Cumbre Vieja Volcano -- Potential collapse and tsunami at
La Palma, Canary Islands
Steven N. Ward
Institute of Geophysics and Planetary Physics, University of California, Santa Cruz California, USA

Simon Day
Benfield Greig Hazard Research Centre, Department of Geological Sciences, University College, London, UK

Abstract. Geological evidence suggests that during a future
eruption, Cumbre Vieja Volcano on the Island of La Palmamay experience a catastrophic failure of its west flank, dropping 150 to 500 km 3 of rock into the sea. Using a geologicallyreasonable estimate of landslide motion, we model tsunami
waves produced by such a collapse. Waves generated by the
run-out of a 500 km3 (150 km 3) slide block at 100 m/s could
transit the entire Atlantic Basin and arrive on the coasts of theAmericas with 10-25 m (3-8 m) height.

1. Lateral collapse of island volcanoes -- Atsunami wave source
Lateral collapses of oceanic island volcanoes rank amongst
the most spectacular natural events on Earth. Although nosuch lateral collapse punctuates the historical past, residualdebris found on the seafloor evidence their abundance in recent geological time. Moore (1964) first identified the remains of lateral collapses off the flanks of Hawaii. Since then,dozens have been recognized adjacent to island volcanoes innearly every ocean (Moore et al. 1994; Keating and McGuire,2000). These observations constrain not only the geography
and frequency of lateral collapses, but also their magnitude(up to 5000 km 3 of material), extent (to 300 km length) andferocity (underwater speeds to 140 m/s).
Tsunami list among the many hazards associated with lateral collapses. Admittedly, direct geological evidence of tsunami in identifiable deposits (Moore and Moore, 1984) orcoastal erosion features (Young and Bryant, 1992) are controversial. Still, history has documented large and damaging
tsunami from far smaller lateral collapses of stratovolcanoes
in island arc environments (Johnson, 1987; Satake and Kato,
2001). Prudence dictates that the lack of abundant wavecaused signatures associated with collapses of oceanic island
volcanoes be viewed as more a function of scanty preservation
than evidence that these events do not produce tsunami.
Hazard from collapse tsunami may be particularly important in the Atlantic Ocean because of both the number of active oceanic islands there and the recent proposals (Day et al.,1999a, 1999b) that at least two of these volcanoes show signsof incipient instability. It seems timely then, for this paper toinvestigate the consequences of tsunami waves induced by acollapse of one of these unstable volcanoes -- Cumbre Vieja
on the island of La Palma, Canary Islands (Figure 1).

2. Geological evidence for a future collapse of
the Cumbre Vieja
During most if not all of the past 125ka, Cumbre Vieja has
been the most active volcano in the Canary Islands (Carracedo et al., 1999). Subaerial Cumbre Vieja forms the southern third of the island of La Palma (Figure 2), rising 2 km
above sea level with average slopes of 15° to 20°. The early
Holocene has seen major changes in Cumbre Vieja. Day et al.

Figure 1. Inset. Canary Island chain off the western coast of
Africa. Above. Location of La Palma Island, home to Cumbre
Vieja volcano. As evidenced by the abundant landslide deposits strewn about their bases, the Canary Island volcanoes
have experienced at least a dozen major collapses in the past
several million years.
(1999a) observe that, over the last several thousand years, thedistribution and orientation of vents and feeder dykes within
the mountain have shifted from a triple rift system (typical of
most oceanic island volcanoes) to one consisting of a single
N-S rift with westward extending vent arrays. They argue thatthese structural re-organizations are in response to evolving
stress patterns associated with the growth of a detachment
fault under the volcano's west flank. Coincident with the mostrecent eruption of the Cumbre Vieja in 1949 (Bonelli Rubio,1950), the steeply inclined headwall section of this detachment surfaced as a west-dipping normal fault along the crestof the volcano (see Figure 2). The scarp extended 4 km with amaximum offset of 4 m. The appearance of surface rupture isominous because: (1) initial subsurface development of adetachment fault, (2) its later propagation to the surface, and(3) ultimate slide block failure, typically sequences landslide
development (Martel and Muller, 2000). Detailed examinationof the 1949 rupture and geodetic measurements in the period
1994-1998 (Moss et al., 1999) indicate that the fault has been
inactive since 1949. Inactivity is not unexpected however,
because the triggering of flank instability on steep volcanoes
generally requires additional destabilizing influences such as
dyke emplacement or pressurization of trapped groundwater(Elsworth and Voight, 1995). These events often accompany a
volcano's eruptive phases.
This line of reasoning leads us to believe that a future eruption near the summit of the Cumbre Vieja will likely trigger aflank failure. To estimate the surface extent, subsurface geometry and total volume of such a failure we turn to geological evidence and comparisons with existing lateral collapse
scars. Because the breadth of the Holocene structural changes
in Cumbre Vieja appears to have affected the entire subaerial
edifice, Day et al. (1999a) conclude that the developing detachment now underlies most if not all of the western flank of

