An earthquake, also known as a quake, tremor or temblor, is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time.
At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacement of the ground. When the epicenter of a large earthquake is located offshore, the seabed may be displaced sufficiently to cause a tsunami. Earthquakes can also trigger landslides, and occasionally volcanic activity.
New Zealand capital shakes 'like jelly' in powerful quake
Agence France-Presse in Wellington
A powerful earthquake rattled New Zealand's main cities yesterday, sending terrified office workers fleeing. Central Wellington shook "like jelly", but authorities reported no major damage.
The 6.5-magnitude quake struck at 2.31pm near an area where a series of quakes hit last month, the US Geological Survey said. It was felt from Christchurch in the South Island to Auckland in the North Island.
The USGS said the quake was centred five kilometres east of the town of Seddon at a relatively shallow depth of 9.9 kilometres. Seddon is near Blenheim at the top of the South Island.
A cluster of major aftershocks measuring up to 5.9 followed but no tsunami alert was issued.
The tremors caused violent jolts in Wellington, where office workers dived under their desks for cover as buildings swayed and police had to rescue several people trapped in lifts.
"Lots of aftershocks. 'Beehive' wobbling around like a jelly, but all OK," Economic Development Minister Steven Joyce said on Twitter, referring to New Zealand's distinctive parliament building in the capital.
Local resident Juli Ryan tweeted: "That was pretty wild. I was sitting in my parked car watching buildings shake like leaves."
Other witnesses said the powerful shaking felt like a jackhammer and left them struggling to stay on their feet.
"I feel a bit queasy, it was swaying so much. I waited about 10 seconds and got under my table, then we decided to get out," said Sam Stanley, who works in the New Zealand stock exchange building.
Workers poured into the streets of Wellington as aftershocks continued to rock high-rise blocks through the afternoon, with businesses telling staff to leave early and get out of the downtown area, causing gridlock in the capital.
Trains were also out of action due to potential danger from buckled rails, leaving many commuters stranded. There were power cuts to parts of the South Island. However, authorities said while there were a few incidents involving broken glass, no major injuries or damage were reported.