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Summer 2011

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark      

September 6, 2011

Sorting out the scales of magnitude

The Mineral, Va., 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 ten times more jarring than the previous step.

But for earthquakes larger than 3.5, seismologists now use the moment magnitude scale, which represents 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 ten 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 worse than a 5.0, but the energy released is a thousand times greater.

The diagram at right uses spheres to compare relative amounts of energy released by earthquakes. Each sphere represents the volume of TNT that would be required to release as much energy as a quake of the specified magnitude. The Washington Monument is thrown in for scale.

Energy equivalence of three earthquakes.

SOURCES: Michael Germeraad, Earthquake Engineering Research Institute; U.S. Geological Survey; University of Arkansas at Little Rock

Comparing the moment magnitude of three earthquakes.