Which statement best describes how magnitude and intensity differ in earthquakes, and why are both important?

• A. Magnitude measures local building damage; intensity measures energy release at the source; both are independent.
• B. Magnitude measures total energy released at the source on a log scale; intensity measures ground shaking observed at a location and varies with distance and geology; both determine hazard and damage.
• C. Magnitude measures ground shaking at a location; intensity measures energy release at the source on a log scale.
• D. Magnitude and intensity are the same measure but used in different regions.

Answer: (B)

The key idea is that magnitude and intensity describe different aspects of an earthquake. Magnitude measures the total energy released at the earthquake’s source and is reported on a logarithmic scale, giving a single number that reflects how powerful the quake was overall. Intensity, on the other hand, describes the ground shaking actually felt at a specific location and can vary widely depending on how far you are from the source, the depth of the quake, and the geology of the ground you stand on. Because the energy release is a property of the event itself while the shaking at any location depends on distance and path effects, both are crucial for understanding hazards and potential damage. A large quake far away might produce modest shaking in a city, whereas a smaller quake near soft ground can cause severe damage locally; that difference illustrates why magnitu­de informs the overall size of the event and intensity informs the impacts at particular sites.