CANBERRA, Feb. 24 (Xinhua) -- Australian earth scientist on Thursday explained what causes the ground to liquefy, tilting buildings and wreaking damage after an earthquake.
A massive 6.3 earthquake on Tuesday afternoon hit New Zealand South Island's largest city of Christchurch for the second time in less than six months.
The quake struck at a shallow depth of just four kilometers with an epicenter around 10km outside Christchurch, killing at least 76, and 238 people are still missing.
As the first images of the Christchurch earthquake came through, people can see large expanses of water over roads, footpaths and other flat areas.
According to University of Melbourne's earth scientist Professor Mike Sandiford, what looked like flooding in the image was actually soil liquefaction, a phenomenon that sometimes occurs with earthquakes.
Sandiford said liquefaction happens when a quake shakes up water sitting in loosely packed sediment about 10 to 20 meters below the ground, turning the usually solid soil into a liquid.
However, Sandiford said it does not happen after every earthquake.
For liquefaction to occur, the affected ground has to be made up of loosely packed sediment, and there must be a lot of water sitting in the spaces, or pores, separating the individual sediment grains.
"If you have a city on solid rock, like granite or something, it will never liquefy because there is no liquid in the pores," he told ABC Science on Thursday.
He went on to say the process happened after earthquake on Christchurch is because the city was established on an alluvial plain, built out over time by river deposits of sands and muds, which have never consolidated.
Meanwhile, Sandiford said the closer the area is to the epicenter of the earthquake or the greater the magnitude of the quake, the more liquefaction is likely to occur.
With this latest quake, Christchurch was close to the epicenter, he notes, so even though it was not a large earthquake, there was a lot of liquefaction, which leads to a devastating impact to the city.
Liquefaction was famously happened after a magnitude 7.5 quake in the Japanese city of Niigata in 1964, leaving buildings intact but sinking at angles into the ground.