Tsunami and Disaster Risk
AdvancedA tsunami is a series of giant waves raised when submarine earthquakes suddenly displace a huge body of water — jet-fast and under a metre high in the deep ocean, surging into walls of water near the coast. Monitoring, forecasting, warning and evacuation must all work; 3.11 and the 2004 Indian Ocean case show both sides.
- A tsunami comes from sudden displacement of the whole water body by seabed earthquakes — not from wind
- In the deep ocean it flies like a jet (about 700 km/h) under a metre high; near the coast it slows, piles up and becomes a wall of water
- An abnormal rapid retreat of the sea is a natural warning — run to high ground immediately
文體小知識：自然系統說明文——剖析海嘯的生成、傳播與破壞機制，以及監測預警系統如何環環運作，是「機會與風險」單元中機制最複雜的一課。
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1A tsunami is not an ordinary wind wave but a series of giant waves raised when a whole body of water is suddenly displaced; the Japanese word means 'harbour wave'. The commonest cause is a submarine earthquake, when the seabed fractures and shifts, lifting or dropping the water above; undersea eruptions and huge landslides into the sea can also trigger tsunamis, though less often.
2In the deep ocean a tsunami may have a wavelength over 100 km but a height of less than a metre, so ships hardly notice it. Its speed depends on water depth — about 700 km/h in water 4,000 m deep, as fast as a jet airliner — allowing it to cross an entire ocean within hours.
3As a tsunami nears the coast, the shallow seabed slows the wave, following waves catch up and energy piles together; the height surges into walls of water ten metres or several tens of metres high. The sea often withdraws far first, exposing the seabed, then floods ashore with destructive force, sweeping away buildings and infrastructure.
4On 11 March 2011, a magnitude 9 earthquake struck off north-eastern Japan; the tsunami reached the Sanriku coast within about half an hour, with run-up heights of about 40 m. It overtopped breakwaters, destroyed houses and knocked out the cooling systems of the Fukushima Daiichi nuclear plant; about 18,000 people died or went missing. The 2004 Indian Ocean tsunami killed about 230,000.
5Tsunami risk depends on wave height, coastal population and infrastructure, and the capacity for warning and evacuation. Pacific countries have run a tsunami warning centre since 1949, using seismometer and tsunami gauge networks; the Indian Ocean only built a similar system after the 2004 disaster. Evaluating every link of such systems is the core training of this lesson.
1Summary 重點摘要
A tsunami is a series of giant waves raised when submarine earthquakes suddenly displace a huge body of water — jet-fast and under a metre high in the deep ocean, surging into walls of water near the coast. Monitoring, forecasting, warning and evacuation must all work; 3.11 and the 2004 Indian Ocean case show both sides.
2Vocabulary cards Vocabulary
A series of giant waves caused when submarine earthquakes, eruptions or landslides suddenly displace a large body of water.
An earthquake whose epicentre lies on the seabed; displacement of the seabed shifts the water above — the commonest cause of tsunamis.
The distance between two successive crests; in the deep ocean a tsunami's wavelength can exceed 100 km.
The highest elevation a tsunami reaches as it floods inland — a key measure of tsunami size.
The international centre set up in 1949 to watch the Pacific and issue tsunami warnings to member states.
An instrument recording ground shaking; within minutes of a quake it can locate the epicentre and estimate magnitude.
Moving coastal residents to high ground or inland before the waves arrive — the last line of defence for saving lives.
Disasters triggered by the main event, such as fires, nuclear accidents and epidemics, which prolong its effects.
The degree to which an area suffers from a hazard, given its population, buildings, awareness and capacity to respond.
3Step by step Steps
- Step 1: The chain of causesA tsunami needs three things: a huge disturbance, a large body of water and sudden displacement. The classic case is a submarine earthquake: locked rocks on a plate boundary suddenly rupture, heaving one stretch of seabed up and dropping another, lifting or lowering hundreds of millions of tonnes of water; gravity then spreads the energy out as a series of long waves. For example, in the 2011 '3.11' quake the subduction zone ruptured over a huge area and the seabed shifted tens of metres in moments — the direct trigger of the giant tsunami.
- Step 2: The invisible long wave in deep waterIn deep water a tsunami has a wavelength over 100 km and periods of ten-plus minutes, yet stands less than a metre high; ships feel only a gentle rise and fall. Its speed depends on depth — the deeper the water, the faster the wave — about 700 km/h at 4,000 m. For example, the 2004 Indian Ocean tsunami reached Sri Lanka about two hours after the quake and Somalia in about seven, so distant coasts were struck all the same.
- Step 3: Transformation and surge near the shoreNear the coast, shallow water slows the wave from hundreds of km/h to a few dozen; following waves catch up, the wavelength shortens, energy concentrates and the height surges above 10 m into a wall of water carrying sand and debris, which can push several kilometres inland. The sea often withdraws abnormally first, exposing the seabed — a vital natural warning. For example, in Phang Nga, Thailand in 2004, many tourists walked onto the exposed seabed and lost their ten-odd minutes to escape.
