Tsunami threat sparked by earthquakes not over after first waves pass - experts

Friday, 5 March 2021

People are ignoring the tsunami warning and continuing to engage in activities at the southern end of Orewa beach in north Auckland.

New Zealand's tsunami threat remains despite the first expected tidal surges passing for many parts of the country, experts have warned.

National Institute of Water and Atmospheric Research (NIWA) scientist Emily Lane said those told to evacuate “definitely cannot relax yet”.

The two separate earthquake clusters on Friday.

Often, Lane said, initial waves are smaller – with later waves increasing in size.

“These things can take time to build up. The wave arrivals are just the first wave arrivals,” Lane said.

**READ MORE:

University of Auckland associate professor Kasper van Wijk says people should not relax just because first wave times have passed.

* Earthquake cluster and tsunami warnings: What you need to know

* Tsunami alert for New Zealand after third earthquake, 8.1M in Kermadec Islands

* Tsunami warning: Threat to NZ passes after 7.7 magnitude earthquake in New Caledonia

A map issued by the National Emergency Management Agency shows the areas due to be affected by tidal surges caused by tsunami

Kasper van Wijk, an associate professor in the Department of Physics at Auckland University, said tsunamis can cause damage beyond the first or direct wave because of how waves interact with currents and tides.

“This is why the alerts extend beyond the time of the first wave,” he said.

Make-up of shoreline affects wave size

Lane said the make-up of a shoreline can multiply or reduce the size of the waves.

“Often the places where there are larger tsunami responses are places where you get resonance in bays or harbours,” she said.

“Every bay and harbour have resonances to do with how long it takes waves to travel to the far end and back.

“If the period of the tsunami [time between peaks] happens to be similar to the period of the bay [the time for a wave to travel there and back] it can reinforce itself. This can exacerbate the waves.

“Other locations that can exacerbate things are v-shaped bays narrowing towards the top – this can focus the tsunami energy to the top of the bay.”

Van Wijk said widespread evacuation orders were partly a symptom of greater understanding and monitoring of tsunami threats.

“Given the size of this earthquake and the proximity to the coast being careful is totally valid.

“What you are seeing is just us being a bit better prepared as we learn from previous events.”

The largest of the three Kermadec earthquakes today, which measured 8.1 magnitude, held roughly 30-times the energy than the Gisborne event that woke up much of the country.

This presented an elevated risk despite its distance from land.

Estimated times of first waves striking

Auckland East – 11:35 am

Auckland West – 11:48 am

Bluff – 2:41 pm

Dunedin – No information

East Cape – 10:31 am

Gisborne – 10:09 am

Great Barrier Island – No information

Greymouth – No information

Jackson Bay – No information

Kaingaroa Chatham Islands – 10:51 am

Lottin Point – 9:49 am

Lyttelton – No information

Marlborough Sounds – No information

Milford Sound – No information

Mount Maunganui – 10:19 am

Napier – 10:45 am

Nelson – No information

New Plymouth – 12:29 pm

North Cape – 10:18 am

Picton – No information

Port Tauranga –10:26 am

Stewart Island – No information

Timaru – No information

Waitangi Chatham Islands – No information

Whanganui – 12:29 pm

Wellington – 11:06 am

Westport – No information

Whangarei – 10:34 am