An international team that drilled almost a kilometre deep into New Zealand’s Alpine Fault has found extremely hot temperatures and high fluid pressures.
According to Nature.com, these surprising underground findings have broad implications for understanding what happens in the buildup to a major earthquake, and may represent the discovery of a new type of geothermal energy resource.
The Alpine Fault is one of the world’s major plate boundaries and New Zealand’s most hazardous earthquake-generating fault. It runs for 650 kilometres and ruptures (on average) every 300 years, producing an earthquake of about magnitude 8. The Deep Fault Drilling Project, has given the team of more than 100 scientists from 12 countries, the opportunity to take a closer look at the fault as it builds up to its next rupture.
Two holes were drilled into the Alpine Fault, and the team’s second attempt made it 893 metres deep. As the scientists drilled deeper, the temperature increased rapidly, at a rate of 15 degrees Celsius per 100 metres in depth. This is much higher than the normal rate of three degrees celsius per 100m in depth. At a depth of 630 metres, the water at the bottom of the drill hole was hot enough to boil, it if had been allowed to rise to the surface. The high pressures at depth stop it from boiling.
Whilst the hottest boreholes on earth are found in volcanic regions, there are no volcanoes near the Alpine Fault. So why is it so hot?
Scientists explain that there are two processes which cause the extreme underground conditions at their drill site. An earthquake on the Alpine Fault has two geological effects – mountains pushed higher and the shaking breaking up the rocks.
During an earthquake, fractured rocks come down in landslide and carried into the sea. Eventually, hot rocks from great depth, are transported to the surface without enough time to cool. Heat is then transported from depth by the rock movement. Alternatively, the other process that helps explain the team’s findings is the rock fracturing, which allows rain water and snow melt to percolate downwards into the mountains so fast that it can move heat towards the valley. This is where the water wells up and discharges. The flow needs to be fast enough so that the heat is not lost along the way; water flowing through the rock concentrates the heat and raises fluid pressure between the valleys.
The results of The Deep Fault Drilling Project are important beyond New Zealand. Other faults around the world may experience similar conditions that have never been investigated. Due to the results, the team can now describe and estimate conditions on a geological fault that will rupture as an earthquake, which in turn, will help scientists develop better computer models of earthquake rupture.