(For related story on Monday's quake, see [nT148237)

July 16 (Reuters) - A strong earthquake of 6.8 magnitude killed at least three people in Japan on Monday and injured more than 500.

Following are details about measuring the strength of earthquakes and related issues.

* Magnitude measures the size of an earthquake by the energy released at the source of the quake, and is determined from readings on seismographs.

Richter-type formulas, the most commonly used, read the frequency and amplitude of the seismic waves, which are vibrations from earthquakes.

* The Richter scale was named after seismologist Charles Richter, who first developed this method of measuring quakes in 1935.

The scale is logarithmic. Each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value. So a magnitude 7.0 quake releases about 900 times more energy than a magnitude 5.0 tremor.

The scale is also open-ended. A quake of magnitude 2 is usually said to be the smallest normally felt by humans.

* The largest recorded earthquake occurred in Chile on May 22, 1960. It measured 9.5 and triggered a tsunami that swept across the Pacific Ocean, killing scores of people in Hawaii, Japan and elsewhere.

The quake that triggered the 2004 Indian Ocean tsunami was magnitude 9.15.

* Among other measurements is moment magnitude, which is a measure of the amount of energy released -- an amount that can be estimated from seismograph readings.

* Japan also uses a seismic intensity scale from 1 to 7 that measures the strength of seismic motion, and usually gets stronger the closer you get to the epicentre of an earthquake.

An earthquake that is 1 on the Japanese scale is felt by only some people in a building. At 7, people find it impossible to move at will and most furniture moves violently.

* The epicentre is the point on the earth's surface vertically above the focus, or hypocentre, the point in the earth's crust where the earthquake is triggered. (Source: U.S. Geological Survey (www.usgs.gov)

AlertNet news is provided by