Alpine fault: tiny quakes can tell us a lot about earthquake maker next break

The Alpine Fault is a geological fault that stretches almost the entire length of the South Island of New Zealand. YouTube / Science GNS

Tremors too weak to be felt could help scientists learn more about the next major break along the South Island’s high-risk Alpine Fault.

As part of a new three-year project, scientists have started installing the densest network to date of earthquake recording equipment along the fault, which stretches about 800 km from the side. west of the South Island between Milford Sound and Marlborough.

By recording small shakes taking place in the rift, the team, led by Professor John Townend of Victoria University and Dr Caroline Holden of SeismoCity Ltd, hope to reveal where slips could take place in a future big. earthquake.

They also hope to record “seismic paths” between the fault and urban centers on the South Island in order to better predict the amount of earthquakes that can be expected during a major earthquake.

Several terabytes of data will be recorded over the next 18 months and then closely analyzed.

The fault, marking the border between the crustal plates of the Pacific and Australia, generates earthquakes of magnitude 8 to 8.2 approximately every 260 years.

The most recent of these dates back to 1717, and scientists expect the next major breakthrough to occur in a few decades.

Scientists are deploying the equipment – much of which is on loan from the Portable Array Seismic Studies of the Lithosphere facility in New Mexico – around the Franz Josef area next week, before heading to Haast.

Along with these, the project will build on more than a dozen existing seismometers operated in the Southern Alps by Dr Emily Warren-Smith of GNS Science and Dr Calum Chamberlain of the University of Victoria.

Townend expected the study – supported by the Marsden Fund and dubbed the Southern Alps Long Skinny Array, or Salsa – to significantly improve our understanding of fault behavior.

The Alpine fault, marking the border between the crustal plates of the Pacific and Australia, generates earthquakes of magnitude 8 to 8.2 approximately every 260 years.  Photo / GNS Science
The Alpine fault, marking the border between the crustal plates of the Pacific and Australia, generates earthquakes of magnitude 8 to 8.2 approximately every 260 years. Photo / GNS Science

“Once we have collected all the data, we can look at the characteristics of very small earthquakes and consider what they tell us about future large earthquakes and how the three-dimensional geological structure of the South Island affects earthquakes, ”he said.

“The methods we use allow us to analyze earthquakes throughout Tai Poutini Westland and southern and central New Zealand in response to many more earthquake failure scenarios than can be expected. studied using conventional approaches.

“This will provide us with solid estimates of the tremors that will occur during future large Alpine fault earