A booklet for Students and the Community
Chapter 9
Tsunami Hazards and Disasters
FACTS ABOUT TSUNAMIS
Tsunamis are seismic sea waves. Their name is a Japanese word meaning ‘harbour waves’. They are often incorrectly called ‘tidal waves’ (rare events linked to tides). A tsunami is a series of travelling ocean waves of extremely long length and period. They are caused by under-sea events like earthquakes, landslides and volcanic eruptions; or by other natural events such as ocean meteorite impacts, or large landslides into bays or lakes. (The effect in a lake is known as a ‘seiche’). A tsunami is like the effect of a stone landing in a pond. Waves radiate out in ever-widening circles. As a tsunami crosses a deep ocean, length from crest to crest may be 150 km but the height may be under a metre, unfelt by ships nor seen from the air, and may reach speeds up to 1,000 km/h. The waves’ energy extends from deep ocean floor to surface.
Effect on Coastal Regions
As tsunamis enter shallow water near coastlines, wave speeds drop as energy is compressed into much shorter distances. Wave heights increase dramatically up to 40 metres, threatening life and property, as they strike the shore with devastating force (see diagram). Between multiple waves, which may vary in size, sea-level may actually drop well below normal. Great destruction may arise even though the tsunami may have been caused thousands of kilometres away.
Where Do They Occur?
Pacific Region - The majority of tsunamis occur in the Pacific Ocean, although they have occurred in all oceans of the world. In our region, areas most at risk include many Pacific Islands, New Zealand, and the heavily-populated coasts of Japan and Indonesia.
Australian Occurrences - On average, a tsunami is recorded in Australia every two years, but most are too small to be seen by people. The tsunami threat to Australia varies from ‘low’ for most of our coastline to ‘medium’ along the northern half of WA (see map, page 27). A small one struck WA in 1994 (see Case Study 2). In the 1980s a tsunami reached Darwin at low tide, which fortunately cancelled out most of its force. Evidence also exists of large tsunami impacts on our south-east coast, but before European settlement. The largest actually recorded in Australia was in August 1977 at Cape Leveque, WA, with a ‘run-up’ of 6 metres (ie wave travelled inland to where the ground was 6m above sea level).
History of Tsunami Disasters
The worst tsunami disaster followed the volcanic eruption of the island of Krakatoa in 1883 when 35 metre waves crashed into Java and Sumatra, drowning 36,420 people! In 1896, a large tsunami killed 27,000 in Japan. Destructive tsunamis of recent times included one of Chilean origin in May 1960, drowned 1,000 people in Chile, Hawaii, the Philippines and Japan. From 1970 to 1997, tsunamis caused over 9,000 deaths, including 2,700 on the Indonesian island of Flores in December 1992.
PACIFIC WARNING SYSTEM
The Pacific Tsunami Warning Centre (PTWC) in Hawaii is the Headquarters of the Operational Tsunami Warning System and works closely with other regional national centres in monitoring seismological and tidal instruments around the Pacific Ocean, to evaluate and issue warnings for potential tsunami-producing earthquakes. The PTWC is operated by the United States National Weather Service. A similar centre is now planned in Australia.
TSUNAMIS - CASE STUDIES
1.
Hilo, Hawaii, 1960
Following a sub-ocean earthquake near Chile in 1960, tsunamis were predicted to cross the Pacific Ocean and strike many island nations to the north including Hawaii (about 15 hours after the shock). That tsunami struck with such force, it tore 2 tonne boulders from a sea wall and bent parking metres to the ground. It caused 61 deaths and millions of dollars damage in and near Hilo. Since then, evacuation plans and warning sirens have been developed. Here is a graphic eye-witness account by a scientist, observing the phenomenon at Hilo. He has by this stage observed two large surges which peaked at two metres above the normal sea-level. Water had begun to retreat following the second wave until it was approximately 1.5 metres below normal height. The time is just after midnight..........
‘At first there was only a dull rumble, like a distant train coming from the darkness. By 1.02 am all we could hear was the loudening roar as it came closer through the night. Our eyes searched for the source of the noise..........A pale wall of tumbling water was caught in the dim light.
By 1.04 am a 6 m vertical front of in-rushing water churned past our lookout..........Dull grating sounds were heard as buildings ground together and sharp reports as power poles snapped. Soon, most of the island of Hawaii was in darkness.’
2.
Java, Indonesia and North-Western Australia, 1994
On 3 June, 1994, a tsunami took over 200 lives along the Indian Ocean coastline of Java and impacted on the northern WA coast three to four hours later. A small tsunami was recorded at Broome, King Bay, Onslow and Carnarvon. Several vessels close to shore were affected, including two ships transferring fuel, resulting in an oil spill. Water carried hundreds of fish, crayfish, rocks and coral inland for a distance of two to three hundred metres. The area was almost deserted but the tsunami was heard at nearby caravan parks and described as sounding ‘like the roar of a train’. An eyewitness at Onslow said that from a calm sea she saw two large waves appear, the second of which was estimated to be two to three metres high.
TSUNAMI SURVIVAL (Safety Rules)
If you hear that a strong earthquake has occurred, stand by for a possible tsunami emergency, and be prepared to move from low-lying coastal or lakeside areas to high ground at short notice. All tsunamis are potentially dangerous and destructive.
When an official warning is issued, it means a tsunami actually exists. Some tsunami victims have dismissed such warnings as false alarms and died as a result.
Approaching tsunamis are sometimes preceded by a rapid rise or fall in sea-level. This is nature’s warning and should be heeded. Vessels should head for deep water immediately.
A small tsunami at one point on the shore can be extremely large a few kilometres away so don’t let the modest size of one make you lose respect for all.
As a tsunami is not a single wave, stay out of danger areas until an official ‘all-clear’.
Never go to the shore to watch a tsunami. If you can see it, you are too close to escape.
Cooperate with your local emergency authorities if asked to evacuate.
STUDENTS - FIND FURTHER INFORMATION
As a project on tsunamis, find out more about:
the tsunami warning system in the Pacific Ocean;
tsunami disasters of the 20th century; and
countries that are likely to be affected by a tsunami hazard.
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EMERGENCY MANAGEMENT AUSTRALIA, PO
Box 1020 Dickson, Australian Capital Territory 2602, AUSTRALIA Telephone: + 61 (0) 2 6266 5402 Facsimile: +61 (0) 2 6266 5029 Email: ema@ema.gov.au AUSTRALIAN EMERGENCY MANAGEMENT INSTITUTE,
Mount Macedon Road, Mount Macedon, Victoria 3441, AUSTRALIA |
| This page was last updated May 31, 2001 |
