The Richter scale

By John Callan of Geological and Nuclear Sciences Ltd (GNS)

Most people have heard of the Richter scale - the measure of the strength of an earthquake. But what does it measure and what does it mean?

The Richter scale, or magnitude, is related to the amount of seismic energy an earthquake releases at its epicentre. It does not measure its effects.

The scale was devised by Californian seismologist Charles Richter in 1935 as a useful way of comparing one earthquake with another.

Richter discovered he could work out the size of a quake by using the height of the biggest wave on his seismograph and the time difference between the arrival of the primary and secondary waves of the earthquake.

The two waves travel at different speeds and are quite distinct on a seismograph. The bigger the gap between the primary and secondary waves, the further away the earthquake.

The Richter scale is not linear, but is based on power-of-ten logarithms. That means for each increase of one unit, there is a 30-fold increase in energy.

For example, the energy released in a magnitude 6.3 quake is 30 times greater than the energy released in a magnitude 5.3 quake. And a magnitude 7.3 quake is 1000 times more powerful than a magnitude 5.3 quake. Similarly, a magnitude 7 quake releases about a million times more energy than a magnitude 3.

A magnitude 5 earthquake releases as much energy as the Hiroshima atomic bomb -- the equivalent of 15 kilotons of TNT. A magnitude 6 earthquake is equivalent to 30 Hiroshima bombs.

New Zealand can have as many as 20 earthquakes a year that are between magnitude 5 and 6.

Each year about 16,000 earthquakes are recorded in New Zealand. Most are small, but about 1 percent are big enough to be felt.

The largest quake worldwide in the last 200 years was a magnitude 9.5 jolt that hit Chile on 22 May 1960.

It caused 5700 deaths and was the first real confirmation that the whole Earth can ring like a bell after a large earthquake.

It caused a huge amount of liquefaction with some vehicles being buried up to their windows.

It also caused a tsunami that travelled across the Pacific and upset shipping movements at the port of Lyttelton.

New Zealand's largest earthquake since European settlement was the magnitude 8.2 quake that occurred in southern Wairarapa on 23 January 1855. It occurred on the Wairarapa Fault and caused widespread damage between New Plymouth and Christchurch. There were no deaths, largely because New Zealand was very sparsely populated at the time.

The surface rupture along the fault extended for 75km. It moved about 12m horizontally and lifted land on its western side by 6m.

Generally humans don't feel earthquakes under magnitude 3.0.

However, in the North Island's volcanic plateau, swarms of small shallow earthquakes -- sometimes numbering several hundred -- are occasionally felt by people living in the Rotorua region.

They seldom cause any damage, but can be alarming for people who have not experienced earthquake swarms before.

Worldwide there are about 9000 earthquakes of magnitude 3 or less each day.

Magnitude is a single statistic like shoe size. "It's a handy label to put on an earthquake to give us an idea of how large it was and to compare it with other earthquakes," says Terry Webb, Natural Hazards Group Manager at the Institute of Geological and Nuclear Sciences Limited (GNS).

"But it doesn't tell us the full story. From a human perspective, it's not just the size of an earthquake, but its depth and location. A large earthquake that is very deep or a long way from populated areas won't necessarily translate into loss of life and devastation."

Dr Webb says the worst possible combination is a large, shallow earthquake located under or near a populated area.

Also lethal is a submarine earthquake that triggers a tsunami that inundates coastal areas. Landslides triggered by earthquakes are another serious hazard.

- NZPA