The effects of earthquake waves at the surface can be measured using an intensity scale.This is an arbitrary scale based on observations of phenomena such as:
- the type and extent of damage,
- whether sleeping people were woken,
- whether items fell from shelves,
- whether the event was felt or heard.
The most common intensity scale used in Australia is the 12-point Modified Mercalli scale. On this scale, intensities up to 5 are felt but cause no damage, while intensities from 6 to 12 cause increasing amounts of damage. A Modified Mercalli Intensity of six is abbreviated as MMI 6.
Modified Mercalli Intensity (MMI) Scale
|1||Not felt. Recorded by seismographs|
|2||Rarely felt, usually only on top floors of high buildings|
|3||Felt indoors, like a passing light truck|
|4||Windows, dishes, doors rattle. Like passing train|
|5||Felt by all. Small objects upset|
|6||Books off shelves. Trees shake. Isolated damage|
|7||Difficult to stand. Many poor buildings damaged|
|8||Significant damage. Branches broken from trees|
|9||General panic. Serious damage. Ground cracking|
|10||Most buildings destroyed. Rails bent slightly|
|11||Rails bent greatly. Pipelines destroyed|
|12||Near total damage. Objects thrown into the air|
Other intensity scales are used in some countries. The Rossi-Forel scale is relatively old, has values from 1 to 10, and is still used in some countries. The JMA (Japan Meteorological Agency) scale from 1 to 7 is used in Japan and Taiwan. The MSK scale is the most recent, has 12 values that approximate but are not the same the Modified Mercalli values, and is used extensively in Europe.Intensity values can be correlated with other measures of ground motion, such as displacement, velocity and acceleration. For example, MMI 6 corresponds to a peak ground velocity of about 50 mm/s.Intensity is easy to use, even for historic earthquakes.
Maximum intensity normally occurs near the earthquake epicentre, with intensity values then decreasing with distance.
An earthquake has a single magnitude, but intensity varies with distance.
Many factors affect surface ground motion, including topography and near-surface geology, especially soft surface sediments. These variations can be considerable, even over short distances. It is common to find intensities ranging by ±1 unit in a neighbourhood, and not unusual to find values ±2 or more.