Analysis: Faults buried deep in the earth make detection a hard task

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.

Similarly, the Canterbury region of the South Island has a history of significant earthquakes. However, the earthquake activity over the past 12 months has taken many people by surprise.

The magnitude 7.1 Darfield earthquake occurred on 3 September last year, some 28 miles west of Christchurch. This quake triggered many thousands of aftershocks, the largest of which was the devastating magnitude 6.3 earthquake on 22 February that resulted in 182 deaths and severe damage in Christchurch.

Aftershocks occur as a natural readjustment to the changes in stress in the Earth caused by a quake and can last for many months or even years depending on the size of the initial shock. But the faults responsible for these quakes were buried below the surface and there was little indication they existed or could result in large earthquakes.

Aftershock sizes typically range up to an order of magnitude less than the main shock, so a magnitude 6 aftershock following a magnitude 7 earthquake is not unusual. However, the location of the magnitude 6.3 earthquake in February was particularly unfortunate in that it was very close to the city centre. This resulted in very high levels of ground shaking in the city itself. It may take Christchurch years to recover from the impacts of these earthquakes.

• Dr Brian Baptie is a seismologist with the British Geological Survey in Edinburgh