Letter: Forces of nature

Sir: To answer Godfrey Keller's query about earthquake scales (letter, 13 September), the Richter scale, commonly quoted in the media, indicates energy released in the initial rupture of the Earth's crust that causes an earthquake.

On this scale, last month's magnitude 7.8 earthquake in Turkey was big but not very special, as about 20 earthquakes between magnitude 7 and 8 on the Richter scale can be expected in any one year.

However, not all big earthquakes cause major damage, whereas some small earthquakes claim many lives. This depends on many factors, such as the depth of the initial rupture, whether the near-surface rocks magnify or dissipate the amplitude of the vibrations, and the quality of building construction.

Possibly of more relevance in news reporting of the scale of devastation caused by an earthquake would be the Mercalli scale. This ranks the intensity of the effects of an earthquake from I (detected by instruments only) to XII (total destruction).

On the Mercalli scale, last month's Turkish earthquake, and a Richter 5.8 earthquake that occurred in the same area on Monday, would both rank XI or XII in the centres of the most heavily damaged zones.

Dr DAVID A ROTHERY

Dept of Earth Sciences

The Open University

Milton Keynes