Fixing the cracks in disaster mitigation
Box 1 | What is an earthquake?
An earthquake occurs when suddenly-released energy moves through the earth in the form of seismic waves. These waves radiate out from the site of energy release (the hypocentre) and, at the earth's surface, are felt as a shaking or displacement of the ground.
What causes them?
Most major earthquakes are natural events. The outermost shell of the earth, called the lithosphere, is divided into ten major and many minor tectonic plates ('tectonic' is a derivative of a Greek work meaning 'to construct and destroy'). These plates are about 100 kilometres thick and 'float' on top of another layer called the asthenosphere. Most major earthquakes occur on and adjacent to the boundaries between plates. The plates move constantly – if slowly – against each other, creating stresses that build up over time. An earthquake occurs when these stresses are suddenly released; the resultant seismic waves radiate in all directions, including towards the earth's surface.
Location, location, location
Location plays a very large role in the potential for an earthquake disaster, with the most vulnerable sites lying along the boundaries of tectonic plates. The majority of the world's earthquakes occur on the Pacific Rim of Fire, which lies on the boundary of the Pacific plate and runs from New Zealand in the southwest, up through the Solomon Islands, New Guinea and the Philippines to Japan, and eastwards across the northern Pacific to Alaska and down the west coast of Canada, the United States, Central and South America. The Indonesian archipelago is also highly susceptible to earthquakes, as are the Himalayan Mountains, which have formed at the boundary between the Eurasian and Indian tectonic plates.
Not all earthquakes occur along the tectonic plate boundaries, however. The earthquake in Gujarat, India, in 2001 is located well away from the nearest plate boundary, and its cause is not well understood. The entire continent of Australia is in the middle of a tectonic plate, with no part of it near a major plate boundary. Australia should therefore be reasonably safe from big earthquakes, but they still occur from time to time. Since 1900, 17 earthquakes with magnitudes greater than 6 have been recorded. In 1989 an earthquake registering 5.6 on the Richter scale killed 13 people in Newcastle, most of them in a single building that collapsed.
Human activities such as mining and the build-up of large masses of water behind dams can also cause earthquakes, although these are usually relatively minor in scale.
Related sites
Plate tectonics (Nova: Science in the news, Australian Academy of Science)
Earthquake science explained (United States Geological Survey)
Plate tectonics, the cause of earthquakes (Nevada Seismological Laboratory, USA)
Seismic deformation (Nevada Seismological Laboratory, USA)
Indian Gujarat earthquake, 26 January 2001 (Global Education, Australia)
Posted October 2006.