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.
See also...
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