5.2 earthquake 'unnerving' for some Whangamata residents

Two thousand and five hundred people reported the largest of earthquakes centred off the coast of Whangamata on Saturday. Photo / Coastal News

A 5.2 magnitude earthquake left Coromandel residents shaken and stirred early Saturday morning and was felt as far north as Whangārei - but not felt underground in Waihi.

GNS Science seismologist John Ristau says a magnitude 5 was "not uncommon" in the Bay of Plenty but the shallower depth of the quake, 80km north of White Island off the coast of Whangamata, ensured it was felt quite strongly.

Some 2500 "felt" reports flooded in to GNS, from as far as Whangarei. It was felt in Auckland, Thames, Waihi, Whangamata, Tairua, Whangapoua and Kuaotunu as people woke to rattling items and rocking beds.

The quake was described as weak by GNS but residents throughout the district who are unused to quakes say they would not agree at that description.

Kathy Ellison in Waihi said she heard a loud thudding noise that sounded like the roof was lifting off.

"Wondered if it was a small tornado moving past, felt it as well. Didn't really know what was happening. I've felt small earthquakes before but this felt different."

Robyn Templeton said it felt like her house was being "flung about". "I wonder why they're calling a 5.2 weak!"

Several more followed of a similar magnitude throughout the weekend and seismologists call this phenomenon a swarm.

"One thing about this earthquake that helped to mitigate damage is that it was well offshore, about 80km to 100km east," says Ristau.

"What's interesting is the swarm type behaviour and we're keeping an eye on it. There have been a few aftershocks that have gone on, several with a magnitude 4.5 which again points to the swarm type nature of it."

OceanaGold workers underground on night shift at Waihi say they did not feel any shaking.

A photo showing the protective infrastructure underground at Waihi Mine. Photo / OceanaGold

"It did, however, register on our blast vibration monitors at 3.47am," says spokesman Kit Wilson.

"It is just as safe being underground during an earthquake as being on the surface. All of our staff are working under 'supported ground' – that is the rock above them is secured with two metre-long bolts which hold steel mesh in place. And, of course, all of our staff are wearing PPE – hardhats, steel cap boots, gloves and eye protection."

On the waterways at Pauanui, Amanda Iggulden said she was preparing to move to higher ground.

"Both my husband Matt and I woke up and I was checking the local Facebook pages waiting for information updates and listening out for the siren to see if we needed to move to higher ground.

"One of the things I was concerned about is sometimes we can't hear the siren or it's very faint."

There was no official warning sent out by Waikato Civil Defence because the magnitude was not great enough.

Magnitude increases greatly, for example a magnitude 6 is 32 times greater, and a 7 is 1000 times greater than a 5.

Ristau said people should not be overly alarmed with this swarm because it was always a possibility.

"The entire Bay of Plenty is all riddled with faults."

He says the earthquake had nothing to do with volcanic activity at White Island.

However earthquakes can trigger events such as undersea landslides, and these are difficult for GNS to pinpoint happening in advance.

In the case of a tsunami-generated landslide, the magnitude of a quake becomes irrelevant.

"It may need just a little bit of shaking to get it started. But that's extremely difficult for us to know in advance. You have to have advanced knowledge of where the landslides are, but it hasn't happened before."

As always, Coromandel residents should be prepared with an evacuation plan in case of tsunami.

Ristau said it was likely that the greatest source of a tsunami will be more distant sources of earthquakes.

"[At] the Kermadec Trench we can get very large earthquakes along there and your warning time is less than an hour, it doesn't leave you much time."

The Civil Defence message is long, strong, get gone - and if you feel shaking of a minute or more or a large quake, do not wait for information but go immediately to higher ground. You should not wait for the continuous siren which is the siren call of an imminent tsunami.

Some places on the Coromandel, such as Onemana, do not have a tsunami siren, says Onemana Fire Chief Jo Adams, who was awake for milking cows when the quake hit on Saturday.

"Everyone at Onemana and Opoutere has a bush telegraph, tell your neighbour, get in your car and hold your hand down constantly on the car horn as you're driving to higher ground."

Whangamata Area School principal Alistair Luke said the quakes were "a bit unnerving" and he was reassuring parents that the school had a procedure to take students immediately up the hill across the road from the school to the top of Chevron Cres.

"The reality is we would be cutting across private property and perhaps not in such an orderly manner as we are in a drill, when we would hold hands in two lines."

- Download the Red Cross Hazards App to set and receive alerts by geographical location. Civil Defence Waikato is on Facebook.