TSUNAMI GENERATING SOURCE MECHANISMS
THE EASTERN CARIBBEAN REGION
on presentation made at the 2004 National Science Foundation
Tsunami Workshop in San Juan, Puerto Rico, and paper published
in the Journal of Tsunami Hazards, Volume 22, Number 2. 2004
eruptions, volcanic island flank failures and underwater slides
have generated numerous destructive tsunamis in the Caribbean
region. Convergent, compressional and collisional tectonic activity
caused primarily from the eastward movement of the Caribbean
Plate in relation to the North American and South American Plates,
is responsible for zones of subduction in the region, the formation
of island arcs and the evolution of particular volcanic centers
on the overlying plate.
Caribbean tectonic plate and its Volcanic Island Arcs
The inter-plate tectonic
interaction and deformation along these marginal boundaries result
in moderate seismic and volcanic events that can generate tsunamis
by a number of different mechanisms. The active geo-dynamic processes
have created the Lesser Antilles, an arc of small islands with
volcanoes characterized by both effusive and explosive activity.
Eruption mechanisms of these Caribbean volcanoes are complex
and often anomalous. Collapses of lava domes often precede major
eruptions, which may vary in intensity from Strombolian to Plinian.
Locally catastrophic, short-period tsunami-like waves can be
generated directly by lateral, direct or channelized volcanic
blast episodes, or in combination with collateral air pressure
perturbations, nuess ardentes, pyroclastic flows, lahars, or
cascading debris avalanches. Submarine volcanic caldera collapses
can also generate local destructive tsunami waves. Volcanoes
in the Eastern Caribbean Region have unstable flanks. Destructive
local tsunamis may be generated from aerial and submarine volcanic
edifice mass edifice flank failures, which may be triggered by
volcanic episodes, lava dome collapses, or simply by gravitational
instabilities. The present report evaluates volcanic mechanisms,
resulting flank failure processes and their potential for tsunami
generation. More specifically, the report evaluates recent volcanic
eruption mechanisms of the Soufriere Hills volcano on Montserrat,
of Mt. Pelée on Martinique, of Soufriere on St. Vincent
and of the Kick'em Jenny underwater volcano near Grenada and
provides an overall risk assessment of tsunami generation from
volcanic sources in the Caribbean region.
Tectonic deformation and active
geo-dynamic processes in the Caribbean region have produced distinct
seismic and volcanic activity sources capable of generating destructive
tsunamis. The historic record documents numerous events in this
region for the last 400 years (Lander et al. 2002, 2003; ETDB/ATL:
Expert Tsunami Database for the Atlantic, 2002). The list includes
tsunamis from distant sources, such as that generated by the
1755 Lisbon earthquake. Eighty-eight tsunamis from local and
distant sources have been reported for the 1489 to 1998 time
period. Several of these were generated by volcanic eruptions
and by collateral volcanic flank failures, debris avalanches,
and landslides. A rough evaluation of the cumulative frequency
of tsunamis was done specifically for the Barbados and Antigua
(Zahibo and Pelinovsky, 2001).
Present Tectonics of the Caribbean Region
found submarine debris avalanches on the sea floor around many
islands in the Lesser Antilles suggest that large scale landslides
and volcanic island flank collapses must have generated tsunamis
in the distant past (Deplus et a 2001). Also, extremely large
paleotsunami activity has been postulated for the Southern region
of the Leeward Lesser Antilles, consisting of the islands of
Aruba, Curacao and Bonaire (Scheffers, 2002) within the tectonically
active Caribbean - South American plate boundary zone and the
West Indies Island Arc. However, it remains to be determined
whether the extremely large boulders and rocks found on these
islands, some at high elevations, are indeed deposits from paleotsunamis
- as speculated and what the source regions may have been.
There have been several numerical
modeling studies of tsunamis in the Caribbean and tsunami travel
time charts for the region have been prepared (Weissert, 1990).
Additionally, historical tsunamis of seismic origin have been
extensively documented and numerically simulated. Some of the
best-studied historic events are the 1867 Virgin Island earthquake
and tsunami (Devill, 1867; Reid and Taber, 1920. Zahibo et al
2003) and the 1918 tsunami in Puerto Rico (Mercado and McCann,
1998). The heights of tsunami waves from distant sources
such as those generated by the 1755 Lisbon earthquake or from
a postulated, massive landslide on La Palma, Canary Islands,
have been estimated with numerical models (Mader, 2001a, 2001b).
The threat of mega tsunami generation from the same postulated
massive slope failure of the Cumbre Vieja stratovolcano on La
Palma and the far field effects in the Eastern Atlantic from
such an unlikely event, have been assessed (Pararas-Carayannis,
Volcanoes of the Eastern Caribbean Island Arc (Modified web graphic
of Lesales, mount-pelee.com)
The flank instabilities
of the island volcanoes in the Lesser Antilles are well known
and adequately documented (Le Friant, 2001). In the last decade,
tsunamis generated by landslides and flank failures that followed
eruptions of the Soufriere Hills volcano on Montserrat Island
and an increased level of activity of the submarine volcano known
as Kick'em Jenny, north of Grenada, have raised concerns about
the generation of destructive tsunamis from these and other volcanic
sources in the region. Because of such concerns, water waves
generated by potential debris avalanches and landslides on Montserrat
were numerically simulated and evaluated (Heinrich et al., 1998,
1999a,b, 2001; Mangeney et al., 2000; Zahibo and Pelinovsky,
2001). Similarly, a gravitational dome collapse of the Soufriere
Hills volcano on Montserrat Island and the resulting pyroclastic
flows that could generate tsunamis were modeled (Hooper and Mattioli,
2001). Finally, numerical simulations of Kick'em Jenny's explosions
were completed (Ginsler et al 2003, 2004).
Scope of Present Study: The
present evaluation extends on these investigations by reviewing
primarily the eruption mechanisms of some of the more active
volcanoes of the Eastern Caribbean Region and the factors that
contribute to their instabilities, massive flank failures, and
tsunami generation. Active tectonics, subduction, block movements
inferred from tectonic studies, seismological and geodetic data,
and the evolution, geochemistry and eruption mechanisms of particular
volcanic centers are reviewed for the purpose of evaluating volcanic
explosivity factors in general - and potential collateral
mass edifice failures in particular - that may result in
tsunami generation. Specifically reviewed are historical eruptions
and flank collapse events of Soufriere Hills on Montserrat, of
Mt. Pelée on Martinique, of Soufriere on St. Vincent,
and of Kick'em Jenny near Grenada. Based on an analysis of the
eruptive and mass failure mechanisms of these volcanoes, the
risk of tsunami generation from such sources is evaluated for
the entire Eastern Caribbean Region. Finally, through analogies
and comparisons of differences and similarities of volcanic compositions
and failure mechanisms, conclusions are drawn about the tsunamigenic
efficiency of such processes, and the near and far field effects
of tsunami waves generated by historical events or that can be
expected in the future from postulated massive edifice flank
collapses of other volcanoes in the Caribbean region and around
SETTINGS DEVELOPMENT OF VOLCANIC CENTERS IN THE EASTERN
generation mechanisms from volcanic sources in the Caribbean
requires a brief review of the tectonic setting of the region
and of the development of volcanic centers. The present review
focuses on the presently volcanically active Eastern region,
where most of the recent tsunamis of volcanic origin have occurred.
Eastern Caribbean Subduction and Volcanic Island- Arc Mechanism
( web graphic of Lesales, mount-pelee.com)
The Caribbean plate
forms part of a region of great geologic and geographic diversities.
It extends from southern N. America to northern S. America. It
includes Central America, thousands of islands, and the oceanic
areas in between.
