13 Jan 2005
Surrounded on all sides by heavily populated continental edges and islands, the spreading wave hit
What was the cause of the tsunami?
A tsunami is a large, seismically generated sea wave which is capable of considerable destruction in certain coastal areas. The wavelength in the open ocean is of the order 100 to 150km and the rate of travel of a tsunami is between 640 and 960 km/h. December’s tsunami was caused by a giant earthquake where one of the earth’s major tectonic plates – the Indo-Australian plate – collides with the smaller Burma microplate at a rate of about 6 cm/year. At this destructive plate boundary, the denser Indo-Australian plate is subducted beneath the overriding Burma plate. Driven by convectional currents in the earth’s mantle, this movement is responsible for producing deep-focus earthquakes that can force the ocean floor upwards, generating tsunamis.
A build-up of pressure over recent decades caused the floor of the
well as the Andaman and
centimetres a year, but this movement does not occur smoothly. There has not been a very large quake along this fault since 1833 — a fact that may have contributed to the huge force of this one. The
Described by the U.S. Geological Survey (USGS) as a megathrust earthquake, the scale of this Magnitude 9.0 event was stunning. A 1200 km stretch of the plate slipped, and it is likely that the average displacement on the fault plane was about fifteen meters. The actual rupture duration on the fault (the time it took for the earthquake to take place on the fault and rupture the entire length) was approximately 3 to 4 minutes. During this time, the earthquake released energy that was the equivalent of 23,000
As a result, the sea floor overlying the thrust fault was uplifted by several meters and it was this sudden change in ocean depth that generated the tsunami, as a trillion tonnes of water overlying the upwardly-moving plate was vertically displaced. This produced a long, low-amplitude wave travelling at up to 900 kilometres per hour in all directions away from the earthquake’s epicentre. Wherever the wave reached shallower water near a coastline, it shortened as friction interfered with the elliptical orbit of the moving water. As it slowed, it gathered into surges that came crashing down on populated coastal areas.
The cost of the tsunami, both in terms of human lives and also in financial terms, is immense. It is the fourth worst natural hazard to have occurred globally within living memory, exceeded only by past flooding along
(1) Population density and distribution Parts of the region are very densely populated after several generations of rapid population growth (caused by high fertility but falling mortality).
(2) Inadequate warning systems and preparation Advance warning of hazards - even if it is only a matter of hours or perhaps minutes - can drastically reduce mortality rates. The successful collection and dissemination of information is therefore vital. Although earthquake scientists knew about the Magnitude 9.0 earthquake within minutes, the absence of monitoring equipment in the ocean itself meant that they did not know whether a tsunami had occurred. Additionally, the region lacks effective warning systems. Thus, even if the tsunami had been spotted sooner, it is uncertain whether effective warnings could have been made (and in LEDCs, many people do not have access to TV or radio, making it difficult to warn remote villages). The event has highlighted the lack of a system to warn
(3) Time of year The tsunami struck on Boxing Day, when numbers of tourists (particularly those from northern and western Europe) were higher than at many other times of the year. This may well have raised the final death toll.
You can download excellent image analyses of the satellite imagery of Sri Lanka and Indonesia from the DigtialGlobe website:
What happens next?
Longer-term developments that geography students and their teachers may want to keep an eye on include:
(1) Secondary hazards Major concerns now exist over the possible spread of disease, particularly in places where fresh water supplies have been contaminated. A major cholera outbreak would constitute a secondary hazard that might send death tolls spiralling even higher.
(2) Aid efforts Already, newspapers and television are reporting the more controversial aspects of emergency aid efforts to help the region (see The Guardian,
(3) Tourism Countries such as
(4) Hazard management The human failure to predict, warn and evacuate will certainly be addressed in the aftermath of the tsunami. For instance, a seismograph designed to detect the earthquakes that cause tsunamis was installed on the Indonesian
AS / A-level notes
It would be surprising if candidates taking geography exams this year did not want to discuss the causes and consequences of this dreadful episode in various contexts perhaps when writing about earthquakes, hazard management, sea-level rises, international aid efforts, refugees, tourism or many other related topics. As a basic guideline, here are a couple of academic areas where good knowledge of recent events in the
(1) Earth systems This event reminds us how much earthquakes can vary in terms of both their size and impact. Consider the deceptively simple question ‘what are the impacts of earthquakes?’ Candidates might emphasise that small shallow-focus earthquakes associated with divergence at the mid-Atlantic ridge cannot equal deep-focus subduction earthquakes in terms of their potential for damage. The world's largest recorded earthquakes have all been megathrust events, occurring where one tectonic plate subducts beneath another.
(2) Hazard causes, impact & management With so much media coverage, there is no shortage of information dealing with the management failings that have come to light in the
Staffordshire Learning Net geographers have responded to the tsunami disaster by creating resources to help pupils understand and make sense of the events. There are resources, also, for Post-16 geography
Geographical Association materials for supporting the sequence of lessons and activities on the tsunami disaster
Teacher Resource Exchange resources related to the Indonesian earthquake and tsunami
Geography Pages has S.E. Asia Tsunami weblinks and resources
Geointeractive downloadable resources for teachers on the tsunami disaster
Juicy Geography has a short discussion paper for KS3/4 which examines the problem of drifting media coverage, broken government promises, different phases of aid, long term aid and the role of individual citizens
Oxfam Cool Planet - Tsunami in Asia brings together educational materials from Cool Planet, and information from the main Oxfam website and external sites, to enable teaching around the current humanitarian crisis in Asia.
Guardian - how to cover the subject of tsunamis
Global Dimension - links to tsunami resources
Teachers in Development Education - Responding to the Tsunami
Other tsunami resources and links
Guardian - interactive guides to the tsunami
USGS – Earthquake in the News
The Times - How the shape of ocean floors can affect speed and height of tsunami
The Why Files - warming to a warning system
Wikipedia - Strongest earthquake in 40 years hits Southeast Asia
Satellite images from DigitalGlobe