Standing in the last back field of a deer farm near Whataroa on the west coast of the South Island, high-visibility orange vest, shovel in hand, Professor John Townend searches 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 can help us learn what to expect in the next big earthquake.
This will be the site of one of the South Island Long Skinny Array (SALSA) seismic sensors – a chain of 55 sensors spaced 10 km apart along 450 km of the alpine fault between Maruia and Milford Sound.
The Alpine Fault is the land border between the Pacific and Australian Plates, stretching almost the entire length of the South Island. Paleoseismological research has shown that the fault has a remarkably regular history of producing large earthquakes; one every 300 years or so. The last major earthquake occurred 1717 – 304 years ago, making it likely that the next severe earthquake on the Alpine Fault will occur within our lifetimes.
SALSA’s goal is to increase our understanding of what will happen in the aftermath of this earthquake. The seismic sensors that John and his team use are capable of detecting a wide range of seismic wave frequencies, including background hum, or ambient seismic noise, which is produced when ocean waves hit land.
Previously, scientists would have removed this “noise” from their data or avoided capturing it in the first place. However, recent progress has shown that collecting this kind of data over a long period of time and analyzing it can provide insight into how seismic waves will travel along the fault and affect ground shaking further afield. It allows scientists to create “virtual” earthquakes – they can model how energy will be transferred without having to wait for a real earthquake to occur.
Although the sensors must first be installed, a gigantic task in itself as the team has to deal with the challenges presented by the Southern Alps – different terrain, difficult access and changeable weather. The full team of 11 people work together for two and a half weeks to install as many sensors as possible. Claire Concannon joins a group as they set up a sensor to find out all about how and why they are doing it.
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The SALSA project is funded by the Marsden Fund of the Royal Society Te Apārangi and involves scientists and graduate students from Victoria University of Wellington — Te Herenga Waka, SeismoCity Ltd., GNS Science, University of Auckland and the universities of Edinburgh, Tokyo and Washington. The project is led by the teacher John Townend from Victoria University of Wellington and Dr. Caroline Holden from SeismoCity Ltd. The project uses equipment and is grateful for the technical and logistical expertise of the Portable Array Seismic Studies of the Continental Lithosphere (PASSCAL) Instrument Center in Socorro, New Mexico.
The SALSA team warmly thanks Te Rūnaka o Makaawhio and Te Rūnanga o Ngāti Waewae, the Department of Conservation, Fox and Franz Heliservices, Aratuna Freighters, Out There Learning and landowners across the region for their support and encouragement. Kia ora mai armadillo. Ngā mihi nui ki a tātou.