Friday 5 March 2021

Magnitude 8.1 Earthquake in the Kermadec Islands.

The United States Geological Survey recorded a Magnitude 8.1 Earthquake at a depth of 19.4 km, in the Kermadec Islands, to the north of New Zealand, slightly before 6.30 am local time on Fridat 5 March 2021 (slighty before 7.30 pm on Thursday 4 March, GMT). The event was felt in the islands, and triggered a tsunami warning in New Zealand, where many people were evacuated from low lying areas on North Island, although in the the event, while some notable waves were observed, no damage or flooding was recorded.

The approximate location of the 5 March 2021 Kermadec Earthquake. USGS.

The event was part of a swarm of Earthquakes that has hit the area over the past 24 hours, with two focus points observed, one in the Kermadec Islands and one off the northeast coast of North Island. Two other Earthquakes with Magnitudes in excess of 7.0 have been recorded during this series of events, one in the Kermadec Islands, and one off the coast of North Island.

 
Earthquakes around New Zealand and the Kermadec Islands on 4-5 March 2021. The event indicated in red is the most recent. USGS.

The Kermadec Islands lie along the boundary between the Pacific and Australian Tectonic Plates. The Pacific Plate is being subducted beneath the Australian Plate along the Tonga Trench, which forms the boundary between these two plates, with the volcanic islands that make up the archipelago being formed as the subducting plate is melted by the heat of the planet's interior, so that lighter minerals rise up through the overlying plate as liquid magma, which fuels the volcanoes that build the islands. This subduction is not a smooth process, with the two plates constantly sticking together until the pressure generated by their movement builds up sufficiently to break them apart, causing a sudden forward jerk we experience as an Earthquake.
 
 
Diagram showing subduction along the Tonga Trench, and how this feeds the volcanoes of the Tonga Volcanic Arc. York University.
 
Further south, New Zealand is also 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. 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, fuelling the volcanoes of New Zealand.
 
The subduction zone beneath New Zealand, and how if fuels Earthquakes and volcanoes. Te Ara.
 
Earthquakes along subductive margins are particularly prone to causing tsunamis, since these often occur when the overlying plate has stuck to the underlying plate, being pulled out of shape by its movement.. Eventually the pressure builds up to far and the overlying plate snaps back, causing an Earthquake and a tsunami. 
 
 
Simplified graphic showing tsunami generation along a convergent margin.NASA/JPL/CalTech.
 
Witness reports of Earthquakes can help scientists to understand these events, and the underlying geologic processes that cause them. If you have felt an Earthquake in New Zealand then you can report it to the GeoNet project, which monitors quakes in New Zealand, here. 
 
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