The Taupō District Council is asking people to stay away from the water’s edge at a popular lakeside reserve while it works out what caused about 20m of foreshore to disappear during a swarm of earthquakes this week.
“Hey folks, we know you’re interested in what’s happened out at Wharewaka but we have serious concerns about the land stability there. We estimate we’ve lost around 20m of foreshore,” the council said in a Facebook post.
“Other agencies are investigating the cause, but it may be a result of land slumping with a resultant wave on the lake, rather than a wave alone.
“Our team is working to organise a temporary fence but in the meantime, to keep everyone safe in this area we ask that you please keep yourself, your children and your vehicles well back from the edge. Thanks.”
Taupō District Council parks operation manager Greg Hadley said they had now closed off the road closest to the water at 4 Mile Bay, Wharewaka, because people were driving right up to the edge.
There were cracks running parallel to the shore which indicated the area was unstable and the weight of vehicles could cause it to collapse further.
GNS Science was studying the area to try and find out what had happened, including putting drones in the air.
”We just said to them ‘when you come up with an idea of what went on, let us know’,” Hadley said.
He said there was anecdotal evidence around other areas of the lake, including Kurutau at the southern end, that suggested a large wave or waves had come several metres more than usual up the foreshore but so far, the most significant damage was at 4 Mile Bay in Wharewaka. He said the lake level was very high at the moment which had likely exacerbated the problem.
Two large, four-person pedal boats that were pulled up onto the grass near the area of erosion, were torn from their chains, washed onto rocks and destroyed by a surge of water, thought to be a tsunami, during the swarm on Wednesday night and Thursday morning.
GNS Science said the Volcanic Alert Level remains at 1 despite the recent increased activity.
A GNS report found the magnitude 5.6 earthquake caused ground movement around Lake Taupō.
“At least 100 mm of horizontal movement was observed at an instrument at Horomatangi Reef,” GNS said in its report.
“GNS is carrying out further analysis of existing data and looking at what additional observations can be collected and analysed.”
More than 350 aftershocks have continued to rattle Taupō and surrounding areas after Wednesday night’s quake.
A shallow 3.9 quake hit 20km southwest of Taupō at 5.37am today at a depth of 5km.
This is the #tsunami as measured by the @niwa_nz water level gauges at Acacia Bay (near Taupō township) and Tokaanu (southern end of the lake) - see second image for locations. It's in NZST which is why it looks like it comes in an hour early. https://t.co/W7PfhJFgVl pic.twitter.com/FPScHYSV1n
— Emily Lane (she/her) (@Emily_M_Lane) December 1, 2022
Other tremors were too weak to be noticeable.
The series of shakes continue after a strong 5.6 magnitude quake rattled central North Island just before midnight on Wednesday.
One of the larger aftershocks recorded was a 4.1 magnitude tremor at 11.47pm on Thursday.
Niwa hydrodynamic scientist Dr Emily Lane showed in a tweet the tsunami as measured by water level gauges at Acacia Bay and Tokaanu.
Latest M4.1 earthquake at 1:53pm today is the 3rd M4.0+ aftershock since the main shock of M5.6 late yesterday within the volcano's caldera. Here is the summary and locations of the Taupō Volcano's M4.0+ shock sequence for the 2022 episode....so far pic.twitter.com/Ep6gtNRDOP
— Hauraki Gulf Weather (@GulfHauraki) December 1, 2022
Lane told the Herald it was interesting a tsunami resulted from an earthquake of this size.
“With these volcanic earthquakes, you will get deformation, it actually deforms the ground underneath the lake.”
She referenced the complex Kaikōura earthquake when referring to what it might look like under Lake Taupō at the moment.
During the 7.8 magnitude shake in 2016, parts of the land in Kaikōura were jolted several metres upwards because of the many fault lines that were activated.
Although it may not be as extreme under Lake Taupō, the ground shifting is what would have caused the wave.
However, Lane said the tsunami was bigger than scientists would have guessed given the magnitude of Wednesday night’s quake.
“One of the interesting things is that an earthquake of this size, in terms of generating tsunamis, that’s pretty small.
“We sort of think that maybe the amount of movement you got was more than what you would expect for an earthquake of that size.”
Lane said she and her team are very interested in talking with locals who have seen any more inundation (the very high tide line) around Taupō so they can continue to piece together the science around last night’s quake.
GeoNet is still questioning if the wave was a tsunami at all.
In a post on its website, it said researchers were “still looking into the evidence of a potential seiche or small tsunami” which have both occurred on the lake previously.
“At this point, we don’t know if this is due to a seiche, where the lake moves back and forth and ‘sloshes’, or a tsunami, caused by a landslide, or some combination of both,” GeoNet wrote.
Taupō Pedal Boats owners Jess Ratana and Kiripiti Bowden were down at the lake’s edge on Thursday morning pulling the two four-person pedal boats off the rocks with a 4WD ute.
Ratana said they were fortunate all of their pedal bikes were intact but the larger pedal boats were the only two they had.
“We pulled them right up last week because of the wind. It was really windy down here. They were on the grass, they weren’t really near the water at all.
“It’s just pulled them out and the wind direction has taken them this way and they have ended up being smashed against the rocks.”
She said they were insured.
“It’s not something we ever thought would happen – an earthquake, and a lake tsunami. I don’t think anyone would have expected that.”
Fire crews were called to the scene around 7.30pm.