The Hikurangi subduction zone: a credible magnitude 8.9 earthquake and tsunami scenario


Leading New Zealand scientists have
designed a credible scenario for an earthquake and tsunami on the Hikurangi Subduction Zone. The scenario, further developed by GNS Science experts, helps
emergency responders to plan and prepare for a magnitude 8.9 earthquake and tsunami on the Hikurangi Subduction Zone which is off the east coast of the North Island. The scenario shows impacts from the earthquake and tsunami for the most effected areas across the North Island’s East Coast in
New Zealand. It is one of many possible scenarios and does not predict the
future. It is unlikely that a Hikurangi Subduction Zone earthquake and
tsunami will happen exactly like this. This scenario is based on a magnitude
8.9 earthquake and shows the potential levels of ground shaking. The earthquake’s rupture begins 70 kilometres off the coast Porangahau, Hawke’s Bay at a
shallow depth of nine kilometres. An earthquake like this would release
around 45 times more energy than the 2016 magnitude 7.8 Kaikoura Hurunui
earthquake. The scenario is run at double the actual speed and the scale bar shows the level of shaking. White shows extreme shaking and red shows
strong to severe shaking. The earthquake rupture continues north and south creating long and strong shaking along the east coast of the North Island.
Gisborne would experience particularly strong and long shaking in this scenario
due to the different rock types in the area that increase the shaking’s
intensity and duration. Wellington would feel ground shaking 90 seconds after the
initial earthquake rupture. Areas further away from the earthquake rupture would
experience strong shaking. For example Auckland though a good distance away
would experience around 30 seconds of severe shaking. As well as the shaking
the earthquake would potentially cause liquefaction, landslides and fires in
some areas. The sudden movement of the sea floor during the earthquake causes a lot of water to move creating a tsunami. The scenario is now showing the tsunami
created by this large earthquake. Within the first few minutes there are rapid changes in sea level along the east coast of the North Island. The tsunami moves in all directions. After the earthquake rupture some parts of the coast may see a rise in sea level shown in red. In others such as Hawke’s Bay in this scenario the sea-level would rapidly decrease,
pulling away from the coast and returning with enormous speed and force
when the tsunami waves arrive. This is shown in blue. This is because the ocean
first draws down and sucks water away from the coastlines to later return.
Close to the shore along the East Coast waves could reach about ten metres above normal sea level in some places. In a few locations where the tsunami is funneled into steep valleys on shore It might reach even higher, possibly up to 20 metres above sea level. In some other earthquake scenarios run-up heights
could be even greater than 20 metres at some locations. Tsunami evacuation zones account for multiple possible earthquake scenarios. In this scenario the speed the tsunami waves travel means there would not be enough time for an
official evacuation warning. The long or strong earthquake is your signal to self evacuate all tsunami evacuation zones. Check your tsunami evacuation zones on
your local civil defense group website. Make sure that you and your family and
others who rely on you know your evacuation route to higher ground or
inland whether you’re at home, at work, at school or out and about. In this scenario
around 13 minutes after shaking begins in Wellington large tsunami waves arrive
at the harbour entrance and extend into the tsunami evacuation zones. It’s
important to evacuate all tsunami evacuation zones in a long or strong
earthquake. The scenario is sped up to show how the tsunami moves in the first
two hours of the earthquake. The tsunami would reach cities as far away as
Auckland, Christchurch and even the West Coast of the North Island. While this and
other science-based scenarios do not predict the future and a single scenario
cannot provide all the answers on its own, they help emergency managers to plan and prepare for events like a Hikurangi subduction zone earthquake and tsunami.
To get prepared visit getready.govt nz To check your tsunami evacuation zones visit your local civil defense emergency management website.

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