Two different metrics are used to express earthquake strength: magnitude (M) and intensity. These are often confused but provide different information.
Magnitude measures the total energy released by an earthquake. There is one magnitude value per earthquake. The Richter scale or moment magnitude (Mw) is used. Values are calculated from seismographic observations.
Intensity describes the strength of ground shaking at specific locations. The same earthquake can produce high intensity near the epicenter and low intensity far away. Japan uses 0-7 (8 levels), while the U.S. uses Mercalli Intensity Scale and other variations.
Example: A magnitude 7.0 earthquake can cause massive damage if shallow and urban, or minimal damage if deep and remote. Risk assessment requires both metrics.