied in the west, Pacific Plate in the east, and Philippine Sea Plate in the north-west area. Developing a complex system of subduction, back-arc-thrusting, extension and major transform zones [24]. The seabed features of Makassar Strait distinctly describe two steep gradient zones as the southern and northern boundaries representing the Pastenoster and Palu-Koro transform fault. ...
... The horizontal grid resolution is 100 m yielding in total 277 grids in x-direction and 295 grids in y-direction. Four tsunami sources were distributed in the Strait with the total volume 4, 8, 70, and 200 km 3 following geographic, oceanographic, and seismic study [24]. These sources are modelled as a rigid body falls into the water and it excludes the deformation effects. ...
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Tsunami modelling of potential landslide-induced tsunami in Makassar Strait is carried out to quantify possible damage to the nearby cities. The numerical model is used to represent the wave generation by using NHWAVE model. The simulations consist of a series of scenarios based on distinct size of the landslide volume. Four landslides with volume 5, 8, 70, and 200 km3 are used as tsunami sources in the initiation stage. The sources are evenly distributed in the strait addressing different landslide location. Maximum wave heights of 1.5 m are found in the area between Palu and Bangkir from case 1 and around Talok from case 2 simulations. The empirical run-up calculation of 7.5 m is estimated at the land for the presented wave height. The value significantly elevates the case 3 and 4 proportional to the volume values. The waves impact more than half of coastline with maximum value found in the Sulawesi side. Interestingly, wide and narrow shelf next to Kalimantan Island plays an important role in reducing the tsunami hazard level.
... Notably, this is an area of particular risk due to its proximity to the announced location of the de facto capital city. This is further exacerbated by a number of additional factors all contributing to the oversteepening and destabilization of the continental slope: sediment erosion and deposition generated by the Makassar Throughflow currents, sediment influx from the Mahakam River, carbonate growth, and frequent proximal seismic events (Brackenridge et al., 2020;Gumbira et al., 2021;Hutchings & Mooney, 2021). ...
... The dimensions of the slide (i.e., its thickness, length and width) are maintained at the default ratio set within TOPICS (to create a set of initial surface conditions to be used by the wave propagation model), with the additional limiting of the maximum thickness (th max ) depending on the location of the slide along the shelf. This is done to account for the varying possible slide thicknesses, in kilometers, as determined by the study of seismic sections (Brackenridge et al., 2020), and are limited simply according to latitude as: ...
... Submarine mass failures are notoriously difficult to catalog, and as such, most knowledge of the slope failures within the Makassar Strait comes from the aforementioned analysis of mass transport deposits from surveys of the seafloor. We use the mapped mass transport deposits published in Brackenridge et al. (2020) to construct a correlation between latitude and landslide volume, from which we calculate a new respective volume for each of the 50 training scenarios. ...
Article
Full-text available
This paper presents a significant advancement in the understanding of tsunamigenic landslide hazard across the length of the Makassar Strait in Indonesia. We use statistical emulation across the length of the continental slope to conduct a probabilistic assessment of tsunami hazard on a regional scale, across 14 virtual coastal gauges. Focusing on the potential maximum wave amplitudes (distance between the wave crest and the still‐water level) from possible tsunamigenic landslide events, we generate predictions from Gaussian Process emulators fitted to input‐outputs from 50 training scenarios. We show that the most probable maximum wave amplitudes in the majority of gauges are between 1 and 5 m, with the maximum predicted amplitudes reaching values of up to 10 m on the eastern coast, and up to 50 m on the western coast. We also explore the potential use of Gaussian multivariate copulas to sample emulator prediction input values to create a more realistic distribution of volumes along the continental slope. The novel use of statistical emulation across a whole slope enables the probabilistic assessment of tsunami hazard due to landslides on a regional scale. This area is of key interest to Indonesia since the new capital will be established in the East Kalimantan region on the western side of the Makassar Strait.
