- Written by Nadia O’Carroll
On 4 September 2010 Christchurch experienced a magnitude 7.1 earthquake followed by many aftershocks. Seismologically the February earthquake is classified as an aftershock of the September earthquake. Although of smaller magnitude than the September earthquake, the February earthquake was more devastating because its epicentre was close to the city, it was far shallower, it had more acceleration and shaking intensity.
Another more damaging aspect of the February earthquake is that it produced 300% to 500% more liquefaction.
Liquefaction occurs when the ground is shaken violently. When the ground is wet, water fills the gaps between the soil particles. When the ground is repeatedly shaken the soil particles rearrange into more dense and compact clumps; this increases the water pressure as the water in the ground moves to flow out of the soil. When the shaking is strong and repetitive this water pressure increases with every shaking cycle.
Eventually this pressure becomes greater than the forces holding the particles of soil together. When this happens the soil loses its strength and structure and behaves like a liquid hence the term liquefaction. The result is a layer of thick silty liquid sludge covering the ground. Sometimes there are craters on the sludge surface which resemble volcanoes, this occurs where the rising liquefied soil is under pressure and must force its way through a concentrated narrow outlet on the surface producing a sand boil or sand volcano.
The amount of liquefaction depends upon a number of factors - the strength and duration of the shaking, the depth of the water table and soil properties such as grain size and density, and confining pressures. Loose unconsolidated sandy and silty soils such as marine, river or floodplain deposits or poorly compacted man made fill are vulnerable to liquefaction.
Liquefaction causes extensive damage, it may result in land slides and fissures, the loss of soil strength means that buildings and structures lose their support, underground pipes become buoyant and float and a thick layer of silt spreads over the ground surface. The only positive aspect of liquefaction is that once the soil liquefies it is no longer able to transfer further earthquake waves from underground to the ground surface.
Unfortunately many areas in Christchurch are vulnerable to liquefaction. It is estimated that more than 100,000 to 150,000 tonnes of sand will have to be removed from Christchurch's eastern suburbs. It is also estimated that some 49 square kilometres of land in Christchurch has undergone liquefaction.
The brave citizens of Christchurch are determined to rebuild their beautiful city under very trying conditions and it is hoped that they achieve the success they deserve.