Measuring earthquakes: magnitude and seismic intensity

AFP English

The magnitude measures the energy released by the rupture of the fault causing an earthquake. Seismic intensity, in contrast, measures surface damage. Here is a videographic explainer VIDEOGRAPHIC

Transcript

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 of 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.

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