Caribbean has undergone many changes during its complex evolution
(AAPG, 2003). There have been plate migrations, hotspot and mantle
plume activity, island arc development and disappearance, subduction
reversals, opening of young oceanic basins, major plate migration,
and major block rotations. The Caribbean Plate is largely oceanic,
although it carries large continental fragments in its western
Currently, the Caribbean plate is moving eastward in relation
to the North and South American Plates at a rate of approximately
20 millimeters per year. The plate movement is responsible for
the zones of subduction along the active boundaries and the formation
of the West Indies Volcanic Island Arc on the overlying plate
in the eastern region. These interactions lead to a moderate
level of inter-plate seismicity and interplate and intraplate
Development of tensional volcanic fore-arc and back-arc
Presently, the Caribbean
region is characterized by convergent, compressional and collisional
tectonic activity, which results in frequent occurrences of earthquakes
and volcanic eruptions. Often, localized landslides and volcanic
island mass edifice failures are collaterally triggered. Most
of these events occur near or along the geotectonically active
plate boundaries and can generate local tsunamis with complex
mechanisms, which represent the characteristics of each particular
source. Seismic events in the Eastern Caribbean are principally
associated with a subduction zone along a north-south line just
east of the main island arc where the North American Plate dips
from east to west beneath the Caribbean Plate. The evaluation
of the active seismic centers and the tsunamigenic efficiency
of subduction earthquakes in the Caribbean region will be presented
in a separate report.
The Evolution of Volcanic
Centers in the Eastern Caribbean Region: Volcanic centers in the region is the by-product
of active tectonic activity. Subduction is responsible for the
evolution of the Lesser Antilles,
an arc of small islands with active volcanoes characterized by
both effusive and explosive activity (Martin-Kaye1969). Specifically,
the down-dip compression on the North American plate caused by
tectonic movement has created the tensional volcanic back-arc,
which is characterized by spreading and shallower seismic activity.
As the fore-arc is driven by the mantle drag toward a trench
the zone of subduction - the resulting compression is balanced
with the slab pull. This flow in the mantle causes the back-arc
spreading (Seno and Yamanaka, 1998). Arc stresses and back-arc
spreading result in increased volcanic activity in the region
and in a greater potential for tsunami generation from subsequent
volcanic mass edifice failures, and other mechanisms, which will
be discussed and evaluated further in subsequent sections.
Evolution of Island-Arc volcano (modified web graphic)
OF VOLCANIC ORIGIN IN THE LESSER ANTILLES REGION
In recent times, Soufriere
Hills on Montserrat, Kick'em Jenny near Grenada, Soufriere of
St. Vincent, and Mt. Pelée on Martinique, are volcanoes
in the Lesser Antilles region that have generated local tsunamis
by renewed volcanic activity and associated flank failures and
landslides (Lander et al 2002). Given the degree of violent volcanic
activity and the flank instabilities of stratovolcanoes in the
region, it is believed that the occurrences of tsunami waves
have been under-reported in historical records, probably because
the effects of such sea level disturbances were either localized
or were overshadowed by greater catastrophes caused by violent
volcanic eruptions The following is a brief overview of some
of the reported historical tsunami events.
The recent historic record documents
several tsunamis at Montserrat Island. Earthquakes in the area
generated some of these, while others were generated by pyroclastic
flows of the Soufriere Hills stratovolcano, by debris avalanches,
and by major flank failures and landslides. Also, the coastal
geomorphology of the eastern part of Montserrat near the Chance
Peak of the Soufriere Hills volcano indicates that massive landslides
must generated local tsunamis in the distant past. According
to the more recent historic record, an earthquake in the region
on September 13, 1824, resulted in a remarkable rise and fall
of sea level at Plymouth. Another major earthquake near Antigua,
reportedly triggered landslides into the sea in Antigua, Montserrat
and Nevis Islands. However, most of the noteworthy tsunamis were
generated recently as a result of renewed activity of the Soufriere
Debris avalanches and pyroclastic (lava) flows associated with
the 1999 eruption of the Soufriere Hills volcano on the island
of Montserrat reached the sea and generated a tsunami. (Photo:
Montserrat Volcanic Observatory)
On December 26, 1997, following a major eruption
of Soufriere Hills a landslide - assisted by pyroclastic flows
- reached the sea along the southwestern coast of the island
and generated significant tsunami waves. (Heinrich et al., 1998,
1999a,b, 2001). Maximum runup of the waves, about ten kilometers
away from the source region, was about 3m, with inland inundation
of about 80 meters. Similar debris avalanches and pyroclastic
flows associated with a 1999 eruption of Soufriere Hills reached
the sea and generated another local tsunami. The height of the
waves in the immediate area ranged from12m but attenuated
rapidly. By the time they reached the islands of Guadeloupe and
Antigua the maximum runup heights were only about 50 cm. The
most recent tsunami occurred on July 12, 2003, following a major
collapse of a lava dome (Pelinovsky et al 2004). A pyroclastic
flow reached the sea and generated a tsunami, which was reported
to be about 4 meters on Montserrat and about 0.5-1 m at Guadeloupe.
Travel time chart of the tsunami generated by the 1999 debris
avalanche at Montserrat Island.
Mt Pelée on Martinique
is another active stratovolcano with unstable flanks composed
primarily of pyroclastic rocks. As such, it must have generated
numerous tsunamis in the distant geologic past. The first reported
violent eruption of Mt Pelée occurred in 1792. The record
does not indicate whether the eruptions caused flank failures
on the island or generated tsunamis. Such events could have occurred
but not reported. Even the recent historic record is unclear.
For example, there are reports of observed sea level agitations
on Martinique in 1767, but it is not known whether these were
tsunami waves generated by a distant earthquake or an island
flank failure. On 30 November 1823 an earthquake in the area
generated a tsunami, which caused damage to St. Pierre Harbor.
In 1824, another earthquake near St. Pierre was probably responsible
for a very "high tide" that reportedly grounded several
ships in the harbor.
Mt. Pelée's eruption of May 8, 1902 killed 29,000 people
and destroyed the city of St. Pierre. Local destructive tsunamis
were triggered by a lahar, a nuée ardente nuee and by
flank failures. (Photograph by Heilprin. taken on May 26, 1902)
In the spring of 1902,
Mt Pelée began erupting again. According to historic records,
as the summit eruptions intensified, the water of the Etang Sec
crater lake heated to near boiling point. On May 5, the crater
rim broke, and extremely hot water cascaded down River Blanche.
The hot water, mixed with loose pyroclastic debris and mud, formed
a massive 35-meter high lahar that reached a speed of
about 100 kilometers per hour. The hot volcanic mudflow buried
everything in its path. Near the mouth of River Blanche, north
of St. Pierre, it hit a rum distillery and killed 23 workers.
The lahar continued into the sea, where it generated 4-5 meter
tsunami waves, which flooded the low-lying areas along the waterfront
of St. Pierre. Subsequently, on May 8, 1902, a catastrophic nuée
ardente cascaded for about 6 km down-slope from the central crater
of the volcano, at a velocity of more than 140 Km per hour, destroying
completely St. Pierre, and killing 29,000 of its inhabitants.
According to the historic record there were only two survivors
one in a prison dungeon. There is not much information
on the tsunami that the nuée ardente must have generated,
as the immensity of St. Pierre's destruction overshadowed everything
There is not much information
about tsunamis generated from eruptions or flank failures of
the Soufriere volcano on St. Vincent Island, although several
must have occurred. The historic record shows that the volcano
erupted violently in1718, 1812, 1902, 1971-1972 and in 1979.
The 1902 eruption was the most catastrophic and killed 1,600
people. The record shows that, on May 7, 1902, a day before the
most violent eruption of Mt Pelée on Martinique, tsunami
like disturbances of up to 1 meter were reported for the harbors
of Grenada, Barbados and Saint Lucia. Although the origin of
these waves is not known with certainty, the most likely source
could have been pyroclastic flows reaching the sea from the violent
eruption of Soufriere volcano on St. Vincent. Alternatively,
the sea level disturbances could have been generated by an unreported
flank failure of Mt Pelée, which was also erupting at
that time. The historic record documents that on May 7, 1902
the submarine communication cables from Martinique to the outside
world were cut.
Volcanoes of Grenada (USGS graphic)
Kick'em Jenny is an
active and growing submarine volcano about 8 km off the North
side of the island of Grenada, which erupted frequently during
the 20th Century (Smithsonian Institution, 1999). There have
been several local tsunamis generated by these eruptions. The
volcano's first recorded eruption reportedly occurred in 1939,
but undoubtedly there were many unreported occurrences before
that date. Since 1939 there have been at least ten more eruptions.
The better known are those that occurred in 1943, 1953, 1965,
1966, 1972 and 1974. The 1974 eruption was major. The last known
major eruption occurred in 1990.
