Caldera chronicles: How big was that earthquake?

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This map shows seismic stations in the Yellowstone region, with numbers of channels indicated by number and sensor type by color. Inverted triangles indicate stations operated by University of Utah Seismograph Stations and squares indicate stations operated by other agencies.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week’s contribution is from David Shelly, seismologist with the U.S. Geological Survey, and Jamie Farrell and James Pechmann, seismologists with the University of Utah.

How do seismologists determine the size of an earthquake? This sounds like a simple question, and it is a fundamental task of earthquake monitoring in Yellowstone National Park and elsewhere.

Although simple in concept, in practice measuring the magnitude of an earthquake can be quite challenging. It’s not as easy as making the earthquake stand on a scale and reading the dial. In fact, methods to compute earthquake magnitudes have evolved considerably over the years and continue to evolve today.

The first earthquake magnitude scale was invented by Charles Richter for Southern California in 1935. Unlike seismic intensity, which measures the strength of shaking

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