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TSUNAMIS
A tsunami, also known as a seismic sea wave or as a tidal wave, is a series of waves in
a body of water caused by the displacement of a large volume of water, generally in an ocean
or a large lake. Tsunamis generally consist of a series of waves with periods from minutes to
hours.
Causes of tsunamis:
They can be caused by earthquakes, landslides on the seafloor, land slumping into the ocean,
large volcanic eruption or meteorite impact in the ocean.
75% of tsunamis are caused by earthquakes, they caused the movement of the overlying water.
8% of tsunamis are caused by underwater landslides.
Only 5% of tsunamis are caused by volcanic eruption.
The rest come from an unknown trigger.
Mechanism of formation:
On the Earth, plates are constantly in motion, moving along each other. These motions can
produce earthquakes and volcanism, which, when they occur at the bottom of the ocean, are
two possible sources of tsunamis.
When two plates come into contact at a region known as a plate boundary, a heavier plate can
slip under a lighter one. This is called subduction.
The energy of that force transfers to the water. The energy pushes the water upward above
normal sea level. This is the birth of a tsunami.
The tremendous force created by the seismic disturbance generates the tsunami's incredible
speed.
A tsunami's ability to maintain speed is directly influenced by the depth of the water. A tsunami
moves faster in deeper water and slower in shallower water.
Effects
Although the impact of tsunamis is limited to coastal areas, their destructive power can be
enormous and they can affect entire ocean basins; the 2004 Indian Ocean tsunami was among
the deadliest natural disasters in human history with at least 290,000 people killed or missing in
14 countries bordering the Indian Ocean.
The effects of a tsunami on a coastline can range from unnoticeable to devastating. The effects
of a tsunami depend on the characteristics of the seismic event that generated the tsunami, the
distance from its point of origin, its size (magnitude) and, at last, the depth of water in oceans
along the coast that the tsunami is approaching.
Small tsunamis, non-destructive and almost undetectable, happen almost every day. They are
very often too far away from land or they are too small to have any effect when they hit the
shore. When a small tsunami comes to the shoreline it is often seen as a strong and fastmoving tide.
However, when tsunami waves become extremely large in height, they attack coastlines,
causing destruction and death. A small wave only 30 centimetres high in the deep ocean may
grow into a wave 30m high as it sweeps over the shore.
Prediction
Tsunamis can’t be predicted with today’s technology. So instead of Prediction Centers, there
are Warning Centers. These centers are in charge of detecting a tsunami as soon as possible
and warn potentially affected areas, as well as media and civil defense agencies, to
evacuate/prepare the area so the damage is decreased. There are different types of centers
which get information in different ways:
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PTWC and ATWC: this are two different tsunami warning centers one placed in the
Pacific Ocean (Hawaiian Islands more specifically) and Alaska. They are continuously
monitoring seismic events and changes in the tide levels.
Buoys and tide gauges: they record minute changes in sea level and are complemented
with seismograph stations (which record earthquake activity)
Deep-Ocean Assessment and Recording of Tsunamis (DART): it uses unique pressure
recorders placed on the ocean floor. Instead of detecting sea level rise like the other
ones, they detect changes on pressure of the water above them.
Y. Tony Song’s prototype: Previous warning methods only relied on how much the
seafloor is displaced vertically. Song theorized that horizontal motions of the continental
slope transfer kinetic energy, which means they contribute on tsunami’s power. He
successfully demonstrated this when after the Chilean earthquake of 2010 he, along
with his partners in the NASA’s Jet Propulsion Laboratory in California, predicted the
size of the resulting tsunami. He relied on data from coastal GPS stations and
information about the continental slope.
Geoscience Australia: they are currently working on a system based on several
seismometers in the Indian Ocean. They are aiming to monitor earthquakes so they can
beam the signals and find the epicenter.
Detecting tsunamis and warning about them is just half of the work when preventing disasters. It
is also important to teach people and governments how to respond to these warnings.
How to respond to these warnings (direct quotes from the Australian Government website: Joint
Australian Tsunami Warning Centre)
Tsunami Watch
Be aware there is a potential tsunami threat. Make preparations and stand by for further
updates. At this stage there is no need for further action.
Tsunami Warning – Marine Threat
Be aware of potential hazards from strong rips, abnormal currents and dangerous waves. Get
out of the water, secure boats and move away from beaches, marinas, estuaries and rock
platforms. Any possible flooding will most likely be limited to the immediate foreshore. Major
evacuations are therefore not required.
Tsunami Warning – Land Threat
Be aware of potential hazards from major coastal flooding, dangerous waves and strong rips
and currents. If your area is advised to be under threat, move to higher ground -10 m above sea
level or 1k m inland.
Cancellation
Even though the tsunami warning has been cancelled, unusual rips, tides and currents may
persist for several hours. If there has been damage due to a tsunami, it may also be sometime
before it's safe to enter affected areas. Emergency services will advise the 'All Clear' when it is
safe to return.
Specific Site Planning Strategies to Reduce Tsunami Risk
1. Avoid Inundation Areas: Construct away from hazard area or locate on a high point.
2. Slow Water: Forests, ditches, slopes, or berms can slow down waves and filter out debris.
The success of this method depends on correctly estimating the force of the tsunami.
3. Steering: Water can be steered to strategically place angled walls, ditches and paved roads.
4. Blocking: Walls, hardened terraces, berms and parking structures can be built to block
waves.
SOURCES:
http://science.howstuffworks.com/nature/natural-disasters/tsunami6.htm
http://www.australiangeographic.com.au/topics/science-environment/2012/05/tsunami-warning-whyprediction-is-so-hard/
http://www.nasa.gov/home/hqnews/2010/jun/HQ_10-139_Tsunami_Prediction_System.html
http://media.bom.gov.au/social/blog/12/tsunami-warnings-how-to-respond/