Canterbury earthquakes

An earthquake near Christchurch in September 2010 started a chain of events still being felt years later. It caused extensive damage to property, and aftershocks also caused injury and loss of life as well as lasting social upheaval. The events around Canterbury are examples of common geological phenomena associated with earthquakes. They are important to study because they are potential hazards to human life and property.

Fault movement

Early on 4 September 2010, a magnitude 7.1 earthquake occurred on an unrecorded fault near Darfield, 40 km west of Christchurch and just 10 km below the surface. Seconds later, the Greendale Fault just to the south ruptured. Along 24 km of this fault, ground on either side shifted horizontally up to 5 m and vertically up to 1.3 m. Aftershocks were concentrated at the east end of the fault towards Christchurch as rock distorted by the original earthquake adjusted to new stresses.

One result of this readjustment was a damaging aftershock (magnitude 6.3) on 22 February 2011, when a fault ruptured very close to Christchurch. This fault came to within a depth of 1 km but did not show at the surface. Between the September 2010 earthquake and mid-July 2012, the Canterbury region experienced:

• 3 shocks of magnitude 6–6.9
• 54 of magnitude 5–5.9
• 431 of magnitude 4–4.9
• over 3000 of magnitude 3–3.9
• thousands of smaller tremors.

Ground movement

The vertical acceleration of the ground during the 22 February 2011 earthquake was the highest ever recorded in New Zealand and one of the highest in the world. Shaking in the centre of Christchurch, which caused a lot of damage, was much stronger than in the 4 September 2010 earthquake, partly because it was much closer to the epicentre. The unusually strong shaking might have been made worse by seismic lensing, with seismic waves being reflected off the hard basalt rocks of the Port Hills. Interference between waves amplified the ground acceleration in some places and reduced it in others.

Peak ground acceleration (PGA) is a measure of movement compared to the acceleration of gravity, which is 1 g (9.8 m/s²). The most severe shaking in the February earthquake only lasted a few seconds but in places reached a PGA of 2.2 g. This was unusually high, especially for an earthquake of ‘only’ magnitude 6.3. In comparison, the magnitude 6.8 earthquake at Kobe, Japan, in January 1995 had a PGA of 0.8 g. The magnitude 9 earthquake near Tohoku, Japan, in March 2011 had a PGA of 2.7 g.

Liquefaction

The severe shaking of the September 2010 and February 2011 earthquakes caused some areas of underground, waterlogged silts to behave like liquid, in a process called liquefaction. This meant the ground could no longer support the foundations of many buildings, causing them to subside. Underground infrastructure, such as water and sewage pipes, was also destroyed. About 400 000 tonnes of silt pushed up through cracks in the ground and covered the surface, causing a huge clean-up problem. Read our article on liquefaction to find out more.

Landslides and rockfalls

Shaking caused damaging landslides and rockfalls on steep slopes and cliffs, especially in Lyttelton, Sumner and Redcliffs. Some rocks shaken loose from high on the Port Hills bounced down hundreds of metres onto buildings below.

Tsunamis

The Canterbury earthquakes did not dislodge enough rock underwater to cause any tsunamis off the South Island. However, the earthquake on 22 February 2011 caused about 30 million tonnes of ice to break off the Tasman Glacier, 200 km to the west. This fell into Tasman Lake and created waves up to 3.5 m high.