Analysis: Latest aftershock was made worse by its shallow depth

THE magnitude 6.3 earthquake that struck Christchurch at 12:51 local time yesterday was the latest in a sequence of many thousands of aftershocks that have occurred in this part of New Zealand since the magnitude 7.1 Darfield earthquake on 3 September last year.

A map of these aftershocks shows that they line up along a 100-kilometre fault zone that runs approximately from west to east, extending into Christchurch itself.

Before the Darfield earthquake there was little indication at the surface that this fault existed and could result in large earthquakes.

The latest shock was only a few kilometres south of Christchurch, much closer than the Darfield earthquake, which was approximately 45km west of the city.

This proximity, together with the shallow depth of the earthquake, resulted in exceptionally high levels of ground shaking in the city, much more than most buildings are designed to withstand, which caused the extensive damage. In addition, the way that the fault ruptured resulted in large vertical ground motions above the earthquake zone.

The nature of the near-surface geology also appears to have been a contributory factor in the distribution of damage, with a great deal of damage occurring in the north-east of the city, where many of the buildings are constructed on poorly consolidated sediments like sands and gravels, which amplify the ground shaking.

Soil liquefaction was also widespread; the ground shaking causing the soil to lose strength and behave like a liquid. This phenomenon is again most often observed in loose, sandy soils.

New Zealand lies on the Pacific "Ring of Fire" at the boundary between the Australian and Pacific plates and is no stranger to large earthquakes.

The Alpine Fault, which runs through the South Island, is the "on-land" boundary between the two plates where they are moving past each other sideways. The slip rate on this fault system is about 3cm a year.

The Alpine fault has ruptured four times in the past 1,000 years, each time producing an earthquake of about magnitude 8. On average, New Zealand can expect several magnitude 6 earthquakes every year, one magnitude 7 every ten years, and an 8 every century.

Previously, New Zealand's most damaging earthquake was a magnitude 7.8 event that struck the Hawkes Bay area in 1931 and killed 256 people. The largest was the 1855 Wairarapa earthquake with a magnitude of 8.2.

• Brian Baptie is a seismologist, at the British Geological Survey in Edinburgh