Surging springs not a sign of volcanic activity
A surge of activity from warm springs around Lyttelton Harbour since last month's earthquake does not indicate underground volcanic activity, GNS Science says.
About six springs near Christchurch have become warmer or started releasing more gas or discharging more water during the past month.
GNS geochemist Dr Agnes Reyes and Dr Bruce Christenson sampled springs after the September 4 and February 22 quakes and found elevated activity consistent with previous observations after quakes in New Zealand and overseas.
Reyes said 10 times the quantity of water was now being released from springs at Cass Bay, Rapaki Bay and Motukarara, although spring temperatures remained the same.
However, gases – about 82 per cent nitrogen and 13 per cent methane – were being discharged "copiously" from the Rapaki Bay springs where normally there were none.
Some gas was also released in Cass Bay after last month's quake, but in much smaller amounts than in Rapaki Bay.
The Cass Bay springs usually discharged from two pipes and seeped along a canal adjacent to the boatshed, but since February 22 a landslide in the bay had exposed several sources of warm water along about 50 metres of land from the playground to the boatshed, Reyes said.
At Ferrymead, warmer waters of about 22 degrees Celsius were now flowing in the spring, although it was possible seasonal variations played a part in that.
The Rapaki Bay springs average 33C, those in Cass Bay 30C and those at Motukarara 26C.
"Thermal waters in Christchurch are mainly groundwaters that circulate underground and get heated deep in the crust and discharge to the surface at 20C to 33C, often mixing with cooler groundwaters and/or seawater near the surface," Reyes said.
The extra liveliness was not a sign of volcanic activity stirring below Lyttelton Harbour and Banks Peninsula, she said.
A West Coast spring sensitive to large quakes has also been affected by last month's quake.
GNS geologist Dr Simon Cox has shown how the temperature of South Westland's Welcome Flat hot springs, more than six hours walk up the Copland River from State Highway 6 into the Southern Alps, dropped about 1C after the September 4 Darfield quake before recovering to 56.7C.
After the February quake, the spring temperature fell 0.4C to about 56.3C and has yet to rise to longer-term average levels.
Wells as far away as Northland and Southland also reacted to the February 22 quake and fluctuated by about 25 millimetres as a result of the magnitude-9.0 quake off Japan on March 11.
Environment Southland senior groundwater scientist Karen Wilson said only confined aquifers were affected by last month's quake, but the September shake also altered unconfined aquifers.