Article
Exchange flows past straits and sills play a vital role in controlling water exchange between parts of the oceans with different properties. We modelled such flows in the laboratory using a long tank with a constricted channel in the center, where a barrier was inserted. After barrier removal, the exchange occurred and the corresponding volume transport was measured. Varying channel geometries and shapes were used to investigate the effects of the geometry and shape on the transport. The scaling analysis made for laboratory non-rotating strait exchange flows served as a theoretical basis, giving an upper bound on the transport. The applications to real cases in nature, such as Indonesian Through-Flow (ITF) and world ocean straits provided a new insight into water exchange between ocean basins separated by bottom topography. The results showed that all the transports were less than the normalized value of 0.87 (uppermost), predicted by hydraulic theory for baroclinic strait exchanges with mixing but no friction. A further reduction in the transport was arguably due to frictional effects along the bottom boundary. For the world ocean straits considered in the present study, the normalized volume transports were in the range 0.40–0.86, with the highest was 0.86 recorded in the Gibraltar Strait, implying a relatively small transport reduction due to friction. For the ITF passages, the normalized transports ranged between 0.56 and 0.83, similar to those found for the world ocean straits. These are, in the first order, consistent with the measured transports in the range 0.78–0.86 from the laboratory experiments.
Chapter
The geological or earth processes and humans interact inevitably, giving impacts to modern society and civilization. Indonesia, a country in Southeast Asia, is prominent to its georisks and has experienced these earth processes that greatly impacted human lives. One of the most overlooked hazards is submarine landslides which pose hazards to the living environments by causing the loss of vital land along the coastline, the destruction of undersea facilities such as cables, pipelines, or oil wells, and, most critically, the damage of coastal communities caused by landslide-generated tsunamis. Assessing and minimizing these risks nearly always necessitates risk estimation in circumstances, where the potential associated with those various hazard events is difficult to gauge. Herein we will present a critical review of the exemplary cases of submarine landslides that trigger tsunamis to Indonesian coastal communities, how these impacted humans and building environments, as well as propose, in terms of human dimension; the interaction, mitigation, and coping mechanism to these earth processes.
Thesis
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Oceanic currents flowing near the seafloor erode, transport and deposit sediments, organic carbon, nutrients, and pollutants in deep-water sedimentary systems. Sediment deposits, which have mainly formed under the influence of these bottom currents (contourites) are high-resolution archives for reconstructing past ocean conditions. However, the interaction of sedimentary systems with oceanographic processes in deep-water environments is not well understood. The main objective of this thesis is to better understand the connection between contourites and the hydrodynamics that generate them in order to improve reconstructions of paleocurrents and sediment transport pathways. To achieve this objective a multidisciplinary study combining multibeam bathymetry, seismo-acoustic data, sediment samples, vessel-mounted Acoustic Doppler Current Profiler data, numerical modelling of ocean currents and three-dimensional flume tank experiments has been conducted. Elongated depressions (moats) and their associated drifts form at the northern Argentine margin on top of relatively flat seafloor surfaces (terraces) next to a steep slope. The main current direction is along-slope, and the speed is higher near the slope and decreases on the terrace basinwards. Flume tank experiments show that a moat-drift system forms if there is a secondary basinward flow near the seafloor. The secondary flow increases with higher speeds and steeper slopes, leading to steeper adjacent drifts. Once the moatdrift system has developed, the secondary circulation is confined by the drift into a helix structure. Simultaneously, the primary along-slope velocity is increased. The measured current speed over the moats from the Argentine continental margin and the Bahamas area is high (up to 63 cm/s at 150–200 m) and decreases by 5-48% over the associated drift. Measurements from 185 cross-sections of moat-drift systems distributed at 39 different locations worldwide show that moats at steeper slopes have a steeper drift and that the angle of the slope side is on average 1.6 times higher than the angle of the drift side. However, no statistical relation is found between latitude and moat-drift morphology or stratigraphy. The flume tank experiments show that moat-drift systems can form solely because of along-slope currents without any additional oceanographic processes as eddies, internal waves or ocean current surface fronts. However, Acoustic Doppler Current Profiler data and the hydrodynamic model from the Argentine continental margin show that eddies near the seafloor can form on a contourite terrace. These eddies might lead to the small erosion surfaces on the Ewing Terrace, even though it is mainly a depositional environment and currents are relatively weak (below 30 cm/s). Experiments show that the migration of the moat-drift system and the formation of internal stratigraphic architecture is a function of current strength in combination with sediment supply. The different stratigraphic types of more erosive and more depositional moat-drift systems have been observed in seismic data. A new sub-classification of moat-drift systems based on their stratigraphy is proposed. In summary, this study provides new insights into the interaction between ocean currents and sedimentary systems. It shows the importance of current strength, current variability (in time and space) and sediment supply for the formation of contourite systems. The combined data suggest that higher speeds and steeper slopes intensify the secondary flow, leading to steeper adjacent drifts. Thus, the morphology and internal architecture of the moat-drift systems can be used as a paleo-velocimeter. Furthermore, this study identifies the need for more in situ measurements near the seafloor.