WARD AND DAY: LA PALMA COLLAPSE AND TSUANMI
the volcano. The unstable block above the detachment extends to the north and south at least 15 km, however its length
may be greater as these edges lack surface expression. The
1949 fault break skirts the crest of the volcano about 8 km
inland and it marks the eastern boundary of the presently unstable zone. Because lateral collapses typically cut across thecrest of volcanoes and into their reverse slopes (e.g. Mount St.
Helens, Voight et al., 1983), we place the head of the future La
Palma collapse 2 to 3 km east of the 1949 rupture (Figure 2).
The western boundary of the unstable block lies hidden underwater. Bathymetric and imaging sonar surveys of older
collapses at La Palma and elsewhere (Watts and Masson, 1995;
Urgeles et. al., 1999) suggest that the toe of the block surfaces
in 1 to 3 km water depth -- about 5 to 10 km offshore.
The best geological evidence that we have paints a Cumbre
Vieja collapse sending down a slide block 15-20 km wide and15-25 km long. The thickness of the slide block is not easily
fixed. Mapping the depth to the detachment surface by locating earthquakes that occur on it has not been possible. No
records exist of seismic activity associated with the 1949
eruption, or the subsequent 1971 eruption at the island's
southern tip. No other tectonic earthquakes of consequence
have struck under La Palma in the last three decades either.
Nevertheless, characteristics of past collapses point to a listricdetachment 2 to 3 km below the summit of the volcano. Toward the west, the surface dips seawards at a shallow angle tointersect the offshore toe. Toward the east, the detachment
steepens sharply to intersect the surface within a few km of themountain's crest. In consideration of everything, we believethat a future flank failure of Cumbre Vieja volcano will dislodge a broadly wedge-shaped slide block as cartooned at thebottom of Figure 2. The volume and mean thickness of rockparticipating in a flank failure depends upon the detailed
shape of the basal surface, but they should fall in the range of
150 to 500 km 3 and 1 to 2 km respectively. The inferred geometry and volume of the expected failure coincide closelywith features of the previous La Palma collapse (~566 ka),remains of which are still visible to the north on Cumbre
Nueva (Day et al., 1999a).

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the source with many, small simple slides. These are thenadded up in a number and combination needed to represent
adequately the spatial and temporal history of the entire landslide. A simple slide element is rectangular, with length L andwidth W. On these elements, a landslide of constant thickness
T and step function time dependence, starts along one widthof the rectangle at rs=0 and t s=0, and runs down its length atslide velocity v r. If r is not too close to the slide, and t>L/v r,then (1) becomes in a non-uniform depth ocean,
surf

uz

(r,t) ≈∞TLW
k (ω)J (ωT(ω, r, r0 ))cos[ωt+X( ω, θ)]∫ dω 0 0u (ω )cosh[(k2π 000 (ω)h(r0 )](2)sinX(ω,θ) sinY(ω, θ)G(r, r0 )SL (ω, r, r0 )×X(ω, θ) Y(ω,θ)

where X(ω,θ)=L(k0(ω)cosθ-ω/v r)/2; Y(ω,θ)=W(k0(ω)sinθ)/2,and θ is angle between the slide direction and the observation
point. The k0(ω) and u 0(ω) are the wavenumber and group velocity now specific to frequency ω in water of depth h(r0) at
the source. The new terms T(ω,r,r0), G(r,r0) and S L (ω,r,r0) in (2)account for changes in travel time, and wave height due togeometrical spreading and shoaling in oceans of variable
depth. Their functional form can be found in Ward (2001). The
advantage to the "simple slide" approach (2) versus the general expression (1) is that the integrals over r0 and t 0 can bedone analytically The downside is that a large number of simple slide elements might be needed.

