Tsunamis

What is a tsunami? 

A special type of wave where an event, often an earthquake, moves the entire depth of the water above it. 

How do tsunamis form? 

Usually triggered by earthquakes
The crust shifts (primary effect) causing the displacement of water above it

Japan 2011 Case study

Cause:

The earthquake that triggered the tsunami occurred at a destructive plate boundary where the pacific plate was being sub ducted under the North American plate. It is thought that a segment of rock around 200km in length, slipped suddenly resulting in an upwards flick of the earth's crust between 5-10m 

Primary effects:

• 12,000 people killed
• Over 20,000 people were killed as the waves swept up to 10km onshore. The high death toll was due to the power of the surge of water which overtopped tsunami defences and flooded areas thought to be safe from tsunamis.
• The ground shook intensely and parts of Japan were shifted 2.4 metres further east. 
• The ground subsided and a 400km stretch of coastline dropped vertically by 0.6 metres, which allowed the tsunami to overtop defences and travel faster and further inland. 
• The saturated soil lost strength and stiffness and began to behave like a liquid (soil liquefaction). This was found to have occurred in areas of reclaimed land around Tokyo, damaging more than 1000 buildings. 
• 2 million people were left homeless as a result of the tsunami + earthquake
• 4 trains were swept away from their tracks 
• A cruise ship carrying 100 people was washed away by the sheer force of the waves
• A dam in the north of Fukishima burst its banks, this caused many more homes to be swept away 

Secondary effects

• The earthquake caused a tsunami which reached a maximum height of 39 metres at Miyako city and travelled up to 10 km inland 
• Tsunami waves up to 2 metres hit the coast of Chile
• There was major coastal flooding as a result of the tsunami flowing inland.
• 500,000 people were forced to live in shelters
• 50km squared of coastal plains were inundated, destroying farmland, settlements and communications
• A total of 200,000 buildings were damaged or destroyed by the earthquake and tsunami 
• The damage to underground gas pipes led to fires which spread quickly and ignited wood carried by the wave 

Immediate responses

• A tsunami warning was issued 3 minutes after the earthquake 
• A meteorological Agency official appeared on TV urging those affected by the quake not to return home because of the possible tsunamis 
• 11 aircraft were sent out within 30 minutes to survey the area and help inform emergency aid decisions
• On 2nd April, Japan and USA joined forces to search for bodies along the eastern coast
• Shelters were set up in schools for those who lived in close proximity to the Fukishima Dalichi nuclear power plant 
• 452,000 were cared for in evacuation centres
• Within 2 seconds of the earthquake being detected, all shinkansen trains in the area stopped. 
• Many NGO's (non governmental organisations) responded to the event. For example, Shelter box sent 1,500 boxes of aid including tents, sleeping bags and bottled water. 
• An exclusion zone was set up around the Fukishima nuclear plant and people were evacuated from the area. 

Long term responses

• A huge reconstruction programme was put in place including the reconstruction of houses, infrastructure, and communication systems
• The Japanese government responded by questioning their huge walls and tsunami floodgates as these did not prove to be very effective
• Sendai Airport was repaired by the US and Japan Army by the 29th March
• By November 2011, 96% of the electricity supply has been restored and 98% of the water supply 
• The government also responded to this disaster by reviewing their methods on predicting earthquakes and tsunamis as well as cutting the time between the disaster and the response by improving communications and sirens