Factbox: Earthquakes and magnitude

(Reuters) - A massive magnitude-8.8 earthquake struck near the coast of south-central Chile in the early hours of Saturday, shaking buildings in the capital Santiago, 200 miles away and triggering a tsunami along the coast.

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.

Most seismologists now use "moment magnitude" for medium to large quakes. The scale is calculated differently from the older Richter scale, but the values produced are broadly comparable.

* 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 had a magnitude of 9.15 and the earthquake that devastated Haiti's capital Port-au-Prince on January 12 was rated at magnitude 7.

* 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 epicenter of an earthquake.

An earthquake that measures 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 epicenter is the point on the earth's surface vertically above the focus, or hypocenter, the point deep in the earth's crust where the earthquake is triggered.

(Source: U.S. Geological Survey (www.usgs.gov) )