The Richter scale is based upon the logarithm of the amplitude of the waves that make up the earthquake; the maths behind this is a bit complex, but the logarithm bit is important. A logarithmic scale records orders of magnitude, that is to say 2 is ten times as much as 1, 3 is ten times as much as 2 etc. this means that a huge range of wave amplitudes can be recorded on the scale without getting into huge numbers. Seismometers cannot directly detect large earthquakes (they break), so the magnitude of larger earthquakes is calculated from readings on several remote seismometers, or in the case of very large earthquakes, by calculating backwards from the amount the rocks have moved. This means that the scale is not absolutely reliable; it is a useful tool for geologists - and journalists - but there can be rival claims for the strongest earthquakes.
The magnitude of an earthquake is measured at its central point (hypocentre), which can be a long way beneath the ground, thus earthquakes higher on the Richter scale are not necessarily the most destructive, those close to the ground are the most dangerous. An alternative system of measuring earthquakes, the Mercalli Scale, measures earthquakes by their effects; this is measured at the epicentre of the earthquake, i.e. the point on the ground above the hypocentre. This has downfalls of its own, since cities that expect earthquakes tend to be better prepared than those that do not; an earthquake that would flatten London or New York can be endured by Tokyo or Los Angeles.
On 22nd February 2011 a second earthquake struck, this time 2 km west of Lyttelton, a town 10 km south east of Christchurch. This earthquake had a magnitude of 6.3 (i.e. it was roughly a tenth as strong as the first earthquake) and occurred at a depth of about 5 km. This was much more destructive, significant soil liquidification occurred in the eastern suburbs of Christchurch (the soil was shaken so much that it behaved as a liquid), leading to considerable damage to property and the loss of 181 lives. Geologists are still undecided as to whether this was an aftershock of the first earthquake or a separate event.
Damage to Christchurch's Catholic Cathedral following the February 22 Earthquake.On the 16th April an earthquake with a magnitude of 5.3 occurred 16 km east-south-east of Christchurch at a depth of 9.2 km. This lead to power outages and further damage to property, but no casualties.
On the 10th of May another earthquake struck the region, this time with a magnitude of 5.3, roughly 4 km beneath the town of Rolleston, 20 km to the south-west of Christchurch. Again this caused damage to property but no casualties.
On the 6th of June a second earthquake struck Rolleston, this time with a magnitude of 5.5 but at a depth of 15 km. This caused further damage to properties, but no serious injuries or loss of life.
On the 13th June an earthquake, with a magnitude of 5.7 struck Taylor's Mistake, 10 km south-west of Christchurch, at a depth of 9 km. This was followed by an earthquake under the sea 10 km east of the city, with a magnitude of 6.3 and at a depth of 6 km. These earthquakes caused considerable more damage to property, and resulted in a number of injuries.
All of this has left the people of Christchurch somewhat concerned for the future of the city. Prior to 4th September 2010 there had not been an earthquake of any size in the area since 1922, and now there does not seem to be an end to them.
New Zealand is a seismically active country; it lies on the border of the Pacific and Australian tectonic plates, a convergent boundary where the Pacific Plate is being forced under the Australian at a rate of about 40 mm a year. The country has over forty active volcanoes, and is has around 15 000 earthquakes a year, though most of these are to small for anyone to notice them.
Earthquake prediction is far from being a precise art, but it is safe to predict that if you are on an active fault and have not had an earthquake for a while, then you are likely to have one. The progress of one plate over another is not a smooth process, rocks rubbing against one-another typically move in stops and starts, as the plates move the rocks catch on one another. Stress then builds up within the rocks until they give way, and the plate rolls on. The long period of time that Christchurch has been without an earthquake implies that the rocks beneath it were likely to be very stressed. The earthquakes that have occurred will have released a lot of this stress, possibly all of it. At some point the earthquakes will stop and life in Christchurch will return to normal, though it is impossible to say if this will be tomorrow or in five years time.
Christchurch's situation is made worse by its location on the south-east coast of New Zealand, as this is where the tectonic boundary is at its shallowest. The boundary between the plates runs roughly south-west to north east along the whole of New Zealand. On the southern coast around Christchurch it is only five to ten kilometers beneath the surface, on the far coast it is closer to 500 km deep. This makes earthquakes on the south-east coast far more dangerous than those on the north-east.
A rough diagram of a cross section through New Zealand's South Island (not to scale), showing the depth of the plate boundary beneath the country.