The U.S. Geological survey estimated that Friday's quake feared to have killed 15,000 in India, measure 7.9 on the scale.

The quake which killed 2,400 people in Taiwan in September 1999 measure 7.6 so does that mean the Indian quake was just slightly stronger than the one in Taiwan?

The answer is no. the Indian quake was almost twice as strong as the one in Taiwan.

As the strength of earthquakes rises in step up the richter scale, each quake releases 31 times more energy than one registered at the previous notch.

Introduce in 1935,the scale is named for American physicist charles F richter of the California institute of technology, who evolved it from patterns discovered by studying more than 200 earthquakes a year.

The richter scale is a more objective, quantitative basis of measuring earthquakes than the other widely used standard, the 12 point mercalli scale, the US geological survey says.

The richter scale does not measure an earthquake's effects, but gives its strength in terms of the energy released as measured by seismographs.

The scale starts at one and has no upper limit: each unit is 10 times greater than the one before. Mathematicians call this method of arranging number on a scale logarithmic.

For instance, the survey says, a magnitude of 5.3 might by calculated for moderate earthquake, and a strong earthquake might be classed as magnitude 6.3 last week's earthquake in India it has classed as major.

But because the richter scale has a logarithmic basis each whole number increase in magnitude on the scale represents a 10-fold increase in an earthquake's measured amplitude,

A quake's strength is ranked on the basis of the maximum amplitude of the signal recorded by a seismograph and how far the instruments from the earthquake.

Today, state of the art seismic system use telephone lines and satellites to link to a central computer, which churns out preliminary estimates within minutes.

Earthquakes with a magnitude of about 2.0 or less are called micro earthquakes: people don't usually feel them and they are usually tracked only by local seismographs.

Event ranking about 4.5 or greater or which there may be several thousand every year are strong enough to be recorded by sensitive instruments all over the world.

Great earthquakes, such as the 1964 good Friday earthquake in Alaska, have magnitudes of 8.0 or higher. The san Francisco earthquake of 1906 has since been calculated at 8.3.

Another in Mexico in 1985 registered 8.1 on the scale. On average, one earthquake of such size occurs some where in the world each year, the US geological survey says.

The richter scale has no upper limit, but the largest known shocks have had magnitudes I the 8.8-8.9 range.

In 1950, a quake measuring 8.6 on the scale struck northeast India, killing hundreds of people.

In a quake measuring possibly nine on the scale, more than 8,30,000 people died when the most devastating earthquake in recorded history hit china's shaanxi province in 1956.

China suffered the worst quake in its modern history on july 28, 1976 when the northeastern city of Tangshan was completely razed, and at least 2,40,000 people, possibly hundreds of thousand more were killed. The richter reading was 7.8.

By contrast, the mercalli scale is more subjective in assessing the impact of earthquakes. For example, an earthquake registering five on the mercalli scale is defined as having made furniture shake and churchbells rings, but having triggered little or no damage.

But an earthquake that registered 12 on the mercalli scale would you have destroyed all manmade objects, and created new topography by forming new lakes, huge falls of rock and major earth faults. Russia also has a 12-point scale, and Japan a seven-point system.(Reuters)

The Times Of India, Ahmedabad
Febuary 2, 2001.