The south-westernmost corner of New Zealand was shaken by a pair of Earthquakes on Thursday 19 January 2012. The first quake struck slightly before 5.50 pm local time (slightly before 6.50 am GMT) roughly 100 km off the coast at a depth of 18 km beneath the seafloor, and measured 5.9 on the Richter Scale. The second occurred slightly after 8.00 pm local time (9.00 am GMT) at a depth of 15 km in roughly the same location, and measured 5.2 on the Richter Scale.
Map showing the location of the quakes, from the United States Geological Survey.
There are no reports of any damage or casualties, and no tsunami warning has been issued, though quakes of this size are likely to have been felt on land. The southwest corner of New Zealand is a very tectonically active area, but it is also very sparsely populated, so the quakes seldom bother anybody.
The quakes appear to have happened on the Puysegur Trench, which stretches 800 km to the south from the southwestern corner of New Zealand's South Island. The Puysegur trench has formed where the Australian Plate is being subducted beneath the Pacific Plate; as the plate sinks into the Earth's interior it drags on the overlying Pacific Plate sticking while pressure builds up then periodically moving sharply, releasing the pressure and causing Earthquakes.
A three-dimensional view of the Puysegur Trench. Image from the Te Ara Encyclopedia of New Zealand.
See also Christchurch shaken by another Earthquake, Earthquake in the Kermadec Islands triggers tsunami alert, The Christchurch Earthquakes and Earthquakes on Sciency Thoughts YouTube.