The Great Wave Off Kanagawa by Hokusai
most famous Japanese Tsunami painting by the 18th Century artist Hokusai,
full name Katsushika Hokusai (1760-1849).
The painting depicts a tsunami passing
in front of Mount Fuji.
definition: Tsunami. Noun. An extremely
large wave caused by movement of the earth under the sea, often caused by
is also the scientific word
for a sea wave/sea surge generated by an undersea landslide¹, volcanic
eruption, asteroid impact not to mention any man-made catalysts.
¹ The Dec 26th catastrophe
(see below) was caused
by an underwater quake.Tsunamis are also caused by
terrestrial or submarine landslides.
Several cases of tsunamis caused by landslides have been recorded history. One such happened ±
2 million years ago when a block of 4 000 cubic kilometres slid off the Hawaii island of Oahu into the ocean causing a giant Tsunami.
A tsunami is not a tidal wave. It has nothing to do with tidal movements.
Sea Wave or Sea Surge would be a more appropriate name if you don't feel easy with "Tsunami"*
*Tsunami is the Japanese word for "harbour wave".
Defences, natural or man made
Although very rarely brought to the general public's notice, unless they reach such magnitudes as Dec 26th catastrophe, local or regional tsunamis do in fact occur several times every ten years, at different spots around the Pacific ocean's "Ring of fire".
fact that the populations of tsunami risk coastal regions have increased sharply
in recent years means that the death and injury tolls are higher today that
it ever was.
The fact also that such regions are now prolific tourist resorts² means
that there is also a highly developed media structure and with the acceleration
in the transmission of media information news of such natural hazards is quasi-immediate
bringing them global coverage.
(² The Gross Domestic Product(GDP) of certain countries is reliant on
tourism. Tourism in Thailand's represents 8% of the country's GDP.)<
Man's implantation in these coastal zones has weakened natural defences leaving the zones open to erosion and destruction that would have otherwise been avoided
had the defences, ex: indigenous mangrove swamps or coral reefs, not been
clear to allow for tourist installations or prawn farming, as is the case
in the Thailand.
Settlements, town and even cities have been built in estuaries and Deltas
whose natural defences mechanisms, that once protected the regions against
such severe natural hazards, have been weakened or totally removed. Dams are
built upstream from these settlements hampering the natural depositing of sediment
and silt in the estuaries leaving them open to erosion by the sea.
has a vast experience with such calamities and has intensively studied them and learnt lessons from them.
The densely populated coastal regions on Japan's numerous islands now possess
Tsunami protection systems such as Tsunami walls* and evacuation plans.
walls are often no higher than
a couple of meters and only provide a relative protection against the
"Average" tsunami but are generally of no real protection against
the more violent Tsunamis. Fortunately the Japanese, being signatory of
the Pacific TWS, are equipped with a highly effective electronic early
warning system. The PTWS (see below) continually
informs the Japanese
Meteo Agency of all tsunami and seismic activity, be it in the immediate
vicinity or more generally around the entire ring of fire as earthquakes
and landslides in Alaska or off the coast of Chile. This information is
vital as such far away activities have as much an impact on Japan as does
the seismic activity immediately off their coast*.
(* Due to the the proximity and convergence of the Philippine& Pacific
tectonic plates, just off the coast of Japan there is a particularly high
seismic activity. As such tsunami warnings may be followed immediately by
the Tsunami themselves, leaving the population little time to evacuate. However,
fortunately the Japanese follow a very rigorous evacuation program and as
a result the mortality rate of the tsunamis is often minimum.)
Other causes of Tsunamis
Not all tsunamis are the result of seismic activity and as such the seismic
activity monitoring alone may not be enough to help warn against tsunamis
generated by other causes. Landslides, asteroids or, unfortunately, human activity
are other causes of tsunamis.
- Landslides. As mentioned above, in
the intro, landslides are another major cause of tsunamis. Landslides can
be caused by earthquakes but also by tides, storm waves or sediment deposition
By definition a landslide is caused by the downward sliding movement of a
mass of sediment or rock. The sliding mass pushes the mass of water before
it and sucks or pulls the water behind it. The frontal pushing effect elevates
the water in front of the sliding land mass and a depression above and behind
the landslide which is immediately filled by the frontal elevated mass of
water thus creating the tsunami, see landslide graphic below.
