Rotorua sinking: Data reveals city subsidence
Areas of Bay of Plenty subsiding up to 2cm a year
Rotorua is sinking. Data obtained by the Rotorua Daily Post reveals the Rotorua caldera area and some suburbs of Tauranga are subsiding up to 2cm a year as part of vertical land movement.
Research carried out by GNS geologist Dr Ian Hamling, Sigrun Hreinsdottir, Stephen Bannister, and Neville Palmer, showed the area from Rotorua down to western and northern Taupo¯ was sinking.
Hamling said that was a “big, big area of subsidence”.
“Everything through that zone is subsiding at rates of 1 to 2cm a year, over a fairly broad region, about 30km in width.”
Hamling said this translated to about 1000sq km, “maybe more”.
The predominant reason for the subsidence was believed to be the cooling of magma 6 to 8km deep in the earth’s crust. The area lies within what is known as the Taupo¯ Volcanic Zone.
“Sometime in the past there would have been magma placed 6-8km deep . . . as it starts to cool down, you go from liquid magma and as it cools to rock it becomes dense and it loses volume, so everything above it sinks,” Hamling said. “It’s a long-lived process.” Hamling has been measuring this for the past 20 years, using GPS radar technology.
“At the rate it’s going, it’s probably not a major issue. Because it’s so broad, it’s not going to affect a home’s foundations. People shouldn’t be worried,” he said.
The subsidence was also likely to affect Rotorua’s lake beds so lakefront properties were not expected to be significantly affected. However, “if you’ve got a coastal property and subsidence with increasing sea levels, then you would probably be concerned”.
Other areas in the Bay of Plenty also sinking include Tauranga’s Sulphur Point area, the city’s Fraser Cove Shopping Centre near Parkvale, and its industrial area of Birch Ave, Judea, albeit “at a fairly low rate”.
“That’s partly areas where you combine the geography, a lot of them are old river deposits, with compaction over time from a lot of buildings,” Hamling said.
Meanwhile, land at Matata¯ and Pikowai was rising.
Hamling said this was likely due to earthquake swarms in the 2000s that resulted in magma intrusions.
Local land shifts were brought up in a Bay of Plenty Regional Council Strategy and Policy Committee meeting earlier this month.
During discussions of climate change, councillor Andrew von Dadelszen referenced the data, which he sourced independently, and called for councillors to be better appraised of such research.
In addressing the Rotorua members of the committee, he said: “By the way Rotorua, you are sinking — all around Rotorua. Not that that should make any difference [regarding sea level rise], I don’t think the sea will get to you there but you are subsiding.
“My point here is this is good information. Our staff didn’t have it. I shouldn’t have to ask for it I don’t think. Our councillors should have this information.”
Geologist Dr Richard Levy told the Rotorua Daily Post Weekend that localised parts of Tauranga were subsiding between 3-5mm a year but, overall, the rate of local vertical movement of land was low.
“However, rates of subsidence and uplift along the Bay of Plenty coastline are highly variable and need to be considered when planning for future sea level change,” he said.
“Importantly, subsidence rates can be high in many relatively flat regions in which we typically choose to live. These flat regions are often filled with sediment that compact and sink as time passes.”
Other reasons for vertical land movement were also detailed in the report Te Tai Pari o Aotearoa — Future Sea Level Rise around New Zealand’s Dynamic Coastline.
The report, authored by 13 scientists and experts including Hamling and Levy, explained some movement was a result of the shape of Earth’s land surface slowly changing in response to the retreat and disappearance of massive ice sheets that covered large areas of the planet during the last ice age, 20,000 years ago.
In some areas, such as New Zealand’s South Island, the land was slowly rising in a process called a glacial isostatic adjustment. However, other areas of land were subsiding as the Earth’s mantle flowed away from these regions. These changes in land shape were causing local sea levels to fall in some regions and rise in others.
Projections indicate that sea levels could rise by as much as 1.2m by 2100 under high emissions scenarios. However, this forecast did not include the influence of vertical land movement, the report stated.
Exposure assessments showed that after 1m of sea-level rise, about 125,000 buildings (with a replacement value of $38 billion) could be exposed to future extreme storm-tide events.
Scientists using global positioning satellite technology and radar systems mounted on Earth-observing satellites have shown parts of New Zealand’s coast were going up a rate of 1cm every year and others sinking by as much as 5mm a year.
The vertical position of New Zealand coastlines was also changing due to the movement of tectonic plates, the report stated.
"At the rate it’s going, it’s probably not a major issue."
GNS geologist Dr Ian Hamling