### 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.

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