3. Landslide Tsunami Model - Generalities
The section above provides a feeling for the size and shapeof the block that may slide into the sea during a lateral collapse of Cumbre Vieja. What magnitude of tsunami might thiscollapse induce? One straightforward means to address thisquestion employs classical, linear wave theory. Consider auniform ocean of depth h. Under this theory, a general verticalbotbottom disturbance (i.e. the landslide) u z (r,t) starting at t=0
stimulates surface tsunami waveforms (vertical component) at
observation point r of (Ward, 2001)∞

u surfz (r,t) = ∫0dω k(ω)∫ dr0 J 0 (k(ω) r − r0 )2πu(ω )cosh(k( ω)h) A(t)(1)

t× ∫ dt 0u˙ botz (r0 ,t 0 )cos[ ω(t − t 0 )]
0

In
(1),
k
is
wavenumber,ωisfrequency
ω={gk(ω)tanh[k(ω)h]}1/2 , dr0=dx 0dy 0, superscript •=∂/∂t andJ 0(x) is the cylindrical Bessel function of order zero. The second integral covers area A(t) that includes all points r0 whereu˙ botz ( r0 , t 0 ) ≠0 for t 0<t. Any number of strategies can be used
to evaluate (1) given a kinematic prescription of the landslideu bot
z (r,t). Mostly, the strategies reduce to finding an efficientmeans to compute the three integrals and to adapting theequation to non-uniform depth oceans. One approach shingles

Figure 2. Map of La Palma Island showing the major geological deposits together with the visible and inferred headscarpsof the potential slide block pictured at the bottom.
Figure 3. Time/space history of the La Palma landslide simulation. The upper left panel shows the "shingling" of the collapse scar (light gray sack) with many simple slides. The
black, dark gray/dotted, and light gray regions map areas
excavated, covered, and unchanged, versus time during theslide. The mass movement took 10 minutes to complete andreached 60 km out to sea.

4. La Palma Landslide Model - Specifics
To model the potential tsunami generated from a collapseof Cumbre Vieja, let's consider a worst case (in terms of slidevolume) -- a 500 km 3 block, 25 km long, 15 km wide, and1400 m thick that breaks away and spills westward into thedeep ocean. Judging from the shape of the previous La Palma
slide and past collapses of similar volume elsewhere in the
Canaries, we suppose that material in the 375 km 2 excavation
will cascade down the steep offshore slope for about 60 km

WARD AND DAY: LA PALMA COLLAPSE AND TSUANMI

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Figure 4. Evolution of the La Palma landslide tsunami from 2 minutes (a, upper left) to 9 hours (i, lower right). Red and blue contours cover elevated and depressed regions of the ocean respectively and the yellow dots and numbers sample the wave height,
positive or negative, in meters. Note the strong influence of dispersion in spreading out an original impulse into a long series ofwaves of decreasing wavelength. See also that the peak amplitudes generally do not coincide with the first wave. Even after crossing the Atlantic, a lateral collapse of Cumbre Vieja volcano could impose a great sequence of waves of 10-25 m height on the
shores of the Americas.
until it reaches the flat ocean floor at 4000 m depth. To run
these distances, slides likely raft on a highly pressurized layerof mud or fault gouge breccia (Day, 1996; Van Wyk de Vries etal., 2001) that reduces basal friction and permits rapid acceleration. We further imagine that the slide block travels as aunit for 15 km out to the slope break before it begins to tearapart. The run-out sheet formed from the disintegrating block
will then thin, expand laterally, and eventually cover a jugshaped region about 3,500 km2.
To compute the tsunami, we cover the postulated collapse
scar with 96 simple slides, 6 km square (upper left, Figure 3).
These elements fire off (some more than once) as the front orback of the slide passes. Each firing of a simple slide calls forevaluation of expression (2). The total tsunami field is thensummed for fixed times and many positions r. The sequence
pictured in Figure 3 mimics the progression of a break-away,
hat-shaped block initially sliding intact down slope, and thendisintegrating into a flow covering 2,826 km2 to 177 m depth.

We chose the latter value so that the volume deposited equals
the volume excavated. Although many kinematic histories canbe proposed, we envision an accelerating block that quickly
reaches a peak velocity of 100 m/s before it disintegrates intoa run-out. Tsunami generation is most efficient when landslide velocity approaches the local water wave speed
gh(Ward, 2001). Because the ocean depth at the La Palma slide
generally exceeds 2000 m, a 100 m/s slide velocity substantially lags the tsunami speed, even near the shore. Proportionally larger or smaller waves could be generated by selecting ahigher or lower peak slide velocity (see Section 6). Note finally that because the slide block originates mostly above sea
level, none of the excavation (black color, Figure 3) was permitted to be tsunami-producing.

