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Geological Society, London, Special Publications
doi: 10.1144/SP361.7
p79-90.
2012, v.361;Geological Society, London, Special Publications
T. Giachetti, R. Paris, K. Kelfoun and B. Ontowirjo
Krakatau Volcano, Sunda Strait, Indonesia
Tsunami hazard related to a flank collapse of Anak
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Tsunami hazard related to a flank collapse of Anak Krakatau
Volcano, Sunda Strait, Indonesia
T. GIACHETTI
1,3
*, R. PARIS
2,4,6
, K. KELFOUN
2,4,6
& B. ONTOWIRJO
5
1
Clermont Universite
´
, Universite
´
Blaise Pascal, Geolab, BP 10448,
F-63000 Clermont-Ferrand, France
2
Clermont Universite
´
, Universite
´
Blaise Pascal, Laboratoire Magmas et Volcans, BP 10448,
F-63000 Clermont-Ferrand, France
3
CNRS, UMR 6042, Geolab, F-63057 Clermont-Ferrand, France
4
CNRS, UMR 6524, LMV, F-63038 Clermont-Ferrand, France
5
Coastal Dynamics Research Center, BPDP-BPPT, 11th Floor, Building 2, BPPT, Jl,
M. H. Thamrin no 8, Jakarta 10340, Indonesia
6
IRD, R 163, LMV, F-63038 Clermont-Ferrand, France
*Corresponding author (e-mail: giachettithomas@club-internet.fr)
Abstract: Numerical modelling of a rapid, partial destabilization of Anak Krakatau Volcano
(Indonesia) was performed in order to investigate the tsunami triggered by this event. Anak
Krakatau, which is largely built on the steep NE wall of the 1883 Krakatau eruption caldera, is
active on its SW side (towards the 1883 caldera), which makes the edifice quite unstable. A
hypothetical 0.280 km
3
flank collapse directed southwestwards would trigger an initial wave
43 m in height that would reach the islands of Sertung, Panjang and Rakata in less than 1 min,
with amplitudes from 15 to 30 m. These waves would be potentially dangerous for the many
small tourist boats circulating in, and around, the Krakatau Archipelago. The waves would then
propagate in a radial manner from the impact region and across the Sunda Strait, at an average
speed of 80–110 km h
21
. The tsunami would reach the cities located on the western coast of
Java (e.g. Merak, Anyer and Carita.) 35 – 45 min after the onset of collapse, with a maximum
amplitude from 1.5 (Merak and Panimbang) to 3.4 m (Labuhan). As many industrial and tourist
infrastructures are located close to the sea and at altitudes of less than 10 m, these waves
present a non-negligible risk. Owing to numerous reflections inside the Krakatau Archipelago,
the waves would even affect Bandar Lampung (Sumatra, c. 900 000 inhabitants) after more than
1 h, with a maximum amplitude of 0.3 m. The waves produced would be far smaller than those
occurring during the 1883 Krakatau eruption (c. 15 m) and a rapid detection of the collapse by
the volcano observatory, together with an efficient alert system on the coast, would possibly
prevent this hypothetical event from being deadly.
Most recorded historical tsunamis have a seismic
origin, but such events may also be triggered by
phenomena related to huge volcanic eruptions,
such as large pyroclastic flows entering the water
(e.g. de Lange et al. 2001; Maeno & Imamura
2007), submarine explosions (e.g. Mader & Gittings
2006), caldera collapse (e.g. Nomanbhoy & Satake
1995; Maeno et al. 2006) or by a large, rapidly
sliding mass impacting the water (e.g. Tinti et al.
1999, 2000, 2006; Keating & McGuire 2000;
Ward 2001; Harbitz et al. 2006; Fritz et al. 2008;
Waythomas et al. 2009; Kelfoun et al. 2010). The
December 2002 17 ! 10
6
m
3
flank collapse of
Stromboli triggered a 8 m-high run-up on the coast
of Stromboli, but had little effect on coasts located
more than 200 km from the collapse (Maramai
et al. 2005). The tsunami generated by the
30 ! 10
6
m
3
Lituya Bay collapse in Alaska in
1958 (Fritz et al. 2001) reached 60 m at 6 km later-
ally from the collapse and 30 m at 12 km. These tsu-
namis had very few fatalities as they occurred either
in isolated locations (Lituya Bay, Alaska) or during
a period of no tourist activity (Stromboli). The
largest lateral collapse of an island volcano recorded
in historical times (c. 5 km
3
) took place during the
1888 eruption of Ritter Island (New Guinea), produ-
cing witnessed waves of up to 10 –15 m at tens to
hundreds of kilometres from the source (Ward &
Day 2003). With 15 000 fatalities, the tsunami
generated by the 1792 sector collapse of Mount
From:Terry, J. P. & Goff, J. (eds) 2012. Natural Hazards in the Asia – Pacific Region: Recent Advances and Emerging
Concepts. Geological Society, London, Special Publications, 361, 79 – 90, http:// dx.doi.org/10.1144/SP361.7
# The Geological Society of London 2012. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics
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Mayuyama in Ariake Bay (Kyushu Island, Unzen
volcanic complex) was the second worst disaster
in Japan, and the second deadliest volcanic
tsunami (after that produced by the eruption of
Krakatau in 1883). The failure was most probably
triggered by a strong earthquake, and its volume
was about 340 ! 10
6
m
3
(Michiue et al. 1999).
