What is curious about this sequence offshore from New Zealand is whether or how the ruptures relate to each other. Indeed, quakes up to magnitude 9 can occur, such as Japan's 2011 Tōhoku earthquake, the undersea earthquake off Sumatra that set off the 2004 Indian Ocean tsunami, and quakes in Alaska in 1964 and in Chile in 1960. Magnitude 8 quakes in these subduction zone settings are not unusual. This is typically where these large earthquakes occur. But when it's at a shallow level, it is only dipping at say 10-20 degrees and creates a lot of friction with the overlying (Australian) plate. These later events had thrust, or compressive, mechanisms, in which one body of rock compresses against another, sliding up and over it during the earthquake.Īs the Pacific plate starts to slide under the Australian plate, it starts off at a shallow angle and then turns along a curved trajectory to finally fall away at a very steep (60 degrees) angle. The other two quakes were about 900km north, but just west of the Tonga-Kermadec trench and at depths of about 56km (for the 6.40am magnitude 7.4 event) and 20km (for the magnitude 8.1 quake at 8.28am). This quake had an unusual mechanism-an element of sideways movement known as strike-slip. The US Geological Survey recorded this event at a depth of 21km, not 95km deep as the first reports in New Zealand suggested. The first 7.3 magnitude rupture struck at 2.27am, 174km off the east coast, where the Hikurangi and Tonga Kermadec systems merge. The Tonga Kermadec subduction zone terminates north-east of the East Cape, where it then becomes the Hikurangi subduction zone. One of the questions seismologists around the world are now trying to answer is whether the three quakes were linked and the earlier ruptures triggered the magnitude 8.1 quake. It spans from just north of the East Cape, some 2600km to the north-east in an almost straight line to south of Samoa. This subduction zone is the longest and deepest such system on Earth. By early afternoon, the National Emergency Management Agency ( NEMA) lifted the evacuation order but stressed that people should stay off beaches and the shore.Īll three earthquakes happened along the Tonga Kermadec subduction zone, where the Pacific tectonic plate dives under and then sinks beneath the Australian plate.
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