5. La Palma Tsunami

WARD AND DAY: LA PALMA COLLAPSE AND TSUANMI
Figure 4 contours tsunami height relative to sea level fromour La Palma landslide model. Within 2 minutes of the initial
failure (Figure 4a), a water dome has built atop the sliding
block to 900 m height -- only somewhat less than the thickness of the block. Within 5 minutes (Figure 4b), the fasttraveling initial wave crest has outrun the now disintegratinglandslide front. The leading wave height has dropped to 500
m after 50 km of travel. Large negative waves now appear behind the leading crest. These are due partly to rebound of theinitial dome and partly due to the passing of the back of theslide block that drops the water column (dark gray squares tolight gray squares in Figure 3). At 10 minutes (Figure 4c), theslide has run its course. The tsunami disturbance has grown to
250 km in diameter and several hundred-meter high waves
have rolled up the shores of the three westernmost islands of
the Canary chain. Note the relatively non-directional characterof the wave pattern and that already, the leading positive wave
(200 m) is no longer the largest. Several negative and positive
ones 2-3 times larger trail behind. From 15 to 60 minutes(Figure 4d-f), waves sweep eastward through the rest of the
Canary Islands and 50-100 m waves make first landfall on the
African mainland. Upon nearing the West Saharan shore, thetsunami waves slow, and crush together (Figure 4f). In contrast, toward the west, a great train of dispersed waves 500 km
across, develops as the tsunami moves into the Atlantic basin.
Peak wave heights (60 m) there show up in the second crest.
From 3 to 6 hours (Figure 4g,h), the tsunami expands acrossthe Atlantic retaining palpable amplitude in an arc subtending
more than 180 degrees. Toward the northeast, Spain and England experience 5 to 7 m waves. La Palma Island itself blocked
most of the radiation in this direction. Vanguards of the tsunami (10 m) first brush North America near Newfoundland.
Simultaneously, larger (15-20 m) waves arrive at the northshore of South America. At 9 hours (Figure 4i), Florida faces
the tsunami, now parading in a dozen cycles or more. In 50 mof water offshore Cape Canaveral, even after being weakened
by geometrical spreading and frequency dispersion, tsunami
from lateral collapses of the volume, dimension, and speed ofthat expected at La Palma could retain 20-25 m height. Shoaling waves do not continue to grow much in water shallower
than their height, so 20-25 m probably reflects the terminal
height of the waves expected on Florida's beaches.

6. Conclusions
Geological evidence suggests that during a future eruption,
Cumbre Vieja Volcano on the Island of La Palma may experience a catastrophic flank collapse. For a 500 km 3 slide blockrunning westward 60 km down the offshore slope at 100 m/s,
our computer models predict that tsunami waves 10 to 25 mhigh will be felt at transoceanic distances spanning azimuths
that target most of the Atlantic basin. Simulations of othercollapse scenarios indicate that for slides that do not run too
close to the tsunami wave speed, peak tsunami amplitude follows roughly in proportion to landslide volume times peaklandslide velocity. (The proportionality is locationdependent, and it holds more strictly for volume and lessstrictly for peak velocity.) Thus, more modest assumptions on
the size and peak speed of the slide make for smaller waves.
For instance, a 250 km3 block running westward 60 km at 50m/s generates tsunami with about 1/4 to 3/8 the amplitude ofthose presented above.
In the past million years, dozens of lateral collapse landslides of a size comparable to the one considered here havebeen shed from volcanic islands in the Atlantic. If our models
are correct, tsunami from these incidents should have washedseveral times over most coasts that have good exposure to thesea. A test of these predictions lies in whether tsunami deposits associated with specific collapses can be identified, dated,

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and widely correlated. Ironically, because of the more favorable preservation conditions underwater, evidence of collapse
tsunami may be more widespread on the continental shelf thanon land. Still, the low-lying, tectonically stable, non-glaciatedmargins of west Africa, the southeast United States and northeast Brazil, together with the Bahamas carbonate platform,should be particularly suitable sites for geologists to search
for footprints of these occasional visitors.

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S. N. Ward, IGPP, University of California, Santa Cruz, CA 95064
USA (ward@uplift .ucsc.edu).
S. Day, Benfield Greig Hazard Research Centre, Department of
Geological Science, University College, London, United Kingdom(Received March 2, 2001; revised May 11, accepted June 27, 2001.)
Copyright 2001 by the American Geophysical Union
Paper number 2001GL000000.
0148-0227/01/2001GL000000$09.00

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