The 1939 and 1974
eruptions ejected columns above the sea surface. At the peak
of the July 24, 1939 eruption - which lasted more than 24 hours
- a cloud rose 275 meters above the sea surface (Tilling, 1985;
Seismic Research Unit Website, Univ. of West Indies, 2001). The
event was witnessed by a large number of people in northern Grenada.
Kick'em Jenny's 1939 eruption also generated a series of tsunami-like
waves, which had amplitudes of about 2 meters in northern Grenada
and the southern Grenadines. The waves probably reached the west
coast of the Barbados, but were not noticed as their heights
had attenuated significantly.
OF TSUNAMI GENERATION FROM VOLCANIC SOURCES
Based on what appear
to be debris avalanches or toes of large scale landslides on
the ocean floor, it has been postulated that mega-tsunamis were
generated in the distant geologic past by massive volcanic flank
failures in the Canary, Cape Verde and Hawaiian islands, as well
as elsewhere in the Atlantic, Pacific and Indian oceans. Pararas-Carayannis
(1992, 2002, 2003) evaluated mega-tsunami generation from prehistoric
and postulated massive landslides and flank failures of oceanic
basalt shield stratovolcanoes such as Kilauea, Mauna Loa, Cumbre
Vieja, Cumbre Nueva, Taburiente and Piton De La Fournaise, and
from the explosions/collapses of continental stratovolcanoes
linked to catastrophic phreatomagmatic episodes of Plinian and
Ultra-Plinian intensities, such as those that occurred at Krakatau
and Santorin. Some volcanic source mechanisms of tsunami generation
were realistically modeled in estimating the near and far field
wave characteristics (Mader, 2001; Le Friant, 2001; Gisler 2004).
However, there has not been adequate evaluation of tsunami generation
from eruptions and flank failures of unstable basalt/andesitic
Caribbean volcanoes, which are neither truly oceanic nor continental
and have different geochemistry and eruption styles and processes.
Mechanisms of Shield Volcanoes
Oceanic, basalt shield
volcanoes have different styles of eruption, thus their mechanisms
of flank failures and of tsunami generation differ from those
of volcanoes along continental boundaries. Most of the basaltic
stratovolcanoes have Hawaiian styles of eruptions, which usually
involve less explosivity, and passive lava flows because of lower
silica, gas content and ejecta viscosity. Occasional sudden gas
releases may produce explosive lava fountains and unstable pyroclastic
deposits. Small scale hydromagmatic explosions can also occur
near the coast. Examples would be those that formed the Diamond
Head and Coco Head craters on the island of Oahu, in Hawaii.
However, most of the eruptions of shield volcanoes are usually
confined near summit calderas or slong flank craters and vents.
Resulting slides from unconsolidated pyroclastics usually involve
relatively small volumes of material, which rarely reach the
sea to generate waves of any consequence. However, destructive
tsunamis can be generated from massive volcanic edifice failures
of larger blocks which may be triggered by large scale magmatic
chamber collapses, erosion, gaseous pressure, phreatomagmatic
and forced dike injection, or by isostatic and gravity induced
kinematic changes (Pararas-Carayannis 2002). Any of these processes
can trigger large volcanic mass failures of shield volcanoes,
alone or in combination with other mechanisms.
Mechanisms of Caribbean Volcanoes -
There is plethora
of geologic evidence indicating that volcanoes in the Caribbean
region have generated tsunamis, recently and within the last
100,000 years, by a variety of mechanisms. Destructive tsunami
waves were generated by violent sub-aerial and submarine eruptions
and accompanying earthquakes, by caldera and submarine flank
collapses, by subsidence, by atmospheric pressure waves, by lahars,
nuées ardentes, pyroclastic flows, or debris avalanches.
Also, tsunamis must have been generated from gravitational mass
edifice failures due to the characteristic flank instabilities
of the volcanoes in this region - even in the absence of obvious
triggering events. For example, earth tides could trigger such
Evaluation of flank
instabilities of Caribbean stratovolcanoes and their potential
for tsunami generation requires a closer examination of the styles,
intensity and geometry of eruption mechanisms, of precursor events,
of the time history of volcanic episodes, of the geochemistry
and composition of the lava and ejecta, as well as an assessment
of tectonic processes in the region which result in volcanic
arc stresses, back-arc spreading and an increased level of volcanic
activity. Small scale flank collapses which result in tsunami
generation are a standard phase in the evolution cycles of Caribbean
volcanoes (Young 2004.). The following is a review of different
factors for Caribbean volcanoes that contribute tor eruptions
episodes, which may range in style and intensity from Strombolian
to Vulcanian/Plinian, but are not as catastrophic as the Plinian
and Ultra-Plinian episodes of the Krakatoan/Santorin variety.
Specifically reviewed in the next section are precursor events
and eruption processes of some of the previously mentioned active
volcanoes of the Lesser Antilles processes that result
in moderate flank failures and the generation of tsunamis or
tsunami-like waves. Although the present analysis focuses on
mechanisms of flank failures of active Caribbean stratovolcanoes,
it should be pointed out that the processes and mechanisms that
are described are similar to those occurring at many other volcanoes
around the world. Furthermore, it should be pointed out that,
in contrast to tsunami generation from seismic sources, which
cannot be predicted, the generation of tsunamis from volcanic
sources can be forecasted with proper monitoring of precursor
events, of volcanic activity and of flank instabilities. The
following sections provide an analysis of factors that contribute
directly to eruptions, flank instabilities and failures of Caribbean
volcanoes and indirectly to the generation of destructive sea
CONTRIBUTING TO VOLCANIC EXPLOSIVITY, STRUCTURAL FLANK INSTABILITIES,
MASS EDIFICE FAILURES, DEBRIS AVALANCHES AND TSUNAMI GENERATION
IN THE CARIBBEAN REGION
Tsunamis can be generated
by volcanic caldera and lava dome collapses, by vertical, lateral
or channelized explosive activity and by the associated atmospheric
pressure perturbations, pyroclastic flows, lahars, debris avalanches
or massive volcanic edifice failures. Contributing tectonic factors
include island arc volcanism that overlies a subduction zone
and which can result in the most catastrophic types of eruptions.
Many additional specific factors determine the eruption style,
higher explosivity, the generation of pyroclastic flows, the
structural flank instabilities, the slope failures and the debris
avalanches that characterize the mainly andesitic Caribbean volcanoes.
The following is a description of these factors for the most
active volcanoes in the Caribbean region. However the same factors
apply to all basaltic/andesitic volcanoes around the world that
border tectonic boundaries.
Whether a volcano
will have effusive eruptive activity or explosive type of bursts
will depend primarily on geochemical factors. The build up of
pressure of volatile gases within the magmatic chambers determines
a volcano's eruption style, explosivity, flank instability and
potential for subsequent tsunami generation.
Variations in the
chemical composition of volcanic eruptive effluents result from
differences in the mineralogy and the bulk composition of mantle
material, differences in depth at which melting occurs, differences
in percentages of melting at the source, and alterations in composition
as the magma rises to the surface (Wright and Helz, 1987). The
chemical composition of molten rocks in the magmatic chambers
of a volcano and the different abundances of elements, particularly
silica, determine the viscosity of effluents that can rise to
the surface. Magmas, which are low in silica and rich in iron
and magnesium, like the basalts of Hawaiian types of volcanoes,
are very fluid. Basalt contains anywhere from 45% to 54% silica,
and generally is rich in iron and magnesium. Dissolved gases
escape prior to an eruption, thus resulting in a subsequent effusive
eruptive activity associated with relatively gentle lava flows.
Because of the low viscosity, lava flows travel great distances
from the eruptive vents, and produce broad, shield-shaped volcanoes.
Kilauea, in Hawaii, is an example of such a volcano.
By contrast, the chambers
of stratovolcanoes along continental margins contain magma which
is composed primarily of andesite or dacite, that are relatively
high in silica and low or moderate in iron and magnesium. Andesite
may contain anywhere from 54 to 62 percent silica. Thus magma
material tends to be very viscous. Gases are trapped and cannot
escape until the magma enters the volcanic conduits leading to
the surface. The reduction in pressure, allows the gas bubbles
in the magma material to nucleate and expand. When the outward
pressure exerted by the gas bubbles exceeds the strength of the
lava material above it, the volcano erupts violently. Because
of the high pressure of the expanding volatiles, the lava is
fragmented to produce volcanic ash and pyroclastics that are
ejected out of the volcanic vent at high velocity. Most of the
ash travels far but pyroclastics accumulate near the vent, thus
producing a steep-sided stratovolcano with very unstable flanks.
