Tsunamis

Tsunamis:

• A tsunami is a series of ocean waves that are caused by large, sudden vertical movements on or near the seafloor.
• The word 'tsunami' is a Japanese term that means 'harbour wave'. Tsunamis are commonly called tidal waves, but in fact the tide has nothing to with starting a tsunami.
• Tsunamis can travel up to 400-950 kilometres per hour - almost ten times faster than car speeds allowed on highways. Successive peaks can be anywhere from five to ninety minutes apart. The wave train that reaches the coast can range from 30 metre high breakers to barely noticed ripples - but in the open ocean, tsunamis have relatively small heights. A tsunami wave can have major effects on coastlines a thousand km across open ocean.
• A tsunami is different from normal ocean waves. Wind-made ocean waves cause the water to move down to about 150 meters depth at most. In contrast, the passage of a tsunami involves the movement of water all the way to the seafloor.
• This means that the speed of a tsunami is controlled by water depth - as the wave approaches land it reaches increasingly shallow water and slows down. Compared to the front of the wave, the rear is still in slightly deeper water (so it is going slightly faster) and catches up. The result is that the wave quickly 'bunches up' and becomes much higher.
• The highest tsunamis occur if they encounter a long and gradual shallowing of the water, because this allows enough time for the wave to interact with its surroundings.
• The first sign of an approaching tsunami can be either a rise or fall in the water level.

• The largest tsunamis are powerful enough to throw 100-tonne blocks of coral onto shore or carry a boat a few kilometres inland. The danger of tsunamis is increased by their rarity - it is easy for communities to become complacent about the risk to their region.

• Death tolls can be large following the worst tsunamis, and may reach the tens of thousands. In some cases, many more people have been washed out to sea and never found. People are often deceived when the 1st wave recedes leaving fish on shore to catch or survivors to rescue, only to be killed by the usually bigger subsequent waves. Ongoing casualties may arise from lack of fresh water and massive displacement of populations.
• examples of tsunamis:
  • Apr 2007: tsunami from a Richter 8.0 undersea earthquake hits Solomon Islands killing at least 52 and displacing 6,000 people.
  • Dec 2004, an Indonesian undersea earthquake measuring Richter 8.9, the biggest since 1964, raised the sea floor for some 1000km & caused massive tsunamis to hit coastal areas from Sri Lanka, India, Thailand and Sumatra destroying coastal resorts & killing at least 250,000 and destroying almost everything for up to 4km inland in many cases. Unfortunately, no-one paid attention to the tremors they felt in Phuket 90min before the tsunami hit. The tsunami caused casualties as far away as Kenya!
  • 1998: > 2200 killed in PNG
  • 1976: > 8000 killed in SW Philippines
  • 1964: 120 killed in Alaska
  • 1960: massive earthquakes (Richter 9.5) and tsumanis kill 5000 in Chile
  • 1896: > 27,000 killed in Japan
  • 1883: Krakatoa eruption produces tsunami & kills > 36,000
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  • 1755: > 60,000 killed in Portugal.
  • c1500: massive tsunami hits east coast Australia bringing sand kilometers inland.
  • It is thought that a tsunami from the massive Santorini volcano was responsible for the "parting of the Reed Sea" which is said to have allowed Moses to cross it but then the waters returned killing the Egyptian soldiers.
• Infrequent but powerful tsunamis can pose a significant threat to the people and property in coastal areas. However, early-warning systems have helped to reduce the risks.

  How do tsunamis occur?

• Most tsunamis are caused by large earthquakes at the seafloor, when large slabs of rock are forced to move past each other. Earthquakes cause parts of the seafloor to raise or drop suddenly - the overlying water must move too. The resulting wave moves outwards and away from its source.
• Underwater landslides can cause tsunamis, and so can land that slumps into the ocean. Landslides happen when slopes become too steep to withstand gravity.
• Less commonly, volcanic eruptions can initiate a tsunami - this happens in several ways:
  • if an underwater volcano erupts, the hot lava may heat the surrounding seawater quickly and explosively
  • massive flows of volcanic debris such as ash can travel down the side of a land volcano and into the ocean, pushing water outwards as it does so
  • the top of an underwater volcano may collapse downwards, so that the overlying water also drops when compared to water that is further away
• Large meteorite impacts that occur at sea can trigger tsunamis too.

  Do tsunamis reach Australia?

• On average, Australia records one tsunami every two years and most are small - often, only a close look at tidal records shows that a tsunami has reached our shores.
• Most of Australia's coast has a 'low' or 'moderate' threat of tsunami. A low threat is assigned where little damage has been reported following any tsunamis, or where no tsunamis have been reported at all. An area with a moderate threat has reported tsunamis that reached maximum heights above sea level of 1-3 metres. Maximum heights reached by tsunamis reported in high-threat areas have ranged from 4 to 32 metres, or even more.
• Since 1883 several potentially damaging tsunamis have occurred along the northwest coast of Australia. In the last 1000 years the Australian coast may have experienced large tsunamis because shells, coral and large boulders have been found 20-30 metres above sea level and several kilometres inland.
• there is a theoretical risk that a rupture of the Alpine Fault on the west coast of the South Island of NZ may cause a earthquake of over Richter 8 which could send a tsunami some 2-3hrs later to Australia's east coast including Sydney. Major ruptures of this fault occur every 500yrs or so, the last one was in 1453AD.
• in Dec 2004, a sub-sea earthquake south of Tasmania measured Richter 8.5 but fortunately produced horizontal shift and thus a tsunami did not hit Hobart.