Asteroids. Asteroids are another cause of tsunamis, the most famous example
probably being the asteroid that crashed into the Yucatan ± 65 millions
years ago. However, tsunamis created by asteroid impacts are not as rare as
that in fact Asteroid generated Tsunamis are recorded every year. Example.
In 1998 an asteroid generated tsunami killed more than 2000 people in New
Human activity. Helas human activity is a another cause of Tsunamis. In
an effort to understand severe natural climatic or geological hazards certain
countries now have the technology to re-create calamities such as Hurricanes
specialists are starting to voice the theory that the Dec 26th earthquake
was a man made simulation gone wrong and that one these numerous countries
was trying to artificially re-create an earthquake. This theory is given a
certain credibility as although Indonesia is on the Western rim of the "Ring
of fire" * the Island of Sumatra has itself never experience an earthquake
of such a magnitude. In fact one of the reasons why the Indian ocean does
not have a protection system as exists in the Pacific ocean, is because the
seismic activity in the Indian ocean is minimum, verging on the non-existent!
(* As Indonesia is part of the Pacific "Ring of Fire" the Japanese
PTWS centre duly registered the seismic activity off Sumatra before the quake
actually occurred. The only problem was that the PTWS centre had no pre-established contact or resources
in or around the Indian ocean basin to warn local or regional authorities of the calamity that was soon to hit the region.)
man made sources of tsunamis can be attributed to undersea drilling that may
accidentally trigger seismic activity or underwater nuclear testing such as
the French nuclear testings in the South Pacific before they were "Officially"
stopped in the mid 1990s.
The mechanics of a Tsunami
1- Seismic Tsunami
Click on image to see full size
2- Landslide Tsunami
Most tsunami originate around the Ring of Fire, a zone
of volcanoes and seismic activity 32,500 km long, that encircles the Pacific
Ocean, from the Americas to Asia (see graphic below) although tsunami
may happen anywhere in the world, such as the in the Atlantic.
- Fact: Since 1990 82 tsunamis
have been reported, principally within the Pacific Ring of Fire
- Fact: A major submarine
slope failure in the N. Atlantic could give rise to a tsunami large
enough to flood major cities on the coast of America or Europe.'
- Fact: Mount St Helens
(present on the map below). It has been calculated that if the
Mount St Helens' eruption, in 1980, had happened underwater it would have
created a 'tsunami/tdal wave' with waves over ± 300 meters
high/ 900 feet high!!!
- Fact: A tsunami can have
a wavelength (distance between wave crests) in excess of 100km and with
up to an hour between them. tsunami travel at great speeds across
an ocean with hardly any energy losses and are barely noticeable out at
- Fact: Over the deep Pacific
Ocean, a tsunami travels at about 500mph. If an earthquake happened
in Los Angeles, a tsunami could hit Tokyo quicker than than it
takes to fly between LA & Tokyo.
- Fact: tsunami
waves hitting coastlines have shifted 20-tonne rocks hundreds
of metres inland
Pacific Ring of Fire
Click on image to see full size
Exceptions to the rule
notable exception to the above mentioned rule happened approximately 7200
years ago, off the Norwegian coast, when an underwater landslide, called
the 'Storegga' slide created a tsunami reaching
West to Iceland and South to the Shetland Isles, totally devastating habitants
and fauna of the affected regions*.
*NB. The British Geological Society, based in Edinburgh, evaluate the
risk of submarine landslides and resulting tsunamis in the region,
today, sufficiently serious to undertake extensive studies to gauge the consequences
of such a natural hazard occurring today. While the impact of the 'Storegga'
slide, ±7200 years ago, were sufficiently important, from a physical
point of view, it's socio-economical impact was minimum. Today it's consequences
would be radically different and the fact that the B.G.S. have recorded
various other submarine movements underlines the importance of such a risk
exception is The Marmara sea, in the eastern Mediterranean, off the Turkish
coast, leading to the Bosphorous and the Black sea, regularly witnesses
tsunamis following earthquakes, such as happened in 1999.
another exception, forecasted this time, and published on BBC.com, anticipates
that a flank of the Cumbre Vieja volcano on the Canaries Las Palmas island
is unstable and could plunge into the ocean, creating a landslide generated
Tsunami (see above). Swiss researchers
have modelled the landslide and estimate that half a trillion tonnes of
rock falling into the water all at once would create a wave 650 metres
high (2,130 feet) that would spread out and travel across the Atlantic.