An example of such a volcano is Mount St. Helens in the State
The volcanoes of the
Lesser Antilles in the Caribbean region are mainly andesitic
stratovolcanoes, characterized by both effusive and explosive
activity (Brown et al. 1977). However, even effusive activity
may culminate into explosive activity at later stages of an eruption,
as the more viscous magma material reaches the surface. Explosive
eruptions of Caribbean volcanoes may last for hours and will
result in greater volcanic cone instabilities. Intensities and
stypes of explosive episodes may range from Strombolian, Vulcanian
to even Vulcanian/Plinian. During such eruptions, destructive
sea waves may be generated directly by pyroclastic flows, lahars
and nuées ardentes reaching the sea, and by atmospheric
pressure waves of blast events. Debris avalanches and massive
volcanic flank failures can also generate destructive waves during
an eruption or subsequently.
Collapses of Lava Domes
As a result of the
geochemistry and the higher viscosity of the mainly andesitic
magma, Caribbean volcanic activity often results in the formation
and growth of lava domes near a volcano's summit or along its
flanks (Voight 2000). Rapid lava dome growth, after a quiet volcanic
activity period, is indicative that pressure is building up within
a volcano. Subsequent collapses of lava domes often trigger a
flurry of volcanic eruptions which will vary in intensity and
may be associated with catastrophic pyroclastic flows, lahars,
debris avalanches and large scale flank failures which
can generate directly destructive local tsunamis. In fact the
collapse of a growing lava dome has the potential of generating
directly destructive pyroclastic flows, lahars or debris avalanches,
which, upon reaching the sea, can generate destructive local
tsunamis. Indirectly, such volcanic processes may weaken the
flanks of the volcano, thus flank failures may be retrogressive
and time-dependent. Thus, lava dome growth, chemical and geometric
factors, and the mechanisms of weakening that result in eventual
collapses, are important factors in assessing a volcano's overall
instability and in forecasting a major eruption or even the generation
of a tsunami.
As already indicated,
the sudden releases of gases are responsible for a volcano's
explosivity and flank instability and thus trigger flank failures
which can generate destructive tsunami waves. Additionally, the
direct explosive outflux of volcanic volatiles can create sudden
atmospheric pressure disturbances which, coupled with the sea
surface, can also generate destructive waves.
Significant atmospheric pressure perturbations that are propagated
by buoyancy forces during violent volcanic eruptions can have
periods greater than 270 seconds (Beer, 1974). The periods of
the atmospheric waves is dependent on the volumes of volatiles
and the duration of volcanic bursts. For example, spasmodic volcanic
bursts of Caribbean volcanoes during a violent eruption may produce
extremely large movements of air parcels similar to those produced
by the 1980 Mount St. Helens eruption (Mikumo and Bolt, 1985;
Tilling 1984; Tilling, et al 1990) or to the 1992 Pinatubo eruption
(Tahira et al., 1996). Eruptive phases consisting of numerous
eruptive pulses may last from a few hours to days. Explosive
pulses may last from a few seconds to minutes and may vary in
intensity depending on the impulsivity of the degassing source
(Newhall & Self, 1982). Such volcanically generated atmospheric
pressure waves can generate several destructive tsunami-like
waves of longer periods with heights, which will not attenuate
as rapidly as those from other wave generation mechanisms.
Blast Geometry Factors
geometry of eruption blasts is a factor that will also determine
a volcano's flank instability and its potential for massive failures,
avalanches or subareal or submarine landslides and subsequent
tsunami generation. Generally, volcanic blasts mechanisms can
be vertical, lateral-direct or channelized. If the blasts affect
directly a body of water, then more destructive local tsunamis
can be expected.
Vertical Blast Mechanisms
- More typical for
Caribbean volcanoes are the vertical type of blast mechanisms,
which will deposit unconsolidated pyroclastic debris and build
up cones with unstable flanks. Gravity forces may eventually
lead to cone collapse or other failures, which in turn may trigger
lahars, larger flank landslides or debris avalanches, usually
on the steep-sided stratovolcanoes. As previously discussed,
strong blasts can also generate atmospheric pressure perturbations
which, depending on duration and intensity, may couple with the
sea surface to form additional destructive waves of varying periods.
Distribution of deposits from the May 18, 1980 lateral and channelized
blast of Mt. St. Helens (after Tilling, 1984)
Lateral and channelized
blasting effects can be far reaching if they occur near a body
of water. A large lateral blast from an erupting Caribbean island
volcano could be extremely destructive and could also generate
much more destructive local tsunami waves. The May 8, 1902, explosion
of Mt. Pelée that destroyed the city of St. Pierre on
the island of Martinique was such a lateral blast. Lahars and
nuées ardentes generated destructive local tsunamis. The
distribution of pyroclastic deposits on the western flank of
the underwater volcano Kick'em Jenny indicates that such lateral
blasting occurred there in the past. There is also a major escarpment
that suggests other types of massive slope failures.
OF VOLCANICALLY- INDUCED TSUNAMI GENERATION IN THE LESSER ANTILLES
ISLANDS OF THE CARIBBEAN
In view of the above-described
complex factors, proper numerical modeling of tsunami generation
from volcanic sources requires consideration of explosion dynamics,
of the time history of blast events, of the geometry of sudden
explosions, and of water displacements caused by both movements
of mass and by atmospheric pressure waves.
tsunamis generated at Montserrat, Martinique, St Vincent, and
Grenada Islands in the Lesser Antilles region were briefly reviewed.
Obviously, the mechanisms of tsunami generation from volcanic
sources can vary significantly even within the same geotectonic
region. In the following sections, the eruptive processes at
work for active volcanoes such as Soufriere Hills, Mt. Pelée,
Soufriere and Kick'em-Jenny will be reviewed and analyzed, as
well the time history of major specific eruptions that resulted
directly or indirectly in tsunami generation.
Hills Volcano on Montserrat Island - Eruptive Processes and Mechanisms
of Tsunami Generation
The Soufriere Hills,
located on the southern part of Montserrat Island, is a very
active, primarily andesitic stratovolcano (Rowley1978), which
is the predominant type of explosive volcano in the world. Its
present elevation is 915 m. The first known historical eruption
was in 1995. Since then there have been several more eruptions
in the late 1990s (Hooper and Mattioli, 2001). The volcano is
presently very active. All of its eruptions have been associated
with earthquake swarms , lava dome collapses, steam explosions,
ash falls, pyroclastic flows and debris avalanches.
The volcano's flank
instability and its potential for landslides and tsunami generation
result from the composition of its magma which is very sticky
and has a high content of dissolved water. In fact, eruptions
appear to be linked with rainstorms and high earth tides. When
the volcano erupts it tends to form a steeply sloped peak made
of alternating layers of lava, block, and ash. Thus, the slopes
of the volcano become unstable and susceptible to massive landslides
and debris avalanches, some of which can reach the sea and generate
local tsunamis. In fact several have occurred in the last few
Eruptive Processes of the Soufriere
Hills Volcano: To
understand better the tsunamigenic potential of the Soufriere
Hills volcano on Montserrat, we must review further its eruptive
processes. Two types of eruptive mechanisms characterize this
volcano, both of which have the potential to generate local tsunamis.
In both cases, magma inside the volcano is driven up by buoyancy
and gas pressure, which may vary depending on its viscosity.
In one type of eruption,
the volcano will explode and shoot molten rock violently into
the air in the form of dense clouds of lava fragments. The larger
and heavier fragments tend to fall back around the volcanic vent
which may become increasingly unstable as the eruption progresses.
Often, the accumulation may run down slope as ash flows, while
some of the finer particles may be carried by the wind. Often,
both ash and pyroclastic flows can trigger debris avalanches
and larger landslides and slope failures on the volcano ( DeGraff
1988) that can generate small local tsunamis.
The Island of Montserrat, the Soufriere Hills volcano and the
town of Plymouth.