Tsunami run-ups ranged from 8 to 24 m on the
opposite side of Ariake Bay (Tsuji & Hino 1993).
The 26– 28 August 1883 Plinian eruption of
Krakatau Volcano, and its subsequent tsunamis,
caused more than 35 000 casualties along the
coasts of the Sunda Strait in Indonesia (Self &
Rampino 1981; Simkin & Fiske 1983; Sigurdsson
et al. 1991a, b). This eruption was one of the most
powerful and devastating eruptions in recorded
history. Many tsunamis were produced during this
approximately 2 day eruption, the largest one occur-
ring after 10 a.m. on the 27 August (Warton & Evans
1888; Yokoyama 1981). The leading wave reached
the cities of Anyer and Merak on Java after 35–
40 min, and after approximately 1 h for the city of
Bandar Lampung (Teluk Betung) on Sumatra. A
tide gauge located near Jakarta (Batavia Harbour,
Java) registered the wave arrival approximately
140 min after its inferred initiation at Krakatau
Island. Using the tsunami run-ups determined
along the coasts of Java and Sumatra (Verbeek
1885), the tsunami heights before run-up were esti-
mated to be about 15 m at the coastline all around
the Sunda Strait (Symons 1888). The generation
mechanism of these 1883 tsunamis is still contro-
versial and several processes may have acted suc-
cessively or together (Self & Rampino 1981;
Yokoyama 1981; Camus & Vincent 1983; Francis
1985). Based on low-resolution numerical simu-
lations, Nomanbhoy & Satake (1995) concluded
that a series of submarine explosions over a period
of 1– 5 min was the most probable source for the
major tsunami. Nevertheless, pyroclastic flows
formed by the gravitational collapse of the eruptive
columns are also a possible source for most of the
tsunamis observed before and during the paroxysm
(Carey et al. 1996; de Lange et al. 2001).
Nearly 45 years after this 1883 cataclysmal erup-
tion, Anak Krakatau (‘Child of Krakatau’ in Indone-
sian) emerged from the sea in the same location as
the former Krakatau, and has since grown to its
current height of more than 300 m (Hoffmann-
Rothe et al. 2006). It exhibits frequent activity,
still posing a risk to the coastal population of Java
and Sumatra, and for the important shipping routes
through the Sunda Strait. Following the active
phase of Anak Krakatau in 1980, a permanent
volcano observatory was established in Pasauran
on the western coast of Java, about 50 km east of
the Krakatau Archipelago. A short-period seism-
ometer placed on the volcano flank, visual control
and daily seismic event statistics are used to deter-
mine the current alert level, on the basis of which
Indonesian authorities decide about preventive
measures, sometimes prohibiting tourism around
the archipelago (Hoffmann-Rothe et al. 2006).
One possible major hazard emerging from Anak
Krakatau would be a tsunami triggered by a collapse
of its flank, as the volcano is partly built on a steep
wall of the caldera resulting from the 1883 eruption.
A small tsunami (c. 2 m high) was experienced on
Rakata Island in October 1981 during an awakening
of Anak Krakatau (Camus et al. 1987). In the
present study, we numerically simulate a sudden
southwestwards destabilization of a large part of
the Anak Krakatau Volcano, and the subsequent
tsunami formation and propagation. We show
results concerning the time of arrival and the ampli-
tude of the waves produced, both in the Sunda Strait
and on the coasts of Java and Sumatra. We then
discuss the relationships between the morphology
of Anak Krakatau, the locations of the surrounding
islands, the bathymetry of the strait and the
triggered waves.
Geography, population and infrastructures
in the Sunda Strait
The Sunda Strait, in which Anak Krakatau Volcano
lies, has a roughly NE– SW orientation, with a
minimum width of 24 km at its NE end between
Sumatra and Java (Fig. 1). Its western end is deep
(,21500 m), but it shallows significantly as it
narrows to the east, with a depth of only about
20 m in parts of the eastern end, making it difficult
to navigate due to sandbanks and strong tidal
flows. The numerous islands in the strait and the
nearby surrounding regions of Java and Sumatra
were devastated by the 1883 Krakatau eruption.
