A devastating earthquake in China's remote Tibet region killed at least 95 people and collapsed "many buildings" on Tuesday. The magnitude measures the energy released by the rupture of the fault causing an earthquake. Seismic intensity, in contrast, measures surface damage. VIDEOGRAPHIC
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00:00An earthquake magnitude measures the energy released by the rupture of the fault causing
00:11the quake.
00:12The magnitude is calculated from the amplitude and duration of ground motion recorded by
00:17seismological instruments.
00:21Magnitude is a logarithmic quantity.
00:22A one degree increase on the magnitude scale corresponds to a release of energy 30 times
00:28greater.
00:29The Richter scale is one of many units of magnitude, but seismologists prefer to use
00:34the moment magnitude scale.
00:36There is a relationship between the magnitude of the quake, the size of the fault and the
00:41shift between the two blocks separated by the fault.
00:44Seismic intensity, by contrast, measures surface damage.
00:48It characterizes the severity of ground shaking in a given location by considering the effects
00:53of the shaking on people and man-made structures.
00:56The depth of the quake, as well as the type of rocks crossed by the seismic waves, influence
01:01the intensity.
01:02For the same quake, the intensity can vary from one place to another.
01:05Two quakes with the same magnitude can have different maximum intensities and vice versa.
01:11Intensity depends on the distance from the seismic focus and decreases away from it.
01:16The scales range from 1 in regions where the shake is imperceptible to 12 where ground
01:22structures are destroyed.