Parts of Tauranga are sinking, including the Sulphur Point tip of the Te Papa peninsula. Photo / NZME
Parts of Tauranga are sinking, including the Sulphur Point tip of the Te Papa peninsula. Photo / NZME
Parts of Tauranga are sinking and neighbouring Rotorua is joining them.
Data obtained by the Bay of Plenty Times Weekend reveals some parts of Tauranga and the entire Rotorua caldera area are subsiding up to 5cm a year as part of vertical land movement.
These were Tauranga's Sulphur Point area,the city's Fraser Cove Shopping Centre near Parkvale, and its industrial area of Birch Ave, Judea, which were highlighted in a Bay of Plenty Regional Council Strategy and Policy Committee meeting earlier this month.
Research carried out by GNS geologist Dr Ian Hamling, Sigrun Hreinsdottir, Stephen Bannister and Neville Palmer confirmed the local subsidence.
Hamling said Sulphur Point was reclaimed land so some subsidence was expected. The other places involved areas of land that used to be old river deposits, and where there had been compaction over time from a lot of building work.
The predominant reason for the subsidence was believed to be the cooling of magma in the earth's crust. The area lies within what is known as the Taupō Volcanic Zone.
"Sometime in the past there would have been magma placed 6 to 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, using GPS radar technology, has been measuring this for the last 20 years.
"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.
However, "if you've got a coastal property and subsidence with increasing sea levels, then you would probably be concerned".
Despite this, land at Matatā and Pikowai is, unlike Rotorua and parts of Tauranga, rising.
GNS Science geologist Dr Richard Levy. Photo / NZME
Hamling said this was likely due to earthquake swarms in the 2000s that resulted in magma intrusions.
The Bay's vertical land movement was brought up in a regional council meeting on August 4.
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.
"There are only three areas of Tauranga subsiding and they are: Sulphur Point - which is understandable as its reclamation; Judea's industrial area - understandable, it's a past wetland; and Fraser Cove Brook St which is industrial and again probably below sea level before development," he said.
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."
Geologist Dr Richard Levy told the Bay of Plenty Times Weekend localised parts of Tauranga were subsiding between 3 and 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."
Bay of Plenty Regional Council member Andrew von Dadelszen who also chairs the Public Transport Committee, says Rotorua is sinking. Photo / NZME
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 final 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 falls 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 (at a replacement value of $38 billion) could be exposed to future extreme storm-tide events.
The Rotorua cauldera area is sinking. Photo / NZME
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.