The eruption drastically altered the topography of
the strait, with approximately 12 km
3
(DRE, dense
rock equivalent) of ignimbrite being deposited
around the volcano (Carey et al. 1996). The small
to moderate volcanic explosions of Anak Krakatau,
which is partly built on the site of the former
Krakatau Island, attract tourist boats that circulate
between the islands of the Krakatau Archipelago.
Some areas have never been resettled since the
1883 eruption (e.g. the SW of Java), but much of
the coastline is now densely populated, especially
in Bandar Lampung (c. 900 000 inhabitants) on
Sumatra, and on the west coast of the Cilegon
District (c. 400 000 inhabitants) in Java (Fig. 1).
Moreover, many of the roads on western Java and
southern Sumatra are located near the sea and at
low altitude (,10 m), as well as important econ-
omic infrastructures such as power stations (e.g.
Labuhan, NE of Merak and SE of Banda
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Lampung), industries (e.g. steel industries in
Cilegon), major harbours connecting Java and
Sumatra (Merak, Bakaheuni), and tourist resorts
(e.g. Anyer, Kalianda). There are also several oil
platforms in the strait, notably off the Java coast.
Such infrastructures would potentially be badly
affected by a tsunami of several metres, as was pro-
duced during the 1883 eruption.
In October 2007, the Indonesian government
planned the construction of a 30 km road and
railway connection between the islands of Sumatra
and Java (the Selat Sunda Bridge), across the
26 km Sunda Strait, at an altitude of 70 m asl
(above sea level). In 2009, the ‘pre-feasibility’
study for this 10 billion dollar project was com-
pleted and the construction is expected to begin in
2012. Owing to the seismic and volcanic activity
in the Sunda region, this project faces many chal-
lenges. Krakatau Volcano is located only 40 km
away from the future bridge. Some of the bridge’s
piles may suffer from tsunamis crossing the Sunda
Strait, therefore such hazards need to be quantified.
Anak Krakatau Volcano: evolution and
actual morphology
Anak Krakatau first rose up out of the sea in 1928,
sited just off the steep NE wall of the basin
formed by the collapse of the 1883 Krakatau erup-
tion caldera. This volcano was built where the
main vent for the 1883 eruption is supposed to
have been located, about midway between the
former craters of Danan and Perbuatan (Deplus
Fig. 1. Shaded relief representation of the DEM (100 m resolution) of Sunda Strait, based on ASTER topography,
GEBCO bathymetry and a digitization of the bathymetric map of Krakatau from Deplus et al. (1995, their fig. 7). This
DEM is the calculation grid used to simulate the Anak Krakatau landslide and the subsequent tsunami propagation
(calculations were made at a resolution of 200 m). The main coastal cities or important infrastructures around the Sunda
Strait are indicated by red diamonds. The black frame around the Krakatau Archipelago corresponds to the limits of
Figure 2b, c. Geographical co-ordinates are in metres.
TSUNAMI HAZARD AT ANAK KRAKATAU VOLCANO 81
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et al. 1995). Between 1928 and 1930, the volcano
receded and reappeared three times until it estab-
lished itself permanently above sea level. In 1959,
an uninterrupted 152 m-high hyaloclastic tuff-ring
developed (Sudradjat 1982) and a lake formed in
the crater. The eruption style was Surtseyan during
the 1928– 1930 period (Stehn 1929; Camus et al.
1987), then Vulcanian until 1960, before shifting
to Strombolian explosions that created a cone reach-
ing 200 m asl in 1981 (Oba et al. 1983). In 1981, a
Vulcanian eruption marked a southwestwards shift
of Anak Krakatau activity (Sudrajat 1982) with
more differentiated volcanic products (acid ande-
sites, dacites) than previously erupted (mainly
basalts and andesites before 1981: Camus et al.
1987). At the time of writing, the latest eruption of
Anak Krakatau, which started on 25 October 2010,
is still ongoing, with dense ash clouds forming
plumes 100 –1000 m high.
Rapid soundings in 1928 have shown that the
western slope of the volcano was considerably
steeper (.288) than the eastern, as a consequence
of its position on the steep wall of the basin and
also of the strong current that is generally running
from SW to NE (Stehn 1929). Deplus et al. (1995)
showed that this slope was still in existence in
1995, and that the successive eruptions had not
resulted in an infilling of the caldera. According to
these data concerning the steep slopes on which
Anak Krakatau is built and the fact that this vol-
cano is growing towards the SW, landslides along
its southwestern flank cannot be excluded (Deplus
et al. 1995). Such a landslide would be directed
southwestwards into the 1883 caldera and would
trigger waves that would propagate into the Sunda
Strait, possibly affecting the Indonesian coasts.
