Central New Zealand was rattled by a Magnitude 4.9 Earthquake at a depth of 16km beneath the Cook Strait, which separates the nation's North and South Islands, at about 11.10 am local time on Thursday 1 August 2013 (11.10 pm on Wednesday 31 July, GMT), according to the GeoNet project, which monitors quakes in New Zealand. This was followed 45 minutes later by a second quake with a Magnitude of 4.2, in roughly the same spot. These are moderately large quakes and shallow, with the potential to cause problems. In this case no damage or injuries have been reported, but the quakes were apparently felt across much of the country.
The approximate location of the 1 August 2013 Cook Strait Earthquakes. 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.
A Magnitude 6.5 Earthquake took place beneath the Cook Strait on 21 July 2013, since when there have been a series of smaller quakes (some of them still quite large) in the area, which are thought to be aftershocks of the initial quake.
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 (for the 11.10 am quake) or here (for the 11.55 quake).
See also Pumice raft linked to Havre Seamount, not Monowai, Pumice raft suggests eruption from Mount Monowai, Volcanic activity on White Island, New Zealand, Eruption on Mount Tongariro and Movement beneath the Tongariro Volcanic Complex.
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