Which statement best contrasts moment magnitude (Mw) with traditional local (Richter) magnitude for large earthquakes?

• A. Mw uses seismic moment and energy release; Richter saturates at large magnitudes.
• B. Mw is based on ground motion intensity and is ideal for small earthquakes.
• C. Richter magnitude is a better measure for large earthquakes.
• D. Mw is calculated from the depth of focus.

Answer: (A)

Moment (or moment) magnitude is designed to reflect the total energy released by an earthquake by using seismic moment, which depends on the fault area, average slip, and rock rigidity. This makes Mw scale with the size of the rupture in a way that stays meaningful even for very large earthquakes. In contrast, traditional local (Richter) magnitude is tied to the amplitude of ground shaking recorded on a specific instrument and calibrated for smaller events. As earthquakes become larger, the simple amplitude measure stops increasing in a way that reflects their true energy release, leading to saturation. So Mw, grounded in seismic moment and energy release, avoids this saturation and provides a consistent measure for big quakes, while local magnitude tends to underestimate them.