Methodology
Digital elevation model used and scenario
envisaged
The collapse of the Anak Krakatau Volcano was
simulated on a digital elevation model (DEM)
obtained by merging the ASTER (Advanced
Spaceborne Thermal Emission and Reflection
Radiometer) topography (c. 30 m resolution),
bathymetric maps (one from Dishidros Indonesian
Navy and a Sunda Strait navigation chart) and
the GEBCO (General Bathymetric Chart of the
Oceans) bathymetry (c. 900 m resolution) of the
whole Sunda Strait region (Fig. 1). In addition,
the bathymetric map of the Krakatau Archipelago
from Deplus et al. (1995, their fig. 7) was digitized
and added to the DEM in order to obtain a better res-
olution of the zone where the collapse occurs and
where the waves are initially produced (Fig. 2b).
The final DEM produced, which is the calculation
grid used for the numerical simulation, is a
1500 ! 1300 pixel grid with a spatial resolution of
100 m (Fig. 1). In order to maximize on the best
spatial resolution available to register the initial
waves produced, some of the simulations were per-
formed on a portion of the grid centred on the land-
slide event. Owing to the long calculation times we
down-sampled the grid by a factor of 2 (i.e.
750 ! 650 pixel calculation grid and a spatial resol-
ution of 200 m) for the simulations of tsunami
propagation over the entire Sunda Strait area.
Some level lines of the DEM were modified to
build the sliding surface of the hypothetical land-
slide; that is, to define the hypothetical collapse
scar. This was done so that: (1) the upper end of
the scar is broadly defined by the limit between
the older tuff-ring and the new cone (Fig. 2a, c);
(2) the base of the scar lies at the bottom of the
1883 caldera (Fig. 2a); and (3) the scar is horseshoe-
shaped (Fig. 2c). The scar is oriented southwest-
wards, with an average slope of 8.28 (Fig. 2a) and
a width of c. 1.9 km, defining a collapsing volume
of 0.280 km
3
. This scar probably cuts the NE wall
of the 1883 caldera, but this cannot be clearly
traced on the DEM as no precise bathymetric data
immediately following the 1883 eruption are avail-
able. In our simulation, the debris avalanche is
released in a single event.
Numerical model
We used the numerical code VolcFlow (Kelfoun
et al. 2010; Giachetti et al. 2011) to simulate both
the Anak Krakatau landslide and the tsunami propa-
gations. A full explanation of the code and equations
is given in the previously cited papers. This code is
based on the two-dimensional (2D) depth-average
approach, modified to incorporate 3D interactions
with greater accuracy; both the landslide and the
sea water being simulated using the general
shallow-water equations of mass conservation and
momentum balance. In the model, the water interacts
with the bathymetry/topography and floods onto the
land, but waves breaking and other complex
second-order 3D effects are not taken into account,
and sediment erosion and transport are also ignored.
We simulated the water propagation using
a density of 1000 kg m
23
and a viscosity of
0.001 Pa s. As emissions from Anak Krakatau are
mainly composed of scoriaceous material with a
basaltic (common) to dacitic (rarer) chemical com-
position (Sudradjat 1982; Camus et al. 1987), we
used a density of 1500 kg m
23
to simulate the land-
slide. Kelfoun et al. (2010) and Giachetti et al.
(2011) showed that the rheology used to simulate
the landslide propagation may be important when
dealing with second-order variations of the profile
T. GIACHETTI ET AL.82
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and amplitude of the triggered waves. Thus, we
tested four sets of rheological parameters to simu-
late the debris avalanche propagation: a commonly
used Mohr –Coulomb frictional law with a basal
friction angle of 18 or 28 (hereafter referred to as
rheologies 1 and 2, respectively) and a constant
retarding stress of 5 or 10 kPa (rheologies 3 and 4,
respectively). Although the Mohr –Coulomb fric-
tional law is often used in granular-flow dynamics
because it represents the behaviour of deposits at
rest and of sand flows in the laboratory, the constant
retarding stress appears to be better adapted to the
reproduction of the extent, thickness on all slopes
and some morphological features of natural deposits
(e.g. Dade & Huppert 1998; Kelfoun & Druitt
2005). Figure 3a shows that the surface area
covered by the simulated debris avalanche deposits
varies depending on the rheology used (the numeri-
cal deposits obtained using rheologies 1– 2 and
rheologies 3–4 are quasi-identical and are thus
drawn together). Figure 3b presents the water
surface displacement recorded using a gauge
placed approximately 15 km southwestwards from
the landslide scar (black diamond in Fig. 3a), in
Fig. 3. (a) Simulated debris avalanche deposits obtained using rheologies 1 –2 (grey) and 3 – 4 (black hatching and
black) to simulate the landslide propagation. (b) Simulated water surface displacement recorded at the gauge located in
Figure 2a (black diamond). This figure shows that the waves produced are very similar, whatever the rheology used to
simulate the landslide propagation.
