Tsunamis - an underwater Earthquake hazard

Tsunamis are also called seismic sea waves.  Whereas most waves that we experience day to day are created by both the gravitational pull of the moon on the sea and wind acting upon the sea, tsunamis have other causes.  They can be created by landslides, underwater volcanoes and more commonly underwater earthquakes.  The Boxing Day tsunami of 2004 in the Indian Ocean was created by an underwater earthquake.

The Tsunami of December 26^th 2004 killed over 220,000 people.  This was an enormous disaster of which most people were completely oblivious to on that Boxing Day morning.

Why the Tsunami happened

Just after dawn on the Sundra trench a massive earthquake occurred on the ocean floor.  Here 2 plates driven by convecting heat in the Earth’s interior are grinding past one another.  When the stresses that build up are released a massive earthquake occurs.  The Indo-Australian plate is being subducted under the Eurasian plate at the Sundra Trench, and has been doing this for 20million years.  This zone has stick slip frictional properties, which means that it drags the upper plate down with it, deforming the upper plate.  Eventually the stress becomes too much and the upper plate snaps back – causing the earthquake.  This section was just near to Aceh province and is 1200km long, from western Sumatra up to the Nicobar and Andaman Islands.  The motion generated was 20m of the continental plate over the oceanic plate.  Most of this was horizontal but some was vertical, most of the movement was West or East.  Seismographs in California showed how the earthquake lasted over 4 minutes and measured 9.0 or the logarithmic Magnitude scale.  The energy goes up by 30 every level you go up on this scale. 

When an earthquake occurs under the ocean its seismic energy can dissipate through the Earth's crust or as shockwaves through the ocean water.  The earthquake occurred close to the surface, only 30km deep, and caused tsunami initiation.  The movement of the sea bed upwards displaces billions of tonnes of water above it, some sea floor falls and water rushes in to replace it.  The uplifted water collapses and rushes out radially outwards at a thousand km per hour.

A timeline of destruction

20 minutes before the water arrives Banda Aceh feels the force of the earthquake and buildings collapse.  Across in the Pacific, at the Hawaii Pacific Tsunami Warning Centre pick up the seismic signals just one minute after the quake.  Any earthquake above magnitude six results in a pager being sent to the scientists of the centre.  The scientists used data from numerous sensors (including water level gauges), but they are only in the Pacific Ocean and have been used to help people for the past 50 years.  A system such as this did not exist in the Indian Ocean.  The scientists warned of a tsunami and issued a tsunami bulletin.

10 minutes after the earthquake in Sumatra (Aceh province) the wave is approaching at 600miles an hour, in the open ocean boats were hardly affected.  As the wave neared land the wave grew by a process of amplification.  The front of the wave slows down as it approaches the shore, but the back of the wave is still going its original speed, so the wave builds in size.  Before it hits land, the approaching wave sucks water in from its leading edge exposing the sea bed at the coast for up to a mile; this led many people to their deaths in the disaster.  

Tsunamis rush in as a surge rather than as a classic curved wave, and they have incredible wave lengths of hundreds of kilometres meaning that they keep coming in for 5 minutes or more.  The mass caused huge damage –one cubic meter of water has a mass of one ton, every 1.5m of coastline was exposed to 1000tons of water.  The water was also not clean; it had rocks, cars, and other debris which caused damage.

15minutes after the quake Northern Sumatra becomes the first place to be hit.  It caused utter devastation.  Banda Aceh was completely destroyed, with barely any buildings left standing.   The height of the wave here was colossal, higher than the coconut trees.  Ships were thrown up onto the land, and the cement works were destroyed.  ¾ of the tsunamis victims died in Sumatra.

30 minutes after the quake, 7000 people are killed in the Nicobar Islands

45 minutes after the earthquake the scientists in Hawaii are still unaware of the tsunami.  They issue an upgraded tsunami warning.

1 hour after the earthquake in Thailand, people still do not know that people have died in Indonesia.  People climbed upon the tops of hotels to survive.  The Geomorphology (shape and orientation) of the coastline determined death tolls along the coast of Thailand, and this was shown along the West coast of the Island of Phuket. The exposed bays which faced the wave bore the brunt of the wave, whereas bays which faced away got of relatively lightly. Surin Bay and Beach had a steep beach and protecting headland, and was relatively untouched.  The water was only 30cm deep there, but in Kamala beach the bay faced the wave, had a very shallow gradient allowing the wave to build up and had low land behind the bay.  Kamala was devastated. Buildings and decking were ripped up.  Phuket was hit by a second wave 15 minutes later, which was even bigger than the first wave.  Multiple waves in Tsunamis are normal. The second wave rolled around the whole of Kamala bay. As the water retreats back out to sea it pulled cars, debris, fishing boats and people back out with it. 

2 hours after the earthquake, the wave reaches Sri Lanka.  A train was running along the coast from Colombo to Galle.  The train was hit by the wave of water, and split the train up.  The second wave arrived soon after, killing most of the people who had survived the first wave on the train.  1500 people died on the train, 45,000 people died in India and Sri Lanka.

3 ½ hours after the earthquake the Hawaii Tsunami centre are shocked by the devastation, and upgraded their estimate of the size of the quake to 8.9.  There were no contacts that the team could call, so they tried everyone they could who were ahead of the wave.

3 ¾ hours after the quake, the Maldives are only slightly effected.

7 hours after the quake, East Africa is next affected.  News is finally getting to communities and in Kenya there was only one fatality as people were warned.  The wave dissipates throughout the whole ocean across the globe. 

Tsunamis are not new to Asia, Krakatau’s explosion in 1883 generated massive tsunamis.  NASA showed that the tsunami called the Earth to wobble by 2 ½ cm and slightly shortened the day.  The area affected has concerned scientists for years, and a network of deep ocean sensors could have saved thousands of people.  There were no water level sensors, no warning systems and no disaster plans.  This all contributed to the high death toll. 

1. By the end of the day of the 2004 Indian Ocean tsunami, it had already killed 150,000 people. The final death toll was 283,000.
2. The Indian Ocean tsunami travelled as far as 3,000 miles to Africa and still arrived with sufficient force to kill people and destroy property.
3. Many people in Indonesian reported that they saw animals fleeing for high ground minutes before the tsunami arrived – very few animal bodies were found afterward.
4. The risk of famine and epidemic diseases was extremely high immediately following the tsunami – bodies rotting in the tropical heat contaminated food and water sources.
5. 1.7 million people were displaced

The governments of the affected countries reacted reasonably quickly but were reliant in many cases on OUTSIDE AID due to the magnitude of the disaster and the lower level of economic development of the places affected.  India sent naval ships to affected areas, whilst Thailand, Indonesia responded but slowly, given the extent of the disaster. 

The first tasks of the governments and humanitarian aid agencies were to ensure access to food and clean water, and medical care for the injured. The World Health Organization warned that the number of deaths from preventable diseases such as cholera and typhoid could rival the death toll from the disaster itself.

The USGS managed to get a warning to Africa which saved lives

Country governments  helped to rebuild whole community towns, including in Indonesia in Aceh province

An early warning system for Tsunami has now  been put in place in the Indian Ocean, through co-operation from many governments

The UK government's Disasters and Emergency Committee established an emergency fund almost immediately. 

The DEC Earthquake/Tsunami disaster appeal fund stood at £32 million, up £7 million in one night from  the 31^st of December. At one point the DEC was receiving around £15,000 a minute in donations via the phone and Web

Countries sent aid teams immediately

MEDCs such as the UK sent dog teams, forensic experts and equipment to help identify bodies and clean up after the Tsunami

Action Aid raised £13million and immediately sent food aid.

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