Listening to the hum of the Alpine Fault

A team of scientists are installing an array of seismic sensors along the South Island's Alpine Fault. Claire Concannon joins them to find out how and why.

Standing in the last back field of a deer farm near Whataroa on the South Island's West Coast, orange high-vis vest on, shovel in hand, Professor John Townend is looking for the perfect spot - not too wet, not too stony, with a good view of the sun. A good space to dig a hole and bury something that may help us learn what is to come in the next big earthquake.

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This is to be the site of one of the South Island Long Skinny Array (SALSA) seismic sensors - a string of 55 sensors spaced 10km apart along 450km of the Alpine Fault between Maruia and Milford Sound.

The Alpine Fault is the on-land boundary between the Pacific and Australian plates, running almost the length of the South Island. Paleoseismology research has shown that the fault has a remarkably regular history of producing large earthquakes; one about every 300 years. The last large earthquake was in 1717 - 304 years ago, making it likely that the next severe earthquake on the Alpine Fault will occur within our lifetimes.

The aim of SALSA is to increase our understanding of what will happen as a result of that earthquake. The seismic sensors John and his team are using are able to detect a wide range of frequencies of seismic waves, including the background hum, or ambient seismic noise, that is produced when ocean waves hit the land.

Previously, scientists would have deleted this 'noise' from their data, or avoided capturing it in the first place. However, recent advances have shown that collecting this type of data over a long period and running analysis on it can give insight into how seismic waves will move along the fault and affect ground shaking further afield. It allows scientists to create 'virtual' earthquakes - they can model how the energy will transfer without having to wait for an actual earthquake to occur.

First though the sensors have to be installed, a mammoth task in itself as the team must contend with the challenges that the Southern Alps present - different terrain, difficult access and changeable weather. The full team of 11 people are working together over two and a half weeks to fit as many sensors as possible. Claire Concannon joins a group as they install one sensor to learn all about how and why they are doing it.

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Go to this episode on rnz.co.nz for more details