Fig. 2. (a) Cross-section of Anak Krakatau (inset: Fig. 1) and the 1883 eruption caldera. The landslide scar, defined by
modifying some level lines on our initial DEM, is drawn in black. It is orientated southwestwards, with a slope of 8.28,
delimiting a collapsing volume of about 0.28 km
3
.(b) Topography before the simulated landslide, with the location
of the cross-section presented in (a). The caldera resulting from the 1883 Krakatau eruption is clearly visible,
as well as Anak Krakatau, which is built on the NE flank of this caldera. (c) Topography after the simulated
landslide, with the horseshoe-shaped scar clearly visible.
TSUNAMI HAZARD AT ANAK KRAKATAU VOLCANO 83
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the direction of propagation of the major triggered
waves, where the water depth is aproximately
100 m. It shows that the wave profiles and ampli-
tudes created are very similar, whichever of the
four rheologies are used. The maximum amplitude
recorded at the gauge placed approximately 15 km
southwestwards varies between 11 (rheology 2)
and 12 m (rheology 3).
We believe that the similarity between the wave
profiles presented in Figure 3b is due to the initial
geometry of the collapsing volume and the landslide
scar. Indeed, as the collapsing volume is initially
partly submerged and the landslide scar directs the
debris avalanche southwestwards, the initial waves
triggered by the landslide– water impact are poorly
influenced by the rheology used to simulate land-
slide propagation. This rheology, however, plays a
role in the final run-out of the modelled debris ava-
lanche deposits (Fig. 3a). The morphology of the
modelled deposits (not shown here) is very similar
whatever the rheology used because of the dominant
controlling factor of the structure of the 1883
caldera. The rheology used to simulate the landslide
propagation is also responsible for the small
second-order discrepancies existing between the
wave profiles registered, which are amplified over
time (Fig. 3b). However, since in this paper we
focus on the tsunami hazards and not on the simu-
lated morphology of the debris avalanche deposits,
we arbitrarily chose the constant retarding stress of
10 kPa (rheology 4) to simulate the landslide pro-
pagation for the whole calculation grid.
Results
When interacting with the water, the debris ava-
lanche triggers waves whose maximum initial
amplitude is around 45 m, measured approxi-
mately 45 s after the collapse onset at 2.5 km south-
westwards from the landslide scar. The waves
produced then propagate in a radial manner away
from the impact region, reaching the islands of
Sertung, Panjang and Rakata (Fig. 3a) in less than
1 min, with amplitudes from 15 to 30 m. Owing to
the southwestwards propagation of the landslide,
the highest waves are produced in this direction.
The wave profile obtained about 15 km SW from
the landslide scar (Fig. 3b, rheology 4) shows a
first wave with an amplitude of 11.3 m and a
period of around 162 s (wavelength of c. 3.4 km).
This is followed by another 5.3 m wave, with a
smaller period of approximately 60 s (wavelength
of c. 1.3 km). This is then followed by several
smaller and shorter waves, the sea level regaining
its initial position after a few tens of minutes. The
travel time of the first wave is shown in Figure 4,
and is given more precisely in Table 1 for the
main coastal cities and infrastructures located in
Figure 1. The cities situated on the western coast
of Java are all touched by the first wave between
36 and 47 min after the onset of the Anak Krakatau
collapse. The first wave reaches Kalianda and
Bandar Lampung, located on Sumatra, 45 and
68 min after the onset of the collapse, respectively.
Note that everywhere in the Sunda Strait the wave-
length of the simulated waves is always more than
25 times the water depth. This demonstrates that
the use of the general shallow-water equations of
mass conservation and momentum balance to simu-
late the water propagation is appropriate in this case
(e.g. Synolakis et al. 1997).
Figure 5 presents the maximum wave amplitude
registered over 6000 s of simulation. It shows that
the highest waves are mainly concentrated around
Fig. 4. First wave travel time (expressed in min) for the
first 90 min of simulation. Black lines are at 2 min
intervals. Main coastal cities (names in Fig. 1) are shown
by red diamonds. The impact of the sudden increase in
water depth westwards from the Krakatau Archipelago is
clearly seen (see Fig. 1 for bathymetry), the waves being
more rapid than those crossing the shallow strait. BL,
Bandar Lampung; K, Kalianda; M, Merak; A, Anyer; C,
Carita; L, Labuhan; P, Panimbang. The simulation of the
landslide propagation was carried out using a constant
retarding stress of 10 kPa.
