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Predicting earthquakes
by David Bradley

Understanding how earthquakes happen may help to predict them and prevent people being killed in collapsing buildings.
An earthquake with a magnitude of 7.7 on the Richter scale shook India on 26 January, reminding us that we still can't accurately predict when earthquakes will happen. Now two groups of scientists working on opposing theories might soon reach agreement about the causes of earthquakes, enabling them to sound the alarm sooner and save lives.

Gravity or convection
The earth's rocky crust is sliding about on massive sheets of rock 60 km (37 miles) thick called 'tectonic plates'. These plates carry the continents, but scientists still do not understand the forces that drive the movement.

There are two theories about what pushes the tectonic plates. The first says that heating effects, or 'convection', in the mantle layer directly beneath the crust moves the plates, dragging one plate beneath its neighbour. The second theory says that gravitational forces drive the plates. As rock moves upwards at the mid-ocean ridges between plates, neighbouring plates are squeezed together. Gravity then drags the heavier plate beneath the other.

A seismogram recorded by a seismograph in Russia shows earth tremors during the massive Turkish earthquake in August, 1999.

Götz Bokelmann, a geophysicist at Stanford University, California, believes the argument will be settled with a compromise. Recordings of thousands of earthquakes, known as seismograms, from many places around the world have helped scientists to pinpoint the origin and times of each. From these they have built up a kind of X-ray image of the inside of the earth. This has then helped them see how tectonic plates move relative to the mantle below and so identify the direction of the forces involved.

North America, it turns out, is moving because the deeper mantle pulls it along. However, other data show that there are sideways forces on the plates, too, and Bokelmann reckons that there is probably a mixture of both processes working beneath our feet.

"Truth is often a compromise," he says.

A better understanding of plate tectonics will allow scientists to build computer models of what happens just before, during and after an earthquake or volcanic eruption. These catastrophic events happen where tectonic plates collide. With a better computer model scientists will be able to predict when earthquakes and eruptions might happen and so save lives.

Further information about tectonic processes
NB - These links will open in a new browser window.

http://www.cotf.edu/ete/modules/msese/earthsysflr/plates1.html
A good introduction to plate tectonics

http://earthquake.usgs.gov
The US Geological Survey's Earthquake Hazards Program. An excellent site, including up-to-date information on the latest earthquakes and volcanic eruptions.

http://pangea.stanford.edu/~goetz/convection.gif
Plate convection theory shown schematically

http://pangea.stanford.edu/~goetz/San_Andreas.gif
An aerial photo of the San Andreas Fault, perhaps the most famous earthquake area in the world.

http://pangea.stanford.edu/~goetz/home.html
Götz Bokelmann's home page at Stanford University.

http://www.geographyonline.co.uk/geography/news/16_01_01.html
Details of the El Salvador earthquake in January 2001

Activity

Investigating plate tectonics
The continents are in continual motion on the tectonic plates, and the massive forces they impart on the earth's surface cause various catastrophic events, such as earthquakes and volcanic eruptions. They also lead to long-term physical changes.

Find out which mountain ranges have been formed directly through the movement of the continents.
Can you estimate how big are the forces involved in a collision between two continents?
What information would you need to work out an accurate answer?