A
megatsunami is a term used by the popular
media to describe very
large tsunamis. There is no scientific definition of a megatsunami, but
informally the term has been used for tsunamis with waves of height
from 40 m to over 100 m.
They are a highly
local effect, either occuring on shores extremely
close to the origin of a tsunami, or in deep, narrow inlets. The
largest waves are caused by a very large landslide, such as a
collapsing island, into a body of water. They can potentially reach 20
km inland in low-lying regions.
General Mega
Tsunami information
The
astounding heights quoted for megatsunami
waves are due to the
displacement of a very large volume of water movement in a very short
time near a shoreline.
Megatsunamis are
caused by impact, explosive volcanic, or landslide
phenomena. Underwater earthquakes do not normally generate such large
tsunamis; typically tsunamis caused by earthquakes have a height of
less than ten metres at the shore but can affect thousands of
kilometres of coastline.
Known megatsunamis
Megatsunamis
were first hypothesized by
geologists searching for oil in
Alaska in 1958. They observed evidence of unusually large waves in the
nearby deep inlet called Lituya Bay, Alaska. This is an ice-scoured
inlet 220 m deep with an entrance only 10 m wide. The topology of the
inlet is particulary suited to producing local megatsunamis. A nearby
magnitude 7.5 earthquake on July 8 generated a landslide within the
narrow inlet which produced a wave that washed out trees 200 meters
above normal sea level. Comparison with previous photographs indicated
that several hundred feet of ice had been removed from the front of a
nearby glacier by a 520 m high wave. However this quoted figure was not
the height of the open water wave, but the height it tore up the
mountainside due to its force of impact.
In 1963, a
man-made megatsunami occurred as a result of human
destabilisation of a mountain valley. An enormous slab from the side of
Mount Toc, in the mountains north of Venice, Italy, became destabilised
as a result of reservoir filling, and slid into the Vajont Dam
reservoir at 110 km/h, emptying 50% of the water within 10 minutes.
This produced waves some 250 meters high which destroyed several
villages, and killed nearly 2000 people. Remarkably, most of the dam
survived, although it was rendered almost useless by the infill of the
reservoir and structural damage to the I-beams and mechanisms of its
interior.
The geological
record suggests that megatsunamis are rare, but due to
their size and power, can produce immensely devastating effects.
However as with Lituya bay, this is often localized; the most recent
megatsunami known to have a widespread impact which reshaped an entire
coastline occurred approximately 4,000 years ago on Réunion
island, to the east of Madagascar. [1]
In the Norwegian
Sea, the Storegga Slide caused a megatsumani 7,000
years ago. Extensive geological investigations indicate that the risk
of a re-occurance is minimal.
Megatsunami threats
Volcanic
islands (such as Réunion and
the Hawaiian Islands) can
cause megatsunamis to hit other nearby islands in the same chain
because often they are structurally little more than large, unstable
piles of loosely aggregated material heaped up by successive eruptions.
Evidence for large landslides has been found in the form of extensive
underwater debris aprons around them composed of the material which has
slipped into the ocean. In recent years five such debris aprons have
been found in the Hawaiian Islands alone.
Some geologists
speculate that the most likely candidate for the source
of the next large-scale megatsunami is the island of La Palma, in the
Canary Islands, although further research has dismissed the threat.
During the 1949 eruption the western half of the Cumbre Vieja ridge
slipped several metres downwards into the Atlantic Ocean. It is
believed that this process was driven by the pressure caused by the
rising magma heating and vaporising water trapped within the structure
of the island, causing the island's structure to be pushed apart.
During an eruption that is anticipated to occur sometime within the
next few thousand years the western half of the island, weighing
perhaps 500 billion tonnes, may catastrophically slide into the ocean
in a single event. Were this to happen it could in theory generate a
megatsunami, causing local wave heights of hundreds of meters and a
likely height of around 10–25 meters at the Caribbean and the Eastern
American seaboard coast several hours later.
Besides fjords in
Alaska, many locations face threats of localized, but
still potentially dangerous, megatsunami-type waves. Some geologists
speculate that an unstable rock face at the north end of Harrison Lake
in the Fraser Valley in southwestern British Columbia could collapse
into the lake, generating a large wave that might destroy the town and
Harrison Hot Springs resort at the south end.
