So what isLiquefaction?

Liquefaction is a phenomenon in which strength and stiffness of soil is reduced by earthquake shaking or other rapid loading.

It is caused by a violent shake such as an earthquake. This causes the earth to throw out soil making it mix with water or other liquid

Liquefaction occurs when the structure of a loose, saturated sand breaks down due to some rapid shaking.As the structure breaks down, the loosely-packed individual soil particles attempt to move into a denser configuration.
When liquefaction occurs it causes high buildings to fall due to the soils loss of strength and stiffness.

Then the liquid absorbs soil making it reduce it's stiffness and strength which then makes the soil very hard and sticky causing it to get things stuck to it such as cars passing over it and other things that have moved from their places by the violent shake of the earthquake.
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In an earthquake, however, there is not enough time for the water in the pores of the soil to be squeezed out. Instead, the water is trapped and prevents the soil particles from moving closer together. This is accompanied by an increase in water pressure which reduces the contact forces between the individual soil particles, thereby softening and weakening the soil deposit.

Most of the buildings in Christchurch are built on soft soil and mud which is prone to “a large increase in shaking”and with high susceptibility to liquefaction.