What
causes a tsunami?
The most common cause is a seafloor earthquake. Other triggers are undersea
landslides, undersea volcanic eruptions, and meteorite impact. Sudden
changes to the seafloor cause the ocean to flow away from the disturbance,
creating waves.
What cased the Boxing Day tsunami in the Indian Ocean?
A magnitude 9.3 earthquake occurred on the seafloor off the western coast
of Sumatra. It ruptured over a 1200km section of a fault under the sea,
causing a huge area of the sea floor to be raised . This caused many hundreds
of cubic kilometres of sea water to be lifted above its normal level.
It spread out as a tsunami.
Towns on the coast facing away from the quake epicentre, such as Banda
Aceh, suffered huge damage. How can this be?
Tsunami waves can travel around corners. As the huge energy of the tsunami
moves through the ocean, its speed, direction, and behaviour are influenced
by the depth and shape of the seafloor. The waves can be deflected and
slowed by seamounts and other large structures on the sea floor. GNS scientists
use computer models to show how tsunami waves propagate through the ocean
and around New Zealand's coastline.
Did the Boxing Day tsunami waves reach New Zealand?
Yes. Waves up to half a metre in height were recorded at 15 coastal sites
(mostly ports) between 18 and 25 hours after the earthquake occurred.
The tsunami's path to New Zealand was long and winding. It went around
the bottom of Australia and was reflected northward by the Campbell Plateau,
in sub-Antarctic waters. Wave heights and arrival times around the New
Zealand coast varied. This was no surprise to scientists as the ocean
floor around New Zealand is highly variable. The tsunami waves were further
modified when they entered New Zealand harbours, which are different shapes
and sizes. The waves also reached the North Atlantic and North Pacific
Oceans.
How fast do tsunami waves travel?
In the open ocean, tsunami waves travel at 600kph to 700kph. In the deep
ocean, waves from a large tsunami may be as little as 60cm high. They
pass ships unnoticed. As they encounter shallow water, they slow down
to about 30kph and increase in height.
Is a tsunami a dream come true for extreme surfers?
No. A tsunami is not a wave in the classical sense, but a raging torrent
of water that surges inland with enormous power - much more power than
a surfer can handle. Besides the waves do not stop at the shore, so surfers
may find that they crash into buildings or all the other debris caught
up in the surging water. Invariably a tsunami consists of successive surges
or torrents and equally violent return flow to the sea. The first wave
or torrent of a tsunami is not necessarily the biggest. The second or
third or even much later waves may be bigger. Intervals between successive
waves can vary. It may just be minutes, or it could be more than an hour.
How far inland can a tsunami go?
In low-lying coastal areas they can travel a long way inland. In parts
of Thailand, the tsunami went 3 km inland destroying almost everything
in its path. In Banda Aceh, the tsunami surged 6km inland.
How does seawater destroy buildings?
Easily! A tsunami is not just seawater. It picks up a huge amount of debris
which gives it added destructive power. People don't die just from drowning,
many are killed by being hit by debris and heavy objects in the surging
torrent. A retreating tsunami wave can cause as much damage as the initial
forward surge. A tsunami can exert huge forces against the side of a building.
In Thailand, engineers calculated that modern reinforced concrete walls
of hotels were blown out by tsunami waves generating as much pressure
as 3000kg/sqm. This is far greater than earthquake design loadings.
Why weren't people warned in the countries bounding the Indian Ocean?
There is a 24/7 tsunami-warning centre based in Hawaii for the Pacific
Ocean, but the Indian Ocean does not have an equivalent. The Pacific warning
centre was able to put out a warning in time to save lives in Kenya, but
the protocols for informing many other countries were not in place. In
the wake of the disaster, the UN is planning to install a warning system
for the Indian Ocean. The project is being led by Reid Basher, a former
New Zealander who now works for the UN in Bonn, Germany. But some places
were too close to the tsunami source on 26th December for this type of
system to have been effective, even if it had been operating then.
