A common occurrence : Earthquake are a common occurrence on the planet many go unnoticed because they are "minor" or take place in uninhabited areas.

Scientists say earth, on an average witnesses one "great" ( measuring 8 and above on the richter scale ), 18 "major" ( from 7 to 7.9 magnitude ) and 120 "large"( from 6 to 6.9 magnitude ) earthquakes every year. And about a thousand "moderate" quakes a year, with a magnitude between 5 and 5.9.

EW DELHI : thought science may still be baffled about how to accurately predict the time, place or magnitude of earth quakes, the methods to measure a quakes strength have progressed since American seismologist charles Francis richter camp up with a magnitude scale in 1932. A magnitude is usually expressed in terms of the richter scale, as it has evolved since that time.

It is not a physical device but a logarithmic scale based on recording of seismographs, instruments which automatically detect and record the intensity, direction and duration of movement of the ground.

In a logarithmic scale like the richter, which begins at one and is open-ended each units is 10 times greater than the previous one.

In other words, an increase of one unit ( whole number ) on the richter scale signifies a 10 fold jump in the quakes size. Seismologists say " magnitude" and "intensity" are the two ways in which a quake's strength is generally expressed. The magnitude is a measure that depends on the seismic energy radiated by the quake as recorded on seismographs.

The intensity in turn is a measure that depends on the damage caused by the quake. It does not have a mathematical basis but is based on observed effects.

There are some other scale in use like the modified mercalli intensity scale, which measures the intensity of an earthquake's effects in a given locality .

But the richter scale still remains the most widely known and popular scale for measuring earthquakes. Be that as it may, the magnitude of the quake which rocked Gujarat on January 26 immediately became debatable.

While the Indian meteorological Department (IMD) said it measured 6.9 on the richter scale some international seismological observatories put the magnitude at 7.9 IMD officials, who are standing by their figures at present, say the difference was due to different measuring scales much like measuring the same weight in kilogrammes or pounds. " our seismological observatories (IMD has 55 spread all over India ) are located nearer to the site and measured the shocks on the Richter scale . we measured the body waves, while they measured the surface waves" said one. The issue still remains contentious Meanwhile, aftershocks continue to occur near the original epicenter of the Gujarat quake

The Times Of India, Ahmedabad
Febuary 2, 2001.