In the second type
of eruption, the molten rock - which is lighter than the surrounding
solid rock - breaks through the weaker stratigraphic layers and
raises closer to the surface forming a lava dome. When the dome
becomes too steep, or if pressure within builds up, it collapses
and disintegrates, spewing lava and hot ash down the side of
the volcano. Also, it may form a mushroom cloud of ash that can
spread over the island. The collapse of such lava domes releases
pressure and frequently causes subsequent massive eruptions that
can also affect the volcano's flank stability, thus generating
pyroclastic flows, debris avalanches, landslides and massive
flank failures. In fact a combination of the above-described
eruptive processes occurred during eruptions in the summer of
1995. Specifically, on July 18, 1995, Soufriere Hills had its
first recorded eruption in historic times. It begun with a small
phreatic eruption. Periods of intense seismic activity were associated
with ejection of steam and ash, shortly after a new vent opened
southwest of an old volcanic dome known as Castle Peak. The eruption
culminated into a major event on August 21, 1995, when the volcano
begun spouting molten ash, rock, and gas over the island, killing
19 and incinerating the capital city of Plymouth. A strong burst
of steam carried a cloud of ash to an altitude of 7,000 feet.
The eruption triggered several landslides that reached the sea,
but there was no report of any unusual waves being generated.
(Mangeney et al. 1998; Calder et al. 1998).
The Eruption and Tsunamis
of 26 December 1997 and 1999 and 2003:
As already mentioned,
the Soufriere Hills volcano either erupts by exploding and expeling
lava or by dome collapse. Both types of eruption can be destructive
as they can produce dangerous ash hurricanes and pyroclastic
flows, trigger landslides and debris avalanches and thus generate
tsunamis. Although the 1995 eruption and other volcanic processes
that occurred subsequently did not generate a tsunami, apparently
they weakened Soufriere Hills' flanks. This weakness contributed
to the subsequent volcanic flank failures associated with the
eruptions of 1997, 1999 and 2003 which generated tsunamis.
Specifically, on June
25, 1997, after two years of precursory swelling and micro earthquake
activity, Soufriere Hills volcano erupted again. A damaging pyroclastic
flow of ash, gas, and rock killed at least ten people and destroyed
nine villages. A lava dome was subsequently observed which built
up steadily in the volcano's crater for over two months. On 26
December 1997, following the collapse of this lava dome, a major
eruption occurred. The eruption generated ash hurricanes, which
destroyed Plymouth. Both the ash hurricanes and a landslide -
possibly assisted by pyroclastic flows triggered by the dome-collapse
- reached the sea, along the southwestern coast of the island
and generated significant tsunami waves. (Heinrich et al., 1998,
1999a,b, 2001). The maximum runup of the waves was about 3m.
about ten kilometers away from the source region, with inland
penetration of about 80 meters. The volume of the landslide debris,
which generated this tsunami, was estimated to be about 60 million
cubic meters (Lander et al., 2003).
Similar debris avalanches
and pyroclastic flows associated with the1999 eruption of Soufriere
Hills reached the sea and generated another local tsunami. The
height of the waves in the immediate area ranged from12m
but attenuated rapidly. By the time the waves reached the islands
of Guadeloupe and Antigua their heights attenuated considerably.
Maximum runup heights were only about 50 cm.
The most recent tsunami
was produced by the eruption of July 12, 2003 (local date) following
a major collapse of a lava dome (Pelinovsky et al 2004; Young
2004). Pyroclastic flows and a debris avalanche reached the sea
at the end of Tar River Valley on the east coast and generated
this tsunami, which was reported to be about 4 meters at Spanish
Point on Montserrat Island and about 0.5-1 m at Deshaies and
near Plage de la Perle on Guadeloupe where it caused some damage
to fishing boats.
support that debris avalanches and extensive landslides of andesitic
volcanoes will only generate local destructive tsunamis, is supported
by the April 20, 1988 massive flank failure of the northeast
flank of the volcano La Fossa on the Island of Volcano in the
southern Tyrrhenian Sea, in Italy. According to modeling studies
- which were based on photogrammetric techniques conducted in
1981 and 1991 - the large 1988 flank failure of La Fossa involved
a mass with a volume estimated to be about 200,000 cubic meters.
The mass that was detached fell into the sea for about 10 seconds.
A small tsunami was generated in the bay between Point Nere and
Point Luccia on the island. Maximum observed runup height of
the waves was about 5.5 meters at Porto di Levante and presumably
even at Monterosa on Lipari Island. (Barberi, et al 1990; Lander
et al. 2003).
Map of the Island of Vulcano in Italy where a 200,000 cubic meter
massive flank failure on the northeast side generated local tsunami
(after Imbo,1965 and Keller,1980)
Volcano on Martinique Island Eruptive Processes and Mechanisms
of Tsunami Generation
Mt. Pelée on
Martinique is a very active island-arc stratovolcano with unstable
flanks made mostly of pyroclastic rocks (Smith and Roobol 1990).
Its summit elevation is 1397 m. It undergoes similar eruptive
processes as other Caribbean volcanoes and can also generate
destructive local tsunamis by pyroclastic flows, flank failures
or debris avalanches. However, what makes Mt. Pelée unique
has been its unusual lava dome formations, the intensity and
styles of its eruptions and the unusual and violent pyroclastic
flows it can generate (Fisher and Heiken1982). The volcano has
a long history of eruptions in the last 5,000 years (Westercamp
and Traineau 1983). In more recent historic times the volcano
erupted in 1635, 1792, in 1851-1852, in 1902- 1905 (Heilprin
1908) and in 1929-1932 (Perret 1937).
The historic record
documents two extremely violent eruptions in 1792 and in 1902
- associated with numerous other phenomena that followed dome
collapses and by which local tsunamis were generated. The
eruptive processes of Mt. Pelée and the tsunami generation
mechanisms that are described in subsequent sections are based
on what occurred on Martinique in May of 1992 and whatever little
is known about the violent volcanic eruption of 1792.
Eruption Processes of Mt. Pelée
- Eruptions of Mt.
Pelée range in volcanic explosivity intensity from severe
Vulcanian (VEI = 3) - which can occur yearly - to cataclysmic
Vulcanian-Plinian events (VEI = 4) separated in time by many
decades. The Peléeean eruptions - as they are now termed
because of their unique characteristics - are extremely violent
eruption events that often include collapses of ash columns,
and unique pyroclastic flows known as "nuées ardentes"
- a type of pyroclastic avalanche mixture of gas, dust, ash and
other hot glowing incandescent solid particles and lava fragments
- and debris avalanches containing large amounts of ignimbrites
(ash flow tuffs). These unusual pyroclastic flows are usually
triggered after a lava dome collapse.
The Eruption and Tsunamis
of May 1902 A previously stated, an extremely violent volcanic
eruption occurred on Mt. Pelée in 1792. It is very probable
that a tsunami was generated at that time as a result of a flank
failure or pyroclastic flow, but there are no reports documenting
it. However, the 1902 eruption and its associated unusual phenomena
are well documented in the literature (Lacroix, 1904; Heilprin
1908; Fisher et al 1980). The May 1902 tsunamis were generated
by a lahar and a subsequent nuée ardente of a violent
In early 1902, a large
dome of very viscous lava had grown on Mt. Pelée's flank
near its summit, largely by expansion from within. As the lava
dome grew, its outer surface cooled and hardened. There is not
much information on the size of this particular lava dome, but
it could have been as big as that of the Katmai volcano in Alaska,
which collapsed and triggered an eruption in 1912. That dome
had been circular and measured about 250 meters across and 60
meters in height. However, what was reported about Mt. Pelée's
lava dome is that it had cut a large V-shaped notch through the
cliffs that surrounded the volcano's summit crater. According
to reports, the "notch was like a colossal gun sight pointing
directly at the town of St. Pierre".
The 1902 Eruption of Mt. Pelée on the island of Martinique.
The destruction of the town of St. Pierre was caused by a nuée
ardente. (Photograph by Heilprin, 1902).