It would have a wavelength of 30 to 40 kilometres (18 to 25 miles) travelling
westwards across the Atlantic at speeds up to 720 km/h (450 mph) towards
America. The wall of water would weaken as it crossed the ocean, but would
still be 40-50 metres (130-160 feet) high by the time it hit land. The
surge would create havoc in North America as much as 20 kilometres (12
also estimate that the effects on African & European coastlines would
be catastrophic. A wall of water a 100 meters high would totally submerge
extensive regions of the West African coastline and devastate coastal regions
of the United Kingdom, France, Belgium and the Netherlands....
on images to enlargen
tsunami can race across the ocean at ± 500 mph and at sea
its waves are only a few feet high with the waves increasing in energy
and height, often with waves over 100 feet high, when the waves approach
the coast,. Often before a tsunami hits, there is a giant vacuum effect,
and water is sucked from harbours and beaches.
NB. When tsunamis reach the coast they create a vacuum effect, causing sea water to retract from the shore which in turn feeds the
approaching wave, alerting the population of the impending risk, giving them a certain amount of time to evacuate.
can see the bare sea floor, uncovering fish and stranding boats. That is because
waves are made out of crests and troughs. When a trough hits land first, the
water level drops drastically. Usually another wave blasts ashore about 15
minutes later, repeating the action for a while after the fist wave hits the
shore. After a tsunami many people return to their homes, not realizing that
come in groups, and the first not always the largest. Some of the most destructive
damage caused by a tsunami is not from the arrival of the wave itself, but
from the undertow it creates as it leaves land and heads back to the sea,
carrying objects and people with it.
mention that Tsunamis killed more than 50,000 people during the course
of the 20th century. In order to help saves lives, scientists established
the Pacific Tsunami Warning System, based in Honolulu, Hawaii*, Its network
of earthquake detectors and tide gauges detects quakes that may cause a tsunami
and the monitoring of sea floor activity helps give warning of a potential
tsunami and this warning may give as much as a few hours.
organizations, such as the NOAA study and
analyse Tsunamis, as a part of their global surveillance, creating
simulations to better
understand the impact of Tsunamis on the environment.
seriously does the take tsunamis that Hawaiian authorities have issued instructions
on what to do in case of a Tsunami and have elaborated plans
informing people of flood zones and safety zones to which they may retreat.)
certain of the facts and statistics PTWS never issue false or ill
founded warnings about Tsunamis simply because that any
one given area around the Pacific 'Ring of Fire' can, one day or another,
expect a visit from a tsunami and PTWS member states
are kept updated on all developments.
26/12/2004 : A date to remember
26th 2004. At
±10.15AM local time the entire Indian ocean was struck by a series
of Tsunamis resulting from an earthquake just off the north west coast
of Sumatra. Initially estimated at 8.9 on the Richter scale the earthquake
was later upgraded to 9 on the Richter scale. The force of the quake was
so strong that it was felt in Western Africa and with the tsunami reaching
the East African coast, hitting Somalia, some five thousand miles to the
west and causing the loss of over a hundred fishermen.
speaking the Dec 26 earthquake and following Tsunami was the result of a movement
in the Eurasian plate on the western side of the ring of fire but outside
of the Tsunami Warning system's
(TWS) surveillance zone set up around the ring of fire to
monitor such seismic or tsunami activity.
of such buoys dot the pacific ocean and feedback all activity, via GPS satellites,
to TWS monitor stations
the Indian ocean currently possesses no such warning system and the death
toll impacted to the actual tsunami catastrophe, initially estimated at ±
30 000, is now reaching 150 000 as Indonesian authorities reach areas where
communications and tourism are not as developed. The same applies as Indian
authorities access territories such as the, hitherto, little known Andaman
& Nicobar islands*.
This could have been avoided had countries such as India and Indonesia been
member of the International Warning system.
* NB. As with a lot of people, it's unfortunately true that it took the South East Asian earthquake
& resulting tsunami to put places such as the Andaman and Nicobar Islands
onto my personal World map. I had never heard of them before and it's a shame that the only time such
places come to the attention of the general public is when a cataclysm hits them, such as when Cyclone
"Zoe" devastated the Solomon Islands, Dec 28 2002.
Many thanks to the NOAA for above mentioned info, images and links. All material
appearing is for a purely personal and documentary use
01/ 2005 & 11/2007