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Changes in convective activity and hydroclimate over Northeastern Kalimantan are key features to understand glacial to interglacial climate evolution in the center of the West Pacific Warm Pool during the Late Pleistocene to Holocene. We use high-resolution X-ray fluorescence (XRF) scanner-derived elemental ratios in sediment Core SO217-18522 (1º 24.106’ N, 119º 4.701’E, 975 m water depth) recovered from the northern Makassar Strait to reconstruct changes in precipitation-related weathering and erosion over Northeastern Kalimantan over the last 50 kyr. Enhanced seasonality of rainfall and an extended dry season during Heinrich Stadials (HS4 to HS1) and the Younger Dryas (YD) suggest weakening of the tropical convection associated with a southward shift of the tropical rain belt and the annual mean position of the Intertropical Tropical Convection Zone. Increasing sediment discharge and intensification of convective activity occurred during the early to mid-Holocene during an interval of high Northern Hemisphere insolation, elevated atmospheric p CO 2 levels and global warming. Our reconstructions in comparison with regional terrestrial and marine records highlight the high spatial variability of Kalimantan hydroclimate on millennial to glacial-interglacial timescales.
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On Dec. 22, 2018, at approximately 20:55–57 local time, Anak Krakatau volcano, located in the Sunda Straits of Indonesia, experienced a major lateral collapse during a period of eruptive activity that began in June. The collapse discharged volcaniclastic material into the 250 m deep caldera southwest of the volcano, which generated a tsunami with runups of up to 13 m on the adjacent coasts of Sumatra and Java. The tsunami caused at least 437 fatalities, the greatest number from a volcanically-induced tsunami since the catastrophic explosive eruption of Krakatau in 1883 and the sector collapse of Ritter Island in 1888. For the first time in over 100 years, the 2018 Anak Krakatau event provides an opportunity to study a major volcanically-generated tsunami that caused widespread loss of life and significant damage. Here, we present numerical simulations of the tsunami, with state-of the-art numerical models, based on a combined landslide-source and bathymetric dataset. We constrain the geometry and magnitude of the landslide source through analyses of pre- and post-event satellite images and aerial photography, which demonstrate that the primary landslide scar bisected the Anak Krakatau volcano, cutting behind the central vent and removing 50% of its subaerial extent. Estimated submarine collapse geometries result in a primary landslide volume range of 0.22–0.30 km3, which is used to initialize a tsunami generation and propagation model with two different landslide rheologies (granular and fluid). Observations of a single tsunami, with no subsequent waves, are consistent with our interpretation of landslide failure in a rapid, single phase of movement rather than a more piecemeal process, generating a tsunami which reached nearby coastlines within ~30 minutes. Both modelled rheologies successfully reproduce observed tsunami characteristics from post-event field survey results, tide gauge records, and eyewitness reports, suggesting our estimated landslide volume range is appropriate. This event highlights the significant hazard posed by relatively small-scale lateral volcanic collapses, which can occur en-masse, without any precursory signals, and are an efficient and unpredictable tsunami source. Our successful simulations demonstrate that current numerical models can accurately forecast tsunami hazards from these events. In cases such as Anak Krakatau’s, the absence of precursory warning signals together with the short travel time following tsunami initiation present a major challenge for mitigating tsunami coastal impact.
