The drilling project at New Zealand’s Alpine Fault is the first to investigate a major fault that is due to rupture in a big earthquake in coming decades. John Townend/Victoria University of Wellington, CC BY-SA

An international team that drilled almost a kilometre deep into New Zealand’s Alpine Fault, which is expected to rupture in a major earthquake in the next decades, has found extremely hot temperatures and high fluid pressures.

Our findings, published in Nature today, describe these surprising underground conditions. They 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.

Seismic forces building up

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 along the spine of New Zealand’s South Island and we know that it ruptures on average every 300 years, producing an earthquake of about magnitude 8.

The last time the Alpine Fault did this was in 1717, when it shunted land horizontally by eight metres and uplifted the mountains a couple of metres. It is expected to rupture in a major earthquake in the next few decades and, even though this may not happen in the next 30 years or even 100 years, we know that the fault is at the end of its seismic cycle.

Other projects around the world have drilled into major faults, but usually just after a major earthquake. The Deep Fault Drilling Project, which involved more than 100 scientists from 12 countries, gave us an opportunity to take a close look at a fault as it builds up to its next rupture. It is the first time this has ever been done on a major fault that is due to fail in coming decades.

Hot water at depth

We drilled two holes and during our second attempt made it to 893 metres deep. As we drilled deeper, the temperature increased rapidly, at a rate of about 15 degrees Celsius per 100 metres in depth. This is much higher than the normal rate of about 3°C per 100m in depth. At a depth of 630 metres, the water at the bottom of the drill hole was hot enough to boil, if it had been allowed to rise to the surface. The high pressures at depth stop it from boiling.

The hottest boreholes on Earth are mostly found in volcanic regions. We discovered a geothermal gradient – a measure of how fast temperature increases with depth – that is similar to the hottest geothermal energy boreholes drilled into volcanoes of the central North Island; but there are no volcanoes near the Alpine Fault.