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September 6, 2011

The Richter Scale is over: Meet the Moment Magnitude Scale

Screen Shot 2011-09-06 at 12.51.28 PM

From today's Washington Post story:

The Mineral, Virginia earthquake that rattled the East Coast was measured as a 5.8-magnitude quake.

In the past, that number would have been determined simply by how much an earthquake moved the needle on a seismometer. The size of the jolt was then assigned a position on the Richter scale, a logarithmic chart on which each step is 10 times as jarring as the previous step.

But for earthquakes larger than 3.5, seismologists now use moment magnitude scale, which represent the amount of energy released during an earthquake. In use since 1979, the scale factors in a fault's rigidity, the area of its rupture surface and the distance that the Earth moves along the fault.

While the shaking of a 6.0-magnitude quake is 10 times greater than that of a 5.0 — as in the Richter scale — the amount of energy released is 32 times greater. A 7.0 will shake 100 times more than a 5.0, but the energy released is 1,000 times greater.

The diagram above uses spheres to compare relative amounts of energy released by earthquakes. Each sphere represents the amount of TNT that would be required to release as much energy as a quake of the specified magnitude.

"Since 1979" — why are we only finding out about this now, 32 years on?

September 6, 2011 at 02:01 PM | Permalink


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