Quick Dips

Looking Under the Southern Alps

Geologists have been gaining an unusual view of the Southern Alps, with international teams looking at how the mountain range extends 45 km below the surface.

The rocks forming the eastern and western South Island are being compressed together, with the rocks forming the Southern Alps ramping up along the Alpine Fault at a rate of one metre in 100 years. Seismic data from earthquakes and controlled sources show a 45-km deep root in the Earth's crust formed by the compressing mountain-building forces. Observations of very distant earthquakes show that extra dense material occurs under the Southern Alps to over 150 km in depth.

The Alpine Fault can be imaged to 25km in depth as it continues to form the western boundary of the sub-surface Southern Alps. Electrical measurements of fluids and seismic reflections show that the deeper part of the mountains' root is a concentrated zone of deformation with fluids being released from the compressing rocks.

The relatively low rate of small earthquakes on the fault is perhaps surprising in view of the mountain building, but it is similar to the rate on a similar major fault, the San Andreas Fault in California.