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The 28 September 2018 Sulawesi earthquake generated a much larger tsunami than expected from its Mw = 7.5 magnitude and from its dominant strike-slip mechanism. Within a few minutes after the earthquake, the tsunami devastated the seafront of Palu bay, destroying houses and infrastructures over a few hundred meters. Coastal subsidence and slumping at various locations around the bay were also observed. There is debate in the scientific community as to whether submarine landslides and shore collapses contributed to the generation of strong and destructive waves locally. The objective of this study is threefold: first, to determine whether standard seismic inversions could predict the source in the context of tsunami early warning; second, to define a new seismic source built from optical image correlation and based on the geological and tectonic context; third, to assess whether the earthquake alone is able to generate up to 9-m wave heights at the coast. Numerical simulations of the tsunami propagation are performed for different seismic dislocation sources. Nonlinear shallow water equations are solved by a finite-difference method in grids with 200-m and 10-m resolutions. The early CMT focal solutions calculated by seismological institutes show dominant strike-slip mechanisms with a homogenous slip distribution. These sources produce maximum tsunami heights of 40-cm on the coast of Palu city. Two heterogeneous sources are tested and compared: the USGS “finite fault” model calculated from seismic inversion and a new “hybrid” source inferred from different techniques. The latter is based on a segmented fault in agreement with the geological context and built from both from seismic parameters of a CMT solution and the observed horizontal ground displacements. This source produces water wave heights of 4 to 5-m in the Palu bay. The observed inundation heights and distances are reproduced satisfactorily by the model at Pantoloan and at the southwestern tip of Palu bay. However, the “hybrid” source is unable to reproduce the largest 8 to 12-m water heights as reported from field surveys. Thus, even though this “hybrid” source produces most of the reported tsunami energy, we cannot exclude that the numerous coastal collapses observed in Palu bay contributed to increase the local tsunami run-up.
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On 28 September 2018, a 7.7 Mw strike-slip earthquake struck Central Sulawesi at 0.18°S, 119.85°E, ~ 10 km northwest of Palu, the capital of Central Sulawesi Province, affecting more than 230,000 people. The rapid arrival of the wave suggests a tsunami source within Palu Bay. A bathymetric campaign was conducted in Palu Bay and the region south of Cape Manimbaya from 9 to 18 October 2018. One of the goals was to identify a potential source for the tsunami. The bathymetry data show a prominent submarine channel within Palu Bay with sinuosity controlled by the Palu-Koro Fault. The submarine channel continues into deeper water south and west of Cape Manimbaya with a sinuosity index of up to 1.770. Several submarine slumps were identified within the bay, due to the arcuate head scarp, the abrupt change in slope and debris on the seafloor, or the undulating shape of the slope. One possible source location near the mouth of Palu Bay is identified from the estimated volume of displaced sediment. The bathymetry data also show traces of the Palu-Koro Fault and suggestions of uplift of Cape Manimbaya related to activity of the Palu-Koro Fault.
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The Antarctic Circumpolar Current (ACC) spills across the Falkland Plateau into the South Atlantic as a series of high-velocity jets. These currents are a driving force for global overturning circulation, and affect climate by modulating CO2 exchange between the atmosphere and ocean, but their timing of onset remains controversial. We present new evidence of strong currents associated with the Subantarctic Front (SAF) jet since the earliest Oligocene (~34 Ma) based on a widespread erosional surface on the Falkland Plateau, preserved below a 30,000 km2 contourite sand deposit. This is the largest such feature ever to be recognized, and provides the most robust constraint of the initiation of the SAF to date. By contrast, the South Falkland Slope Drift is dominated by contourite mud of Pleistocene-Recent age, substantially younger than previous estimates, indicating a significant decrease in long-term current strength at that time. As ACC strength is primarily a function of the position of the South-Westerly Winds, our data indicates that associated currents are likely to increase substantially in a warming world. Likely implications include increased upwelling and associated carbon flux from the deep ocean to the atmosphere, a positive feedback loop not included in most future projections of atmospheric CO2.
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Plain Language Summary Tsunami hazard assessment is routinely based on assessing the impacts of long‐period waves generated by vertical seafloor motions reaching the coast tens of minutes after the earthquake in typical subduction‐zone environments. This view is inadequate for assessing hazard associated with strike‐slip earthquakes such as the magnitude 7.5 2018 Palu earthquake, which resulted in tsunami effects much larger than would normally be associated with horizontal fault motion. From an extraordinary collection of 38 amateur and closed circuit television videos we estimated tsunami arrival times, amplitudes, and wave periods at different locations around Palu Bay, where the most damaging waves were reported. We found that the Palu tsunamis devastated widely separated coastal areas within a few minutes after the mainshock and included unusually short wave periods, which cannot be explained by the earthquake fault slip alone. Post‐tsunami surveys show changes in the coastline, and this combined with video footage provides potential locations of submarine landslides as tsunami sources that would match the arrival times of the waves. Our results emphasize the importance of estimating tsunami hazards along coastlines bordering strike‐slip fault systems and have broad implications for considering shorter‐period nearly instantaneous tsunamis in hazard mitigation and tsunami early warning systems.
