MEASURING AN EARTHQUAKE’S INTENSITY – THE MODIFIED MERCALLI INTENSITY SCALE

Ken O’Brien, Principal Planner, NJOEM

How does it feel when a earthquake shakes?

... It depends on where you are.

An earthquake can be measured by the amount of energy released. The Richter scale uses Arabic numbers to rate the amount of energy, or its magnitude.

The size or strength of an earthquake may be measured by the intensity or kind of damage that occurs. Intensity depends on your distance from the epicenter and the geologic area. The Modified Mercalli scale measures the earthquake’s effect on people, property and ground damage. Roman numerals are used to rate the intensity and damage.

An earthquake may have a different intensity rating at different locations. Damage is usually lessened with distance from the earthquake’s epicenter. However, damage may depend on the type of structure, construction, or type of soil on which the structure was built. For example, a building on bedrock experiences less movement than a building on loose sediments.

The Modified Mercalli (MM) scale reads as follows:

1. Not felt except by a very few under especially favorable circumstances.
2. People lying down might feel the earthquake. Light suspended objects may sway.
3. People on upper floors will feel it, but may not know it’s an earthquake. Hanging objects swing.
4. People indoors will probably feel it, but those outside may not. Houses may creak.
5. Nearly everybody feels it. Sleepers are awakened. Doors swing, pictures move, things tip over.
6. Everyone feels the earthquake. It is hard to walk. Windows and dishes broken. Books fall from shelves.
7. It is hard to stand. Plaster, bricks and tiles fall from buildings. Small landslides.
8. People will not be able dive cars. Poorly built buildings may collapse, chimneys may fall.
9. Most foundations are damaged. Masonry heavily damaged. Pipes are broken. The ground cracks.
10. Most buildings are destroyed. Water is thrown out of rivers and lakes. Large landslides.
11. Rails are bent. Bridges and underground pipelines unusable.
12. Most objects are leveled. Large objects may be thrown into the air. Large rock masses displaced.

Approximate Relationship Between Magnitude and Intensity