New Zealand's GeoNet Project, which monitors geohazards in the country, recorded a Magnitude 4.7 Earthquake at a depth of 22 km beneath the Cook Straight, which separates the nation's two main islands, slightly after 5.20 am local time on Wednesday 2 October 2013 (slightly after 4.20 pm on Tuesday 1 October, GMT). This is a moderately large quake, and was felt as far away as Christchurch to the south and Whangerei to the north, though there are no reports of any damage or injuries.
The approximate location of the 2 OCtober 2013 Cook Straight 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 Eruption on White Island, Magnitude 6.6 Earthquake and series of aftershocks hit South Island, New Zealand, New Zealand rattled by Earthquakes, Pumice raft linked to Havre Seamount, not Monowai and Pumice raft suggests eruption from Mount Monowai.
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