What does the Moment Magnitude scale quantify in an earthquake?

• A. The depth of the focus.
• B. The distance to the epicenter.
• C. The duration of shaking.
• D. The energy released by the earthquake.

Answer: (D)

Moment Magnitude measures the energy released by an earthquake. It’s derived from the seismic moment, which captures how much rock has slipped, how large the rupture area is, and how stiff the rocks are. This link to the amount of radiated energy makes it a reliable way to compare the size of earthquakes, especially the large ones. The depth of the focus, the distance to the epicenter, or how long the ground shakes are not direct measures of the energy released—they describe other aspects of the event or its effects. So the scale’s purpose is to quantify the total energy that the earthquake released as seismic waves.