Effects of the christchurch earthquakes
The death toll after the February 2011 earthquake reached 185 people, making this the second deadliest natural disaster recorded in New Zealand. Over 6,500 people were injured. Around 10,000 houses required demolition and over 100,000 houses were damaged.
Medway Bridge over the Avon river
More destruction was caused by the February 2011 earthquake rather than the larger magnitude September 2010 for several reasons. The epicentre of the February quake was located only 10km to the south-east of Christchurch and only 5km deep, therefore, causing more intense shaking as compared to the epicentre of the September quake which was located 40km west of the city and 10km deep – away from all major built-up areas. The shallower earthquake caused more damage because a larger percentage of energy released by the quake hit the heavily built-up and populated city centre. Damage to, and collapse of buildings in the February earthquake was greatly increased by the fact that many buildings had already been structurally weakened by the September quake and its aftershocks. Also, the occurrence of liquefaction was much greater following the February quake, causing significant ground movement, the undermining of building foundations and causing severe damage to the area’s water and sewerage systems due to the breaking of pipes.
Some areas of Christchurch are very susceptible to liquefaction because they are built on loose, sandy, silty soils. Other areas of Christchurch are built on clay or gravelly soils which are not susceptible to liquefaction. Eastern parts of the city, which suffered major liquefaction damage after the February quake, were built on a former swamp. The severity of liquefaction damage also depends on the strength of ground shaking. In the February quake, the epicentre was close to Christchurch city so the shaking was strongest and resulted in more liquefaction in this area. By the time the waves reached the Kaipoi and Selwyn districts, which suffered liquefaction damage after the September quake, there was no longer enough energy in the waves to cause major liquefaction damage there again.
Older masonry buildings were most vulnerable and many sustained severe earthquake damage, such as the Christchurch Cathedral. Brickwork, stone and concrete buildings are heavy and brittle and, without modern reinforcements, are susceptible to crumbling or collapsing during earthquake shaking. The peak ground acceleration (PGA) in central Christchurch in the February quake was one of the greatest ever recorded in the world and greater than many modern buildings had been designed to withstand and several buildings collapsed. Many buildings damaged in the February quake were affected because they had already been structurally weakened in the earlier earthquake in September. Buildings in some parts of the city were severely damaged by liquefaction as they were covered in thick layers of silt, water and sewerage from broken pipes and many house foundations cracked.