A Magnitude 6.6 Earthquake at a depth of 8 km hit New Zealand's South Island, 10 km southwest of Christchurch, slightly after 11.35 pm local time (slightly after 10.35 am GMT) on Monday 18 November 2013, according to the GeoNet project, which monitors quakes in New Zealand. There have been no reports of any damage or casualties arising from this event, though it was felt across much of South Island as well as southern parts of North Island.
The approximate location of the 18 November 2013 Christchurch Earthquake. Google Maps.
New Zealand is located on the boundary beneath the Australian and Pacific Plates. Beneath the islands the Pacific Plate is being subducted beneath the Australian Plate. This causes a great deal of friction which causes Earthquakes where the boundary between the two plates is close to the surface; this is to the east of North Island, but onshore on South Island, where it can lead to strong Earthquakes such as the ones felt in Christchurch recently. Technically such quakes also occur where the plate margin is deeper, but these are felt less strongly as the rocks between the boundary and the surface absorb much of the energy, making strong tremors much less frequent on North Island. As the Pacific Plate sinks deeper into the Earth it is partially melted by the friction and the heat of the planet's interior. Some of the melted material then rises through the overlying Australian Plate, fueling the volcanoes of New Zealand.
Witness reports of Earthquakes can help scientists to understand these events, and the underlying geologic processes that cause them. If you felt either of these quakes then you can report it to the GeoNet here.
See also Magnitude 4.7 Earthquake under Cook Straight, New Zealand, Eruption on White Island, Magnitude 6.6 Earthquake and series of aftershocks hit South Island, New Zealand, New Zealand rattled by Earthquakes and Pumice raft linked to Havre Seamount, not Monowai.
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