Why does the seawater recede a long way out, sometimes hundreds of
metres, before coming back in as a tsunami?
Tsunamis are not just moving lumps on top of the ocean surface, they also
include hollows, and sometimes the hollow reaches the coast first. When
this happens, the ocean first draws down and sucks water away from coastlines.
It then rushes back in with enormous speed and force as the lumps arrive.
People who notice the receding water have as little as five minutes to
flee inland to higher ground.
Has
New Zealand been hit by tsunamis?
New Zealand has experienced about 10 tsunamis higher than 5m since 1840.
Some were caused by distant earthquakes, but most by seafloor quakes not
far off the coast.
A nearby coastal seafloor earthquake is the only warning people may get
before a tsunami arrives. Such a tsunami can arrive within minutes - before
there is time to issue a warning. Distant earthquakes give more warning
time. Some tsunamis are turbulent, foaming walls of water filled with
debris and sand that crash ashore and sweep inland. Others are just rapidly
rising or falling water levels over minutes to an hour. Both are dangerous.
Both can travel over land at speeds faster than a person can run. The
turbulent surge is invariably more destructive because of the churning
and faster speed of the water.
How vulnerable is New Zealand to tsunamis?
Quite vulnerable. Tsunami hazard for the Pacific is higher than for other
oceans because of the "Ring of Fire" - the zone of earthquakes
associated with the tectonic plate boundary that bounds the Pacific. In
New Zealand, scientists consider the tsunami hazard from two viewpoints
- Pacific-wide events for which there will be some warning, and "near-source"
tsunami generated by large offshore New Zealand earthquakes, landslides,
and volcanic eruptions. The list of possible local tsunami sources is
long. Because tsunami research is relatively young (compared to other
geological hazards), the state of knowledge about the severity and frequency
of local and distant tsunamis is poor. One of the main challenges facing
tsunami researchers is that evidence of tsunamis does not stay in the
landscape as it is eroded by natural processes. Impacts of past tsunami
are therefore difficult to identify, unless someone was around and wrote
about them at the time.
Does New Zealand have a tsunami warning system? If so, how does it
work?
New Zealand has a warning system for tsunamis caused by distant earthquakes
(such as in South America), but does not have a warning system for tsunamis
caused by local events. Why? Because tsunami generated by local earthquakes
can arrive at the nearest coast before scientists can calculate the location
of the earthquake and issue a warning.
The only country to have a warning system for tsunamis caused by nearby
earthquakes is Japan. This is because they have an automatic earthquake
location system based on hundreds of seismographs, AND because Japanese
scientists have estimated in advance what tsunamis could be caused by
100,000 different earthquakes at 10,000 locations around their coastline.
As far as tsunamis coming to New Zealand from distant places, the Ministry of Civil Defence and Emergency Management receives warning from the Pacific Tsunami Warning Center in Hawaii. Ministry of Civil Defence and Emergency Management is responsible for evaluating the danger to New Zealand, and will ensure national or regional warnings are issued, if necessary. Tsunamis from South America, Alaska, and Japan, take more than 12 hours to reach New Zealand, giving authorities time to make decisions.
What research does GNS do on tsunamis?
GNS works with a number of groups, including NIWA, universities, and private
individuals, to improve the knowledge of tsunami hazards in New Zealand.
Current research includes:
When a tsunami threatens, what should you do?
Locally generated tsunamis
If you are near a beach and feel a strong earthquake or notice the sea
receding unusually, move to higher ground immediately. You may have only
a few minutes.
Long distance tsunamis
What were some of the main lessons that the team of New Zealand scientists and engineers brought back from their week-long visit to Thailand in January 2005?
Tsunami animation download
Follow the link
to learn more about the Indian Ocean Tsunami and download an animation.
Modelling performed by William Power using the MOST software developed
by Vasily Titov at PMEL.