Earthquake Magnitude

Saturday, February 27th, 2010

Last night’s 8.8-magnitude earthquake was much, much bigger than the 7.0-magnitude quake that hit Haiti recently. If you remember your 8th-grade science, the Richter scale is not a linear scale — it’s a log10 scale — so an 8.8-magnitude quake is not 1.26 times as big as a 7.0 quake — it’s 63 times as big.

Of course, that depends on our definition of big. The Richter scale measures amplitude. In terms of total energy released though, each point on the Richter scale implies not just a factor of 10, but a factor of 31.6 — so the recent Chilean quake released 500 times the energy of the Haiti quake.

By the way, we don’t actually use the Richter scale for such large earthquakes; we use its successor, the less colorfully named moment magnitude scale.

Even fairly light earthquakes (magnitude 4.6 or higher) can be detected by seismographs from around the world; thousands occur per year. Major earthquakes (magnitude 7.0 or higher) are rare, but not that rare; about 18 occur per year. Great earthquakes (magnitude 8.0 or higher) are not nearly as rare as you might think; on average, one occurs per year.

The largest recorded earthquake was the Great Chilean Earthquake of May 22, 1960 which had a magnitude of 9.5 — and which occurred not far from last night’s quake.

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