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Eruptive
Processes of Stratovolcanoes of the Lesser Antilles (Islands
of Montserrat, Martinique, St. Vincent and Grenada) - Mechanisms
of Flank Failures and Tsunami Generation
George
Pararas-Carayannis
Excerpts
from presentation at the 2004 National Science Foundation Tsunami
Workshop in San Juan, Puerto Rico , and from recent paper published
in the Journal of Tsunami Hazards, Volume 22, Number 2. 2004
http://www.STHJOURNAL.ORG
and from
other publications and review of the literature.
 INTRODUCTION
Earthquakes, volcanic 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. 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 of the Eastern
Caribbean Island Arc (Web graphic of West Indies University)
Volcanoes in the Eastern Caribbean
Region have unstable flanks. Destructive local tsunamis may be
also generated from aerial and submarine volcanic edifice mass
edifice flank failures, which may be also triggered earth tides,
earthquakes, or simply by gravitational instabilities. Mechanisms,
resulting in flank failure processes of volcanoes and their potential
for tsunami generation - in general - were presented separately.
More specifically, the following report evaluates recent volcanic
eruption mechanisms of active stratovolcanoes of the Lesser Antilles
in the Eastern Caribbean. Specifically discussed and analyzed
are the eruptive processes at work for Soufriere Hills, Mt. Pelée,
Soufriere and Kick'em-Jenny, as well as the time history of major
specific eruptions that resulted directly or indirectly in tsunami
generation in recent times.
Eruptive
Processes of Stratovolcanoes of the Lesser Antilles Islands (Montserrat,
Martinique, St. Vincent and Grenada) - Mechanisms of Flank Failures
and Tsunami Generation
Soufriere Hills Volcano
on Montserrat Island
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 years.
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.
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.
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)
In 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).
Mt. Pelée Volcano
on Martinique Island
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 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).
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 eruptive phase.
 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".
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. Pelée.
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).
A 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.
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 pyroclastics 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
- La Soufrière Volcano
 La 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
(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 1976 eruption
of La Soufrière on St Vincent island (Photograph by Richard
Fiske)
There 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
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 lives.
Eruption Processes
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.
Coincidentally, the 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.
Island of Grenada
- Kick'em Jenny Submarine Volcano
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.
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.
Flank instability: The distribution
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
flank.
 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)
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.
Assessment of the
Tsunamigenic Potential of Future Eruptions of Kick'em Jenny:
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.
 Tsunami
Generation 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 rapidly,
Morphology of Kick'em
Jenny volcano and of an extensive slope failure of unknown age.
(NOAA multibeam submarine survey)
Tsunami Generation
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.
Tsunami Generation
from Submarine Volcanic Dome Collapses:
Similarly, submarine 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 eruptions.
Tsunami Generation
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 St. Vincent.
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