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Tsunami waves severely damaged the densely populated coast of Palu City immediately after the 2018 Mw. 7.5 Sulawesi Earthquake. Among the several tsunami waves that arrived to the city, the two initial waveforms were most likely generated by a landslide at the southwestern shore of Palu Bay, about 5 km away from the one of the city's shopping malls. The authors accurately identified the arrival time and direction of the waves by comparing multiple videos taken by a pilot from the cockpit of a plane and local people who witnessed waves approaching the coast. Although the authors' bathymetric survey only covered a limited area of 0.78 km 2 , it was found that about 3.2 million m 3 of mass disappeared from it, causing a maximum decrease in the seabed elevation of 40 m. A landslide scarp up to 5 m height was also investigated in the southwestern shore of the bay, which seems to be relatively minor compared to the submarine mass failure. Visible clue for liquefaction was not observed at this particular site. A simplified numerical model suggests that the landslide tsunami propagated as an edge wave and split into two separate waves due to the presence of an underwater shallow area just north of Palu City. Both waves arrived to the coast of this city within several minutes: one from NorthWest and the other from the North. Three major waves were witnessed by residents, who felt horizontal and vertical ground movements and heard the sound of an explosion just after the earthquake. Wave splash exceeded the height of trees on the beach. Given the results the authors conclude that any modern early warning system is unlikely to work well against such short-warning time tsunamis, and thus it is necessary for disaster risk managers to consider a way to help people quickly become aware of the potential disaster and evacuate.
Article
The Subantarctic Front (SAF), one of the three main jets of the Antarctic Circumpolar Current (ACC), flows through a narrow gap in the North Scotia Ridge and then north-westward across the continental slope of Burdwood Bank, ~150 km south of the Falkland Islands. There, the SAF flows across a fold-and-thrust belt caused by oblique convergence at the active plate boundary between the Scotia Plate and South American Plate. We here use regional 2D and 3D seismic reflection data to show the interaction of the associated bottom currents with the active margin, particularly to understand the causes and consequences of a number of large submarine landslides located in the adjacent foredeep. Kinematic indicators from the landslide deposits show that they are derived from a single point source located in an embayment on the northern slope of Burdwood Bank, where we identify a large contourite drift deposit. This drift forms the depositional sink for an along-slope sediment routing system driven by currents associated with the SAF, with sediment being eroded from the Burdwood Terrace, transported ~200 km westward, and plastered against the middle-upper continental slope. The contourite drift is undercut by the core of the current, making the slope inherently unstable in this area. Numerical modelling of the landslides and resultant waves indicates the tsunamigenic potential of these events. Modelled peak wave elevations of up to 40 m inundate the southern coast of the Falklands for a ~100 km3 volume landslide, with a recurrence interval of 1 Ma or less. This research highlights preconditioning mechanisms for submarine failure on continental slopes dominated by strong ocean currents, and specifically, oceanographic controls on the frequency, magnitude and location of submarine landslides associated with contourite systems.
Article
Volcanogenic tsunamis are one of the deadliest volcanic phenomena. Understanding their triggering processes, and mitigating their effect, remains a major challenge. On 22 December 2018, flank failure of the Anak Krakatau volcano in Indonesia generated a tsunami that killed more than 400 people. This event was captured in unprecedented detail by high-resolution satellite imagery and eyewitness accounts. Here we combine historic observations with these recent data to—for the first time—interpret the internal architecture of Anak Krakatau, and reconstruct the failure, tsunamigenesis, and regrowth processes observed. We calculate the volume of material initially lost from the volcano flank failure and find that it was relatively small (~0.1 km3) compared to the overall changes observed during the entire eruption, but it was nonetheless able to generate rapid tsunami waves with devastating impacts. The flank failure also changed the eruption style and the upper volcanic plumbing system, with the subsequent explosive eruptions destroying the summit and then partially rebuilding the lost flank. The nature of the flank failure was controlled by the internal structure of the island, and—although regrowth rate will be a primary control on flank failure intervals—the reconfiguring of the volcano’s internal vent network is likely to have re-stabilized it in the medium term. The findings demonstrate that hazard assessments at ocean islands must consider that even small flank failures, during unexceptional eruptions, can have catastrophic consequences.