Earthquake magnitude and intensity

Earthquakes are measured in two different ways: magnitude and intensity

Earthquake magnitude

Earthquake magnitude is a measure of the energy released by an earthquake, or its "size". Because earthquakes vary a lot in size, earthquake magnitude scales are logarithmic. For a one-step increase in magnitude the amount of energy released increases about 32 times. So a magnitude 7 earthquake is 32 times bigger than a magnitude 6 earthquake, and a magnitude 8 earthquake is 1000 bigger.

The Richter Scale was the first earthquake magnitude scale to be devised, in the 1930s. However, the Richter Scale is not very accurate for measuring the size of earthquakes above about magnitude 6.5, so other magnitude scales, such as local or moment magnitude, are now used (although news readers still ofter talk about "the Richter Scale"). These new scales are set up so that the numbers used are similar to the Richter Scale so that they can be easily compared.

Earthquake intensity

Earthquake intensity describes how much ground shaking occurred, or how "strong" an earthquake was, at a particular location. Earthquake waves weaken as they travel away from the earthquake source, so an earthquake generally feels less strong the further away from the source you are.

The intensity of earthquake shaking at a particular location depends on the magnitude of the earthquake (how much energy was released), and how deep and how far away it was. Local topography, geology and soils also influence the amount of earthquake shaking.

There are a number of different intensity scales, but in New Zealand intensity is measured using the Modified Mercalli intensity scale. This is a descriptive scale from 1 to 12 based on how people feel an earthquake, the damage to buildings and their contents, and how the natural environment responds.

The Modified Mercalli (MM) intensity scale

MM 1	Not felt.
MM 2	Felt by peeple at rest on upper floors of buildings.
MM 3	Felt indoors, like a small truck passing; hanging objects swing slightly.
MM 4	Felt indoors by many, like a heavy truck passing; hanging objects swing, windows rattle.
MM 5	Felt outdoors, sleepers awakened, small objects and pictures move.
MM 6	Felt by all, crockery breaks, furniture moves, weak plaster cracks.
MM 7	Difficult to stand, noticed by car drivers, furniture breaks, weak chimneys break at roof line, plaster, loose bricks and tiles fall.
MM 8	Driving is difficult, ordinary masonry is damaged, chimneys and towers fall, some liquefaction.
MM 9	General panic, poor masonry destroyed, ordinary masonry and foundations damaged, liquefaction and landslides.
MM 10	Most masonry structures destroyed. Some well-built wooden structures and bridges destroyed. Dams and embankments damaged, large landslides.
MM 11	Few buildings left standing.
MM 12	Damage nearly total.