According to historic
records, on May 5, 1902, a 35-meter lahar cascaded down the flank
of the volcano and reached the sea. The lahar generated a local
tsunami wave of about 4-5 meters in height, which killed one
hundred people in St. Pierre. Subsequently, at approximately
7:50 a.m. on May 8, 1902, the pressure from within the volcano
reached a critical level. Suddenly, the summit lava dome collapsed
and shattered with a deafening roar, spilling loose fragments
down-slope. The sudden release of pressure triggered by the dome
collapse resulted in an extremely violent eruptive phase of Mt.
large nuée ardente cascaded from the central crater for
about 6 km down the south flank, at a velocity of more than 140
Km per hour. In less than one minute it struck the coastal town
of St. Pierre, destroying it completely and killing 29,000 of
its inhabitants. Only two people are known to have survived.
According to reports (Heilprin 1908; Fisher et al 1980) the directional
blast was so strong that it carried a three-ton statue sixteen
meters from its mount. One-meter thick masonry walls were blown
into rubble. "Supporting girders were mangled into twisted
strands of metal". The heat of the nuée ardente nuee
was immense and ignited huge fires. Thousands of barrels of rum
that was stored in the city's warehouses exploded and burned
in the streets.
Devastation of the town of St. Pierre on Martinique Island by
a Nuee Ardente of the 1902 eruption of Mt. Pelée. (Photograph
by Heilprin, 1902).
There is not much
direct information on the tsunami that the nuée ardente
must have generated, as the immensity of St. Pierre's destruction
overshadowed everything else. However, it was reported that the
nuée ardente continued seaward toward the harbor where
it destroyed at least twenty ships that were anchored offshore.
The American sailing ship "Roraima", which had arrived
only a few hours earlier, burned and all its crew and passengers
perished. The steamship "Grappler" was presumably capsized
by the force of the nuée ardente. However, it is more
than likely that it was capsized by the wave the nuée
ardente generated in the harbor.
Mechanisms of tsunami
generation involving cascading volcanic gases and rapidly moving
pyroclastic flows are not confined to Caribbean volcanoes or
to Mt. Pelée, in particular. There is evidence that similar
hot glowing avalanches of hot gas, dust, ash and pyroclastics
have generated several tsunamis in the distant past in New Zealand
and elsewhere around the world.
St. Vincent Island and the Grenadines (web graphic)
Volcano on St. Vincent Island Eruptive Processes and Mechanisms
of Tsunami Generation
Soufrière is an active and dangerous stratovolcano on
the island of St. Vincent in the Windward Islands of the Caribbean,
with a well-documented history of violent eruptions (Robson,
1965a, Shepherd and Aspinall. 1982). The present elevation of
its summit is at 1220m. There is a lake within the summit crater.
La Soufrière should not be confused with a volcano by
the same name on the island of Guadaloupe.
The 1979 eruption of La Soufrière on St Vincent island
(Photograph by Richard Fiske)
evidence of activity on Soufrière for the last 650,000
years (Hay 1959; Rowley1978). In recent times, major eruptions
occurred in 1718, 1784,1812, 1814, 1880, 1902-03 (Anderson 1784;
Anderson, T. 1903; Flett 1902,1908; Anderson and Flett 1903;
Sapper 1903; Anderson 1908, Carey and Sigurdsson 1978). In the
twentieth century they were major eruptions, in 1971-72 (Aspinall
et al 1972; Baker, 1972; Tomblin et al. 1972; Aspinall 1973;
Aspinall et al 1973) and in 1979 (Shepherd et al 1979; Shepherd
et al 1982; Barr and Heffter 1982; Brazier et al 1982; Fiske
and. Sigurdsson. 1982; Graham and Thirlwall. 1981). The 1812
eruption resulted in many deaths. However, the 1902 eruption
was the most catastrophic of all resulting in the loss of 1,600
of Soufrière - Geologic evidence indicates that
for the past 4,000 years the Soufrière volcano's eruptions
have alternated between explosive episodes associated with the
forceful ejection of fragmented material and pyroclastic flows
to quiet effusion of slow moving lava that forms summit domes
(Earle 1924; Hay 1959; Heath et al 1998). The 1979 eruption is
typical of such variation. It begun quite suddenly with less
than 24 hours of precursor activity. The mechanism of its subsequent
explosive eruption has been well documented (Shepherd and Sigurdson
1982). The first eruptive episode was Vulcanian in character.
It sent a plume of steam and tephra to a height of 20 km. and
lasted a little less than two weeks (Sparks and Wilson. 1982).
The second episode consisted of a quiet extrusion and growth
of a basaltic andesite lava dome (Huppert et al. 1982).
The Eruption and Tsunamis
of May 7, 1902 - There is not much information about tsunamis
generated from eruptions or flank failures of the Soufrière
stratovolcano, although several must have occurred in the geologic
past - and even more recently. The record shows that on May 7,
1902, a day before the most violent eruption of Mt Pelée
on Martinique, tsunamis like disturbances of up to 1 meter were
reported for the harbors of Grenada, Barbados and Saint Lucia.
Although the origin of these waves is not known, the most likely
source could have been air pressure waves from the violent eruption
of Soufrière on that day, or pyroclastic flows and debris
avalanches reaching the sea.
historic record also shows that on the same day - May 7, 1902
- the submarine communication cables from the island of Martinique
to the outside world were cut. Whether these cables routed near
St. Vincent Island is not known. The exact area where the cables
failed is not known. Thus, it is difficult to determine what
caused the cable failures and whether the sea level disturbances
observed at the harbors of Grenada, Barbados and Saint Lucia
had the same source. The waves could have been generated by an
unknown flank failure of Mt Pelée, and the cable failures
by an underwater debris avalanche. On May 5, Martinique had already
experienced a destructive local tsunami generated by a lahar.
Kick'em Jenny Submarine
Volcano near the Island of Grenada Eruptive Processes and
Mechanisms of Tsunami Generation
Kick'em Jenny is a
growing submarine volcano about 8 km off the north side of the
island of Grenada. It is the southernmost active volcano in the
Lesser Antilles volcanic arc and has erupted frequently during
the 20th Century (Smithsonian Institution, 1999). Presently,
the volcano has a circular base of about 5000m, its main cone
has reached a height of about 1300m above the sea floor, and
its summit is only 160 m below the sea surface. The volcano is
growing rapidly at a rate of approximately 4 meters per year.
At this rate the volcano is expected to reach the surface and
form an island in the near future if there is no flank
subsidence or cone collapse.
Kick'em Jenny's first
recorded eruption occurred in 1939, but many unreported eruptions
must have occurred prior to that date. Since 1939 there have
been at least twelve or more events. Most of the historical eruptions
were documented by acoustic measurements, since submarine volcanoes
generate strong acoustic signals that are recorded by seismographs.
Known eruptions occurred in 1939,1943, 1953, 1965, 1966, 1972,
1977, 1988, and in1990. The better-known events are those that
occurred in 1943, 1953, 1965, 1966, 1972 and 1974. The last major
eruption occurred in 1990. Earthquake swarms in late 2001 indicated
renewed activity. The latest eruption occurred on March 15, 2003.
In 2003, during a
survey of Kick 'em Jenny, an inactive underwater volcano was
discovered about 3 km away. It is, now known by the name of Kick'em
Jack. Its summit elevation is 190 m below the sea surface.
Volcanoes of Grenada (USGS map)
The Eruption and Tsunamis
of 1939 and 1974: According to historical accounts and eyewitness
reports from northern Grenada, the July 24, 1939 eruption of
Kick'em Jenny was major and lasted for at least 24 hours. The
eruption ejected a cloud plume above the sea surface. Furthermore
at the peak of the eruption, the cloud plume rose 275 meters
above the sea surface (Tilling, 1985; Univ. of West Indies, 2001).
The eruption generated numerous tsunami-like waves of short period.
These waves had maximum amplitudes of about 2 meters in northern
Grenada and the southern Grenadines, but were almost imperceptible
when they reached the west coast of the Barbados.
Eruption Processes of Kick'em-Jenny: The underwater topography of
the sea floor north of Grenada indicates that Kick'em Jenny comprises
of three small craters and two lava domes all of which
probably share the same magmatic chambers.
