Which scale measures the magnitude of an earthquake based on the amplitude of ground motion?

• A. Richter Scale
• B. Modified Mercalli Scale
• C. Seismometer
• D. Moment Magnitude

Answer: (A)

The main idea here is how magnitude scales relate to the actual shaking recorded by instruments. The scale that measures magnitude from the amplitude of ground motion uses the recorded size of seismic waves on a seismograph and expresses it on a logarithmic number. This is the Richter scale, also known as local magnitude. It was developed to quantify how strong the shaking is at a location by looking at the wave amplitude, and because the scale is logarithmic, a tenfold increase in amplitude roughly corresponds to one unit increase in magnitude, with energy increasing much more rapidly. Modern practice often uses the moment magnitude for large earthquakes, since it reflects the total energy released, but for the specific criterion of amplitude-based measurement near the source, the Richter (local magnitude) scale is the classic fit. The modified Mercalli scale measures how strongly structures and people felt the quake (intensity), not the size of the shaking itself. And a seismometer is the instrument that records the ground motion, not a way to quantify it.