Slightly before 3.20 pm on Friday 23 December 2011 the city of Christchurch in New Zealand was struck by an Earthquake with a magnitude of 5.9 on the Richter Scale, which occurred at a depth of 4.9 km, 15 km northeast of the city. This was followed by a series of aftershocks, the largest of which had a magnitude of 5.0 on the Richter Scale, and occurred 21 km southeast of the city at a depth of 7.6 km, 9 hours after the initial quake. There are no reports of any serious injuries or damage, but a number of buildings had to be closed temporarily, including shopping malls during their busiest weekend of the year, there was localized flooding.
Flooding in the Parklands District of Christchurch. Earthquakes can cause flooding by squeezing water out of soil, as well as by damaging water mains and sewers.
Christchurch has always been prone to Earthquakes, but since September last year (2010) the city has been repeatedly shaken by large quakes. The largest of these quakes, on 22 February this year, killed 188 people and damaged a number of the cities prominent buildings. Nobody is quite sure what the cause of this increased activity is.
New Zealand lies on the boundary between the Australian and Pacific Plates. Beneath the islands the Pacific Plate is being subducted beneath the Australian and is sinking into the planet's interior. As the Pacific Plate passes underneath the Australian friction causes earthquakes. These are most likely to be felt in the east of the country, where the boundary between the planets is shallowest. To the west the boundary zone is much deeper, so that quakes directly caused by inter-plate friction are seldom felt at the surface, but heating of the underlying Pacific Plate by the Earth's interior causes it to partially melt; as this happens lighter minerals rise up through the overlying Australian Plate, causing volcanoes - which can themselves cause Earthquakes.
The positions of the Australian and Pacific Plates beneath New Zealand.