Earthquake Intensity
Earthquake intensity - the simplified scale to describe the effect of the earthquake at ground level
Earthquake intensity - the simplified scale to describe the effect of the earthquake at ground level
The size of an earthquake is often described using magnitude, which is the amount of energy released during an earthquake. However, not all of the energy released in an earthquake will necessarily be felt at the surface, depending on the earthquake's depth. In New Zealand, where earthquakes occur from near the surface right down to a depth of over 600 km, the Modified Mercalli intensity scale is a better indicator of an earthquake's effects on people and their environment.
The original Mercalli scale graded the effects of earthquakes into ten steps. The scale used in New Zealand is a twelve step ranking, hence Modified Mercalli (MM), with 1 representing the weakest of shaking, through to 11 representing almost complete destruction. The descriptions below are a simplified version of the New Zealand Modified Mercalli Intensity scale.
The table shows a simplified version of the New Zealand Modified Mercalli Intensity scale. We have added a generalised Intensity term.
| MMI | Intensity | Description |
|---|---|---|
| 1 | unnoticeable | Barely sensed only by a very few people. |
| 2 | unnoticeable | Felt only by a few people at rest in houses or on upper floors. |
| 3 | weak | Felt indoors as a light vibration. Hanging objects may swing slightly. |
| 4 | light | Generally noticed indoors, but not outside, as a moderate vibration or jolt. Light sleepers may be awakened. Walls may creak, and glassware, crockery, doors or windows rattle. |
| 5 | moderate | Generally felt outside and by almost everyone indoors. Most sleepers are awakened and a few people alarmed. Small objects are shifted or overturned, and pictures knock against the wall. Some glassware and crockery may break, and loosely secured doors may swing open and shut. |
| 6 | strong | Felt by all. People and animals are alarmed, and many run outside. Walking steadily is difficult. Furniture and appliances may move on smooth surfaces, and objects fall from walls and shelves. Glassware and crockery break. Slight non-structural damage to buildings may occur. |
| 7 | severe | General alarm. People experience difficulty standing. Furniture and appliances are shifted. Substantial damage to fragile or unsecured objects. A few weak buildings are damaged. |
| 8 | extreme | Alarm may approach panic. A few buildings are damaged and some weak buildings are destroyed. |
| 9 | extreme | Some buildings are damaged and many weak buildings are destroyed. |
| 10 | extreme | Many buildings are damaged and most weak buildings are destroyed. |
| 11 | extreme | Most buildings are damaged and many buildings are destroyed. |
Common Alerting Protocol (CAP) is a format for exchanging all-hazard emergency alerts and public warnings. CAP describes the severity of impact of a hazard as one of four terms: minor, moderate, severe or extreme.
This table shows how GeoNet classes these CAP Severity terms against the Modified Mercalli intensity descriptions.
| MMI | Intensity | CAP Severity |
|---|---|---|
| 1 | unnoticeable | Minor |
| 2 | unnoticeable | Minor |
| 3 | weak | Minor |
| 4 | light | Minor |
| 5 | moderate | Minor |
| 6 | strong | Moderate |
| 7 | severe | Severe |
| 8 | extreme | Extreme |
| 9 | extreme | Extreme |
| 10 | extreme | Extreme |
| 11 | extreme | Extreme |