The 7.8 magnitude earthquake that shook Fiordland on July 15 was the largest in New Zealand since 1931. It caused power outages and landslides, but fortunately only minor damage and no fatalities. Land around the epicentre was uplifted by a metre, and the South Island twisted so that Dunedin moved 1 cm closer to Australia. What GNS scientists Simon Cox and Delia Strong revealed a couple of weeks ago was that it also cooled the famous Welcome Flat springs on the West Coast.
The figure below, courtesy of Dr Cox, shows the time-series of spring water temperature before and after the earthquake. Temperature occasionally drops during storms, when cold rainfall infiltrates the ground and mixes with the geothermal waters, but this effect is short-lived. At the time of the earthquake, however, the temperature started to drop to a new long-term average temperature — about 1°C cooler.
Figure: Spring water temperature, where it emerges from the ground, shows short-term fluctuations caused by rainfall events as well as a sustained decline following the July 15 earthquake. [Credit: Simon Cox and Delia Strong, GNS Science]
Hot springs like Welcome Flat’s pock-mark the South Island landscape in an arc that reflects the Alpine fault. Groundwater is heated far beneath the Earth’s surface, and follows a tortuous path through fractures and faults before emerging in springs, to the delight of many trampers.
What the GNS temperature data show is that the fractures and faults that constitute the geological plumbing of the springs are not set in, er, stone. The July earthquake rattled the underground plumbing so that when water now emerges at Welcome Flat, there is a greater proportion of cooler rain water compared with warm geothermal water, to the probable dismay of many trampers.
This research is part of a broader study led by Dr Cox, looking at the hydrological responses of earthquakes in general, and what these responses can tell us about what happens leading up to and during an earthquake. If you happen to have noticed any changes in spring flow — either in temperature or amount — he would be keen to hear from you. For more information, see the Deep Fault Drilling Project website.
Reference: Cox S C, Strong D T (2009) Insights to crustal fluid-flow near the Alpine Fault: monitoring experiments at Copland Warm Spring. Abstract and poster presented at the Geosciences 09 Conference, Oamaru, 23-27 November 2009. Geological Society of New Zealand Miscellaneous Publication 128A. ISBN 978-1-877480-07-2. p46.the