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the Krakatau Archipelago, as it is the location of the
triggering event, and their amplitude logically
decreases away from Anak Krakatau. Westwards,
at about 20 km from the landslide, the wave ampli-
tude is slightly reduced because of the strong
increase in water depth, and waves do not exceed
1–2 m when they reach the western edge of the
calculation grid. The highest waves produced are
directed southwestwards and their amplitude dec-
reases when they reach greater water depths in the
SW. However, they still have an amplitude of
more than 3 –4 m when they arrive near Panaitan
and near the southwestern coast of Java (Ujung
Kulon National Park). The presence of the islands
of Sertung and Rakata (Fig. 3a) – NW and SW of
the landslide, respectively – also causes the wave
amplitude to be reduced. The maximum amplitude
of the waves recorded northwards and northeast-
wards is not related to the first wave produced. It
appears that they come from the reflection of the
initial waves off the coasts of Sertung and Rakata
(the former consisting of a high cliff orientated
NNW). However, owing to the numerous inter-
actions of the waves with the four islands of the
Krakatau Archipelago, it is difficult to establish
exactly what happens near the impact point.
Figure 5 also shows some reflections of the waves,
in particular off the western coast of Java.
Figure 6 presents the evolution of the water level
over 6500 s of simulation, recorded at gauges placed
in the sea a few hundreds of metres (,900 m) off
eight of the main coastal cities or infrastructures
of the Sunda Strait (located in Fig. 1). The gauges
were placed in the sea near the coasts to free the sea-
level profiles recorded from the 3D interactions
that the program fails to reproduce in an accurate
manner. The maximum wave amplitudes measured
at these gauges are indicated in Table 1 (the vertical
water depth at each gauge is indicated in Table 1).
The water-level profiles are different from one city
to another, being complicated by numerous reflec-
tions of the waves throughout the Krakatau Archipe-
lago, as well as around the Sumatran and Javanese
coasts. All of the cities are touched by a first positive
wave with amplitude ranging from 0.3 to 2.3 m, but
this first wave is never the highest one. Near Bandar
Lampung and Kalianda, the maximum wave ampli-
tude measured is 0.3 and 2.7 m respectively, and the
coastal cities of western Java are generally affected
by waves with maximums of between 1.2 (Sumur)
and 3.4 m (Labuhan).
Discussion
Influence of the initial parameters on the
wave characteristics
The volume of a debris avalanche and the way it
occurs (e.g. in one go, by retrogressive failures)
are the parameters that most influence the character-
istics of the triggered tsunami (Locat et al. 2004;
Giachetti et al. 2011). In the present case, the
hypothetical scar has a slope of 8.28, for an initial
Anak Krakatau average slope of 24.28 (Fig. 3a).
These values are lower than those observed for
other scars of debris avalanches that triggered tsuna-
mis, like the Palos Verdes debris avalanche (Califor-
nia, scar slope of 108 –178: Locat et al. 2004) or 29
submarine events identified at Stromboli (average
scar slope of c. 258, and pre-failure slope of c. 288
for debris avalanches between 5 and 200 m b.s.l.:
Casalbore et al. 2011). In this study, we decided to
base the structural definition of the hypothetical
scar on the known structural evolution of Anak Kra-
katau: the upper end of the scar being defined by the
limit between the older tuff-ring and the new cone,
and its base by the bottom of the 1883 caldera.
Therefore, our numerical model of Anak Krakatau
involves a debris avalanche volume of 0.280 km
3
.
The definition of a steeper scar (closer to the
values observed by Locat et al. 2004 or Casalbore
et al. 2011) would lead to a more rapid landslide
into the water, and thus possibly to higher waves.
However, a steeper scar would also result in a
smaller collapsing volume (considering the lower
end of the scar as fixed) and thus to slightly smaller
waves. Since in this study our aim is to quantify
the tsunami hazard linked to a realistic partial flank
collapse of Anak Krakatau, we decided to maximize
the volume involved in the debris avalanche (and
Table 1. Travel time and maximum wave amplitude recorded at gauges located close (,900 m) to the main
coastal cities of the Sunda Strait (see Fig. 1)
Bandar
Lampung
(213 m)
Kalianda
(25 m)
Merak
(212 m)
Anyer
(212 m)
Carita
(212 m)
Labuhan
(24 m)
Panimbang
(22 m)
Sumur
(27 m)
Travel time (min) 68 44 47 38 37 40 43 36
Maximum wave
amplitude (m)
0.3 2.7 1.5 1.4 2.9 3.4 1.5 1.2
For each city, the initial water depth at the gauge site is given in brackets.
