Measuring earthquakes: magnitude and seismic intensity
The 7-magnitude quake that struck Morocco late Friday has claimed more than 2,100 lives. 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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00:06 An earthquake magnitude measures the energy released by the rupture of the fault causing the quake.
00:12 The magnitude is calculated from the amplitude and duration of ground motion, recorded by seismological instruments.
00:19 Magnitude is a logarithmic quantity.
00:22 A one degree increase on the magnitude scale corresponds to a release of energy 30 times greater.
00:29 The Richter scale is one of many units of magnitude, but seismologists prefer to use the moment magnitude scale.
00:36 There is a relationship between the magnitude of the quake, the size of the fault, and the shift between the two blocks separated by the fault.
00:43 Seismic intensity, by contrast, measures surface damage.
00:48 It characterizes the severity of ground shaking in a given location by considering the effects of the shaking on people and man-made structures.
00:55 The depth of the quake, as well as the type of rocks crossed by the seismic waves, influence the intensity.
01:01 For the same quake, the intensity can vary from one place to another.
01:05 Two quakes of the same magnitude can have different maximum intensities and vice versa.
01:10 Intensity depends on the distance from the seismic focus and decreases away from it.
01:15 The scales range from 1 in regions where the shake is imperceptible to 12 where ground structures are destroyed.
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