Two-minute topography of the sea floor north of the island of
Grenada, showing the geomorphology of the calderas, cones and
domes, generally known as the Kick'em Jenny volcano (web graphic)
As most of the Caribbean
volcanoes, Kick'em Jenny has had both violent and effusive eruptive
episodes. Eruptions of the volcano have been associated with
magmas, which have ranged in compositions from basalt to basaltic
andesitic. Thus, gently extruded submarine pillow lavas and domes
as well as tephra and other pyroclastics from minor phreatomagmatic
explosions, are present in submarine deposits around the volcano.
and orientation of pyroclastic deposits on the sea floor, primarily
to the west side of Kick'em Jenny, indicate that many volcanic
eruptions must have occurred that have been lateral or channelized
blasts, possibly following the collapse of lava domes. Furthermore
NOAA surveys in 2003 demonstrated the presence of deposits from
a debris avalanche. The geomorphology of the sea floor indicates
that this debris avalanche extends west for 15 km and perhaps
as much as 30 km from the volcano, into the Grenada Basin (Sigurdsson
et al 2004). Also, earlier multibeam surveys of the sea floor
discovered the existence of an arcuate fault escarpment - of
yet unknown age - to the east of the active cone. Because of
its shape and length, this escarpment cannot be related to caldera
subsidence and collapse. Its configuration and the overall geomorphology
of the sea floor suggest that a larger scale subsidence or volcanic
mass edifice collapse occurred in the distant past. It also suggests
that Kick'em Jenny volcano might have been at or above sea level
in the past.
Overall, the volcano's present
rapid upward growth towards the surface of the sea is indicative
of active vertical summit eruptions and the build up of a cone
by deposition of pyroclastics. However, the flanks of this cone
must be very unstable and subject to collapses and the generation
of future debris avalanches, which could slow the volcano's,
present rate of growth. Additionally, hydromagmatic explosions
associated with future eruptions could also result in greater
instability and might
also slow down the rate of growth. Future major eruptions can
be expected to be more violent and to eject sizeable columns
above the sea surface to heights much greater than those of the
1939 and 1974 events. Major future eruptions can be expected
to have considerably higher plume clouds, because of the greater
strength of hydromagmatic episodes as the summit approaches the
sea surface and the inclusion of a higher content of molecular
water in the form of superheated steam - along with ejected
tephra and other fine pyroclastic materials.
Bathymetry and distribution of volcanic deposits from eruptions
of Kick'em Jenny volcano (Web graphic at http://volcano.und.edu/vwdocs/volc_images/north_america/kick.html)
of the Tsunamigenic Potential of Future Eruptions of Kick'em
The frequency of Kick'em
Jenny's eruptions and the volcano's rapid growth toward the sea
surface have raised concerns that future eruptions will generate
tsunami waves with far reaching destructive effects on Caribbean
islands and along the coast of Venezuela. Earthquake swarms in
late 2001 added to concerns that Kick'em Jenny will again have
a major eruption.
Although there is a good probability that several eruptions will
occur in the near future and in fact the latest occurred
on March 15, 2003 - the potential tsunami risk from a future
eruption has been highly exaggerated by the introduction of speculative
and highly unrealistic "worst case" scenarios. Kick'em
Jenny is not Krakatau and does not pose the purported potential
tsunami danger that has been misreported by the media. Kick'em
Jenny is a much smaller volcano than Krakatau and has much smaller
crater dimensions and magmatic chambers. The tectonic interactions
that have produced this volcanic center in the Caribbean are
substantially different than those of Krakatau, which erupted
in 1883 and generated a destructive tsunami, which killed nearly
37,000 people in Indonesia (Pararas-Carayannis, 2003). Kick'em
Jenny's magmatic geochemistry is substantially different. Its
magma composition ranges from mainly basalt to basaltic andesite.
At the present stage of its development, Kick'em Jenny volcano's
small dimensions and geochemistry prevent eruptions of Vulcanian
or Plinian intensity or extremely massive volcanic edifice collapses.
The following is a realistic analysis of Kick'em Jenny's tsunamigenic
potential and future risks.
from Submarine Explosive Eruptions: Even a major explosion at a peak phase of Kick'em
Jenny's eruption would be expected to generate tsunami-like waves,
not as a single event but spread over a period of 24 hours or
more. The periods of these waves will be relatively short and
will range from 1-4 minutes at the most. Because of the short
periods and wavelengths, the wave heights will decay rapidly
with distance. As in 1939, the waves from future eruptions will
be of significance along the north coast of Grenada and along
the western coasts of Isla de Ronde and Isla Calle (Grenadine
Islands), and possibly Tobago, St. Vincent and Barbados, but
not anywhere else in the Caribbean. This conclusion is further
supported by the numerical modeling studies that were conducted
at the Los Alamos National Laboratory (Gisler et al 2004). Specifically,
numerical simulations of Kick-'em Jenny's explosions with the
same 3-D compressible hydro code used for asteroid impacts -
and injecting as much as 20 kilotons of thermal energy at the
apex of Kick'em Jenny's volcanic cone, confirmed that only short
period tsunami-like waves can be generated and that the waves
will attenuate rapidly away from the source. Accordingly, it
is concluded that explosive eruptions do not couple well to water
waves. The waves that are generated from such eruptions are turbulent
and highly dissipative, and don't propagate well.
At the present time,
the depth of Kick'em Jenny's summit and the hydrostatic pressure
above it dampen the energy of eruptive explosions - although
both the 1939 and 1974 events were violent enough to break through
the sea surface. As the volcano keeps on growing towards the
surface, the hydrostatic column pressure above the eruptive vents
will decrease significantly. Future eruptions can be expected
to be more explosive. However, even such future eruptions will
only generate waves of short period and their heights will decay
Morphology of Kick'em Jenny volcano and of an extensive slope
failure of unknown age. (NOAA multibeam submarine survey)
from Submarine Crater Collapses:
Even if one or all
of Kick'em Jenny's three small craters collapse, no major waves
will be generated. For example, when the summit of the submarine
volcano Loihi collapsed in Hawaii during the summer of 1996,
the wave that was generated was of short period and decayed very
rapidly (Mader 2004). The cavity generated by the Loihi collapse
was 1000 meters wide and 300 meters deep, which is much greater
than any potential cavity that could be expected from collapses
of any or all of Kick'em Jenny's craters. The fact that the top
of the Loihi cavity was at 1050 meters depth while the top of
Kick'em- Jenny's main crater is at 160 meters will not be much
of a factor in tsunami wave generation, since the waves will
be of short periods and will behave as shallow waves.
from Submarine Volcanic Dome Collapses:
dome collapses on Kick'em- Jenny will probably trigger major
eruptions perhaps lateral blasts - with the associated
pyroclastic flows and debris avalanches. However, it is expected
that the volume of the ejecta and gases will be relatively small
and that any resulting tsunami-like waves that will be generated
will not be greater than those generated by the 1939 or 1974
from Future Subaerial Volcanic c Collapses, Flank Failures and
Massive Volcanic Edifice Failures:
When Kick'em Jenny
breaks through the sea surface and begins to build in height,
it is expected that its eruptions will be more violent and that
its flanks will be even more unstable than they are now. As with
other active Caribbean volcanoes, waves may be generated by violent
eruptive episodes, from caldera and dome collapses, from pyroclastic
flows, landslides, flank failures, debris avalanches or even
massive volcanic edifice failures. However the tsunami waves
will be of relatively short periods (1-4 minutes at the most).
Although the waves may be significant locally, they will decay
rapidly away from the source area.
Worse Possible Scenario: Based
on the pattern of Kick'em Jenny's eruptive activity, a "worst
case scenario" at the present time would be a repeat of
the 1939 eruption but at the shallower depth of the present summit.
The waves from the 1939 event were about 2 meters in northern
Grenada and the southern Grenadines but substantially lower on
the west coast of the Barbados. A large violent eruption similar
to the 1939 event, at the present depth of summit, can be expected
to generate waves with a probable maximum runup of about 3 meters
in Northern Crenada and the Grenadines, and as much as 1-2 meters
along the west coast of the Barbados, Trinidad, and St. Vincent.
The wave heights along the nearest coastline of northern Bonaire
and Venezuela may be up to 1 meter at the most. When the volcano
breaks through the surface of the sea, the probable maximum runup
could be as much as 4 meters on Northern Grenada and as much
as 2 meters along the west coast of the Barbados, Trinidad, and
OF FUTURE RISKS OF TSUNAMIS FROM VOLCANIC SOURCES TSUNAMI
FORECASTING AND PREPAREDNESS FOR THE CARIBBEAN REGION
The historic record
indicates that Caribbean volcanoes pose a serious threat for
several islands in the region (Robertson1992, 1995). The1902
eruptions of Mt Pelée on Martinique and of La Soufrière
on St Vincent, and the more recent eruptions of Soufriere Hills
on Montserrat, and of Kick'em Jenny in Grenada increased awareness
that tsunamis generated from volcanic sources represent an additional
hazard that needs to be addressed individually for each of the
region's active volcanoes. Fortunately - and in contrast to the
unpredictability of tsunamis of seismic origin - tsunamis generated
form volcanic sources can be forecasted for the Caribbean region
and appropriate measures can be taken. Preparedness for the tsunami
hazard should include the monitoring of precursory eruptive processes
as ongoing presently (Sigurdsson 1981; Shepherd and Aspinall
1982; Shepherd 1989) but, additionally, studies of geomorphologies
and flank instabilities of each individual volcano and the mapping
of risk areas that can contribute to massive volcanic edifice
failures with or without a volcanic triggering event
and thus in the generation of destructive tsunami waves.
Already, as a result
of greater awareness and concerns about the threat of volcanic
hazards in the Caribbean region, several scientific organizations
have already established monitoring stations on several islands.
For example, following the devastating 1902 eruption of La Soufrière
volcano on St. Vincent Island, a surveillance program was initiated.
In 1952, a Seismic Research Unit was established on the island
and a sustained program of volcano monitoring was undertaken
(Fiske and Shepherd. 1990). Similarly, following the 1995 eruption
of the Soufriere Hills volcano on the island of Montserrat, a
monitoring program was established. Additionally, the Universities
of Puerto Rico and of the West Indies have undertaken extensive
monitoring functions and programs. Present volcano monitoring
operations include routine measurements of geological, geophysical
and geochemical parameters and assessments of precursory-to-an-eruption
phenomena. With some small additional effort, these existing
volcano monitoring programs in the Caribbean region can easily
assess future risks for the collateral tsunami hazard, develop
micro-zonation maps of potential tsunami hazard sites along the
coast and help establish programs of tsunami preparedness for
the public. The following sections summarize briefly the importance
of monitoring some of the precursory-to-an-eruption phenomena
as they relate to potential tsunami generation.
Micro-earthquake Activity: Routine
monitoring of a volcano's micro-earthquake activity helps forecast
eruptions (Hirn et al1987). For example, before a major eruption
occurs on Soufriere Hills on the island of Montserrat, the increasing
pressure within the volcano generates a flurry of micro-earthquakes,
which are indicative of magma movement. When this activity peaks
and the focus of micro earthquakes becomes shallower, it becomes
evident that the pressure within the volcano has reached a critical
phase and that a fairly imminent eruption can be expected. Such
monitoring is presently in effect for several islands with active
Monitoring Lava Dome
Formation and Rate of Growth: Measuring the swelling of the
volcano with tilt meters and other geodetic and photogrammetric
means may also indicate if there is intrusion by a lava dome,
the dome's rate of growth, and its potential for collapse. Mature
lava domes, which grow slowly, are usually non-explosive. Similarly
non-explosive are post eruption lava domes, such as the felsic
lava dome known as the Tower of Pelée - extruded in the
waning stages of the 1902 eruption of Mt. Pelée on Martinique.
However, younger, fast growing, pre-eruption extruded domes that
contain lava which has not been completely degassed, may explode
or collapse. The eruption and explosion of Mt. St. Helens in
the State of Washington were preceded by the rapid development
of a lava dome over a three-year period from 1980 to 1983. Also,
a lava dome formed rapidly on the Soufriere Hills volcano's crater
on Montserrat Island over a two-month period prior to the major
eruption of 26 December 1997. The dome formation served as a
natural warning for the residents of Plymouth to evacuate, thus
there was no losses of life. Stations on several Caribbean islands
with active volcanoes, routinely monitor lava dome formation
and rates of growth.
Evaluation of Potential
Lava Dome Collapses: The
periodic explosion or gravitational collapses of the viscous
masses of lava domes can sometimes generate deadly pyroclastic
flows that can reach the sea and generate tsunami waves. Lava
dome collapses were associated with the 1902 eruption and nuée
ardente of Mt. Pelée on Martinique, the 1902 eruption
of Soufriere on St. Vincent Island and the 1997, 1999 and 2003
eruptions, pyroclastic flows and debris avalanches of Soufriere
Hills on Monteserrat Island the latter generating significant
tsunami waves along the southwestern coast of the island.
In view of the above,
it is important to monitor changes of lava domes and their potential
for collapses. Furthermore, since lava dome collapses, particularly
near a volcanic summit may be followed by violent eruptions,
pyroclastic flows and debris avalanches, the expected path of
destruction and potential flank failure sites can be determined
by careful evaluation of the local topography and geomorphology.
Based on such assessments, coastal areas subject to the tsunami
hazard could be identified, microzonation maps can be drawn and
appropriate warning signs be posted for the protection of the
events from volcanic sources in the Caribbean Region have been
under-reported as the immensity of destruction from volcanic
events has overshadowed them. Small scale flank failures are
quite common for most of the active volcanoes in the Caribbean.
Such volcanic sources have the potential of generating destructive
local waves in confined bodies of water and in the near field
environment of an open coast. Local tsunamis can also be generated
by gravity-induced flank failures, even in the absence of eruptive
triggering events. Heavy rains and earth tides appear to play
a significant role in small scale flank failures of unstable
Tsunami or tsunami-like
waves can be generated by a variety of volcanic mechanisms, pyroclastic
flows, debris avalanches, and volcanic edifice mass failures
and by aerial or submarine landslides. Impulsively generated
waves from such complex source mechanisms behave non-linearly
and change significantly away from the source, with varying near
and far field effects and terminal run up heights. However the
wave periods are short and range from 1-4 minutes at most. The
heights of these waves attenuate rapidly with distance because
of relatively smaller source dimensions and shorter wave periods
and do not pose a significant danger at great distances from
the source. Caribbean volcanoes and their associated flank failures
can be forecasted with careful monitoring and programs of preparedness
need to be established.
At the present time. the Soufriere Hills volcano on the island
of Montserrat poses the greater threat of local tsunami generation
in the Eastern Caribbean Region. Its eruptive activity in the
last decade, the rapid rates of lava dome formations and growth
and the associated collapses and eruptive style, indicate ongoing
active volcanic processes that will continue for many years.
Tsunamis can be expected in the near future from both pyroclastic
flows reaching the sea and by flank collapses.
The historic record
supports that Mt. Pelée on the island of Martinique will
continue to pose a threat for a repeat of a Vulcanian-Plinian
episode in the future. When this will happen is not known. However,
given the sophistication of present monitoring programs, any
future activity of the volcano will be properly forecasted and
cautionary measures will be taken. Local tsunamis may be expected
around the island by flank failures of unstable slopes, even
in the absence of a triggering volcanic event.
Also, The historic
record supports that the stratovolcano "La Soufrière"
on the island of St. Vincent poses a very significant threat
for renewed activity in the future. Given the fact that there
is a lake at the summit, there is also a potential danger that
even an eruption of moderate activity may cause a breach on the
crater's rim and trigger a dangerous lahar which may be destructive
and may even generate a local tsunami if it reaches the sea.
Also the instability of La Soufrière flanks pose a threat
of failures and of local tsunami generation even in the
absence of a volcanic eruption. Heavy rains, gravitational forces
and earth tides may be significant triggering factors.
Kick them Jenny volcano
will continue to rise towards the surface and eventually will
form an island volcano. It is possible that its present rate
of growth may be slowed down by cone collapses and subsidence.
The dimensions of the volcano and the style of expected eruptions
and intensities limit the size of tsunamis that can be generated
from future events. A large violent eruption of the Kick'em Jenny
volcano at the present depth of the summit, can be expected to
generate waves with a probable maximum runup of about 3 meters
in Northern Crenada and the Grenadines, and as much as 1-2 meters
along the west coast of the Barbados, Trinidad, and St. Vincent.
The wave heights along the nearest coastline of northern Bonaire
and Venezuela may be up to 1 meter at the most. When the volcano
breaks through the surface of the sea, the probable maximum runup
of a tsunami from a major eruption could be as much as 4 meters
on Northern Grenada and as much as 2 meters along the west coast
of the Barbados, Trinidad, and St. Vincent.
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