- Step 4: Direct and secondary impactsDirect impacts include loss of life, buildings and infrastructure swept away, fishing fleets and ports destroyed, and farmland ruined by salt water. Secondary impacts run deeper: power loss can fail cooling systems and cause a nuclear accident; damaged water supplies bring epidemics; lost livelihoods leave survivors in lasting poverty. For example, the 3.11 tsunami swept away the backup generators of the Fukushima Daiichi plant; the reactors lost cooling and leaked radiation, and more than a hundred thousand people were evacuated.
- Step 5: Monitoring and forecastingWarning begins with monitoring: deep-ocean pressure gauges and buoys track the sea surface while seismometer networks fix the epicentre, depth and magnitude within minutes to judge whether a great submarine quake has occurred. Next comes forecasting: seabed displacement data feed computer models that predict wave heights and arrival times. For example, the Japan Meteorological Agency issued its major tsunami warning about three minutes after the 3.11 quake, and the Pacific Tsunami Warning Center alerted countries all around the Pacific.
- Step 6: Dissemination and evacuationFor warnings to save lives, two more links are needed: dissemination and evacuation. Television, radio, mobile alerts and coastal sirens broadcast instantly, and residents follow set routes to elevated shelters. Any failure — slow judgement, broken communication, blocked routes — magnifies the disaster. For example, in Kamaishi, Iwate, many school pupils survived by climbing to high ground on their own thanks to regular drills — proof that education and practice are the most reliable link.
- Step 7: Failures and reviewWarning systems are not foolproof. In 2011 the Japan Meteorological Agency initially underestimated wave heights, and some residents delayed evacuation after hearing low figures; methods were later revised to urge immediate escape instead of quoting heights. In 2004 the Indian Ocean had no warning system at all: the tsunami crossed the ocean for hours without any alert, killing over 200,000, and an Indian Ocean system was only built afterwards. For example, comparing the two disasters shows that technology, institutions and education must all work together.
4Diagrams Diagrams
5Worked example Example
Data question: after the 11 March 2011 earthquake off north-eastern Japan, the Japan Meteorological Agency issued a tsunami warning within about three minutes and television broke in at once, while coastal residents headed for high ground. Post-event reviews, however, found that early wave-height forecasts were too low, that some communication chains broke down, and that the tsunami exceeded the design height of the Fukushima plant's defences. Evaluate each link of Japan's tsunami warning system.
- Frame the links: monitoring, forecasting, warning dissemination, evacuation response; assess each against the data — strengths first, then limits
- Monitoring and forecasting: seismometer networks quickly identified a magnitude 9 submarine quake and deep-ocean buoys logged real wave heights — strong performance; yet early models underestimated heights, showing the limits of prediction
- Dissemination: a major tsunami warning went out in about three minutes, broadcast nationwide — world-leading speed; but power and communications failed in some villages, leaving some residents uninformed
- Evacuation: many residents escaped in time and school drills proved highly effective; yet waves overtopped the design heights of sea walls, and some shelters themselves flooded — engineering must match the warning system
- Conclude: all four links are indispensable; Japan far outperformed the unwarned 2004 Indian Ocean case, proving warnings save lives, but protection must be designed for the worst case
- Answer: Japan's tsunami warning system can be judged link by link. Monitoring and forecasting: seismometer networks located a submarine quake of magnitude 9 within minutes and deep-ocean buoys supplied real measurements — fast and accurate; yet early models underestimated wave heights, showing a blind spot in prediction. Dissemination: a major tsunami warning was issued within about three minutes and spread nationwide by television, radio and mobile alerts — among the fastest in the world; but cut power and communications meant some villagers never received it. Evacuation: years of drills sent many people to high ground immediately, with school evacuations especially effective; yet waves far exceeded the design heights of sea walls and the nuclear plant, showing that engineering must plan for the worst case. Overall, deaths were far fewer than in the unwarned 2004 Indian Ocean tsunami, proving the four links work as one — but the system must keep being reviewed and improved.
6Common mistakes Mistakes
7Quick check Quick check
Q1What is the commonest cause of a tsunami?
Q2Why is a tsunami hard to notice in the deep ocean?
Q3What happens as a tsunami nears the coast?
Q4Which secondary disasters followed the 2011 tsunami in Japan?
Q5What four links make up a tsunami warning system?
8Recap Recap
- A tsunami comes from sudden displacement of the whole water body by seabed earthquakes — not from wind
- In the deep ocean it flies like a jet (about 700 km/h) under a metre high; near the coast it slows, piles up and becomes a wall of water
- An abnormal rapid retreat of the sea is a natural warning — run to high ground immediately
- Warning has four links — monitoring, forecasting, dissemination, evacuation; 3.11 proved its worth and exposed underestimated heights
- The unwarned 2004 Indian Ocean tsunami killed over 200,000; secondary disasters such as nuclear accidents prolong the impacts