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thus the waves produced) while remaining consist-
ent with the structure of the volcano.
Influence of the bathymetry/topography
on the tsunami characteristics
To define the initial volume that would hypotheti-
cally collapse, we used the available topography
data (ASTER data, spatial resolution of 30 m) for
Anak Krakatau Island. However, there is no
up-to-date high-resolution topography and bathy-
metry data for this volcano, whose morphology
changes rapidly due to its numerous eruptions. For
this reason, we think that high-resolution topo-
graphical and bathymetric surveys of the Anak
Krakatau Volcano should be performed in order to
Fig. 5. Maximum wave amplitude (m) recorded over 6000 s of simulation, using a constant retarding stress of 10 kPa to
simulate the landslide propagation.
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improve the accuracy when defining the initial
conditions of the landslide. Side-scan sonar sur-
veys coupled with INSAR (Interferometric Syn-
thetic Aperture Radar) monitoring may also reveal
evidence of slope instability. The travel time map
of the first wave based on the simulations (Fig. 4)
is consistent with the refraction diagram of the
tsunami caused by the 1883 Krakatau erup-
tion (Yokoyama 1981). However, the wave travel
time estimated may suffer from artefacts in the
600 m off Bandar Lampung
–1
0
1
1000 2000 3000 4000 5000 6000
Time (s)
Sea level (m)
300 m of Kalianda
–4
–2
0
2
4
1000 2000 3000 4000 5000 6000
Time (s)
Sea level (m)
700 m off Merak
–3
–2
–1
0
1
2
3
1000 2000 3000 4000 5000 6000
Time (s)
Sea level (m)
700 m off Anyer
–2
–1
0
1
2
1000 2000 3000 4000 5000 6000
Time (s)
Sea level (m)
900 m off Carita
–12
–8
–4
0
4
8
12
1000 2000 3000 4000 5000 6000
Time (s)
Sea level (m)
700 m off Labuhan
–
9
–
6
–
3
0
3
6
9
1000 2000 3000 4000 5000 6000
Time (s)
Sea level (m)
300 m off Panimbang
–3
–2
–1
0
1
2
3
1000 2000 3000 4000 5000 6000
Time (s)
Sea level (m)
600 m off Sumur
–2
–1
0
1
2
1000 2000 3000 4000 5000 6000
Time (s)
Sea level (m)
Fig. 6. Simulated sea-level profiles (m) registered several hundred metres (indicated on the plots) off eight of the
main coastal cities located in Figure 1. The simulation of the landslide propagation was carried out using a constant
retarding stress of 10 kPa. Time is expressed in seconds after the collapse onset. The water depth below each gauge is
indicated in Table 1.
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bathymetry used for calculations. The inclusion of
bathymetric maps of parts of Sunda Strait in the
constructed DEM allowed us to minimize these
artefacts, but new bathymetric maps of this region
would be useful for a better tsunami hazard
assessment.
Tsunami hazards
Our simulation shows that the first wave produced
has a maximum amplitude of about 45 m. This
height is reached at approximately 2.5 km SW of
the landslide scar (i.e. Anak Krakatau Island),
inside the Krakatau Archipelago. Moreover, the
waves produced reach the surrounding islands of
Sertung, Rakata and Panjang in less than 1 min,
with heights of up to 30 m. These waves could be
a serious hazard for the many tourist boats that
visit these islands every day. Further from the Kra-
katau Archipelago, the wave amplitude decreases in
Sunda Strait and waves are generally smaller than
10 m at a distance of more than 20 km from the
landslide scar. However, these waves could still be
dangerous for the small boats crossing the strait
between the Krakatau Archipelago and the coasts
of Java or Sumatra. It should be noted that the
islands of the Krakatau Archipelago (Anak Kraka-
tau, Sertung, Panjang and Rakata), as well as those
of Sebesi and Sebuku in the NE, those of Legundi
and Siuntjal in the NNW, and Panaitan in the
SSW are uninhabited, and thus the risk is drastically
reduced. Between the two islands of Java and
Sumatra, where the planned bridge is to be con-
structed (see the explanation in the earlier section
on ‘Geography, population and infrastructures in
the Sunda Strait’), the waves do not reach more
than 3.8 m, and the construction should be able to
absorb the strain developed by such a wave.
Our numerical simulation of the sudden collapse
of Anak Krakatau Volcano into the 1883 caldera
shows that all the coasts around the Sunda Strait
could potentially be affected by waves of more
than 1.0 m in less than 1 h after the event. Even
the southern coasts of Sumatra, which are located
more than 40 km to the north of the landslide,
would be touched by the tsunami because of the
numerous wave reflections off the islands of the
Krakatau Archipelago. All of the main cities or
infrastructures of the Sunda Strait would be affected
within 1 h of the collapse. The highest waves regis-
tered off these coastal cities are those near Labuhan
(3.4 m) on the western coast of Java, but most of the
gauges give values of less than 3 m for the highest
wave. These values are far less than those observed
during the 1883 Krakatau eruption, which reached
an average value of 15 m on the coasts of Sumatra
and Java (Symons 1888; Yokoyama 1981), with a
local wave height of up to 30 m. Moreover,
Figure 5 shows that some parts of the coast are
partially protected by the numerous islands in
Sunda Strait (e.g. Rakata prevents the propagation
of very high waves towards the large bay off
Panimbang). Waves become smaller with increas-
ing distance from the triggering event. During the
1883 tsunami, Jakarta was touched by a wave
approximately 1.8 m high about 140 min after the
eruption of Krakatau, whereas Merak and Anyer
were touched by 15 m-high waves. Likewise, the
1883 tsunami also reached locations thousands
of kilometres from the volcano (Choi et al. 2003,
Pelinovsky et al. 2005). Considering that the
maximum wave height recorded off Anyer and
Merak is around 1.5 m in our simulation, we
believe that the tsunami triggered by a flank collapse
at Anak Krakatau would be negligible at Jakarta.
Conclusion
Our numerical simulation shows that a partial desta-
bilization (0.28 km
3
) of Anak Krakatau Volcano
towards the SW would possibly be dangerous on a
local scale (tourist and fishing activities around the
volcano) or even on a regional scale (coasts of
Sumatra and Java). This event would trigger an
initial wave of 43 m that would reach all of the
islands in the Krakatau Archipelago in less than
1 min, with amplitudes ranging from 15 to 30 m,
and would be extremely dangerous for boats in the
Krakatau Archipelago. Waves would then pro-
pagate in a radial manner across Sunda Strait at
an average speed of 80–110 km h
21
, the first
wave reaching cities on the western coast of Java
after 35– 45 min, with a maximum amplitude of
between 2.9 (Carita) and 3.4 m (Labuhan). These
waves would be considerably smaller than those pro-
duced during the 1883 Krakatau eruption (average
wave height of c. 15 m around the Sunda Strait).
Owing to the high population, the concentration
of road and industrial infrastructure along some
parts of the exposed coasts of Java and Sumatra,
and the low elevation of much of this land, the
tsunami might present a significant risk. However,
as the travel time of the tsunami is several tens of
minutes between the Krakatau Archipelago and
the main cities along these coasts, a rapid detection
of the collapse by the volcano observatory, coupled
with an efficient alert system on the coast, could
prevent this hypothetical event from being deadly.
A tsunami preparedness project was initiated in
2006 by UNESCO and the Indonesian Institute of
Sciences (LIPI). However, it should be noted that
the ground deformation of the volcano is not perma-
nently monitored, and the available data (e.g. bathy-
metry) are not sufficient to allow for an accurate
assessment of slope instability.
T. GIACHETTI ET AL.88
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The example of Krakatau Volcano illustrates the
point that tsunamis generated by volcanic eruptions
and flank instability are a neglected hazard. They
represent 25% of all the fatalities directly attribu-
table to volcanoes during the last 250 years (Latter
1981; Bege
´
t 2000). At least 115 volcanic tsunamis
have been observed since 1600 AD (death toll
.54 000), with 36 events during the nineteenth
century and 54 events during the twentieth. Volca-
nic tsunamis can be dangerous because they can
occur with little warning, and cause devastation at
great distances. South Asian and South Pacific
regions are particularly exposed to volcanic tsuna-
mis because of the high density of active volcanoes
located near the coasts (volcanic island arcs).
Systematic monitoring of flank instability and the
integration of tsunamis into volcanic hazard assess-
ments (e.g. maps, evacuation routes) would reduce
the impact of future events.
This work is part of the ‘Vitesss’ project (Volcano-Induced
Tsunamis: numErical Simulations and Sedimentary Signa-
ture) supported by the French National Research Agency
(ANR project 08-JCJC-0042) and whose leader is
R. Paris (Geolab, CNRS). ASTER GDEM is a product of
METI and NASA. We thank two anonymous reviewers
for their constructive reviews of this manuscript. We are
also grateful to Anaı
¨
s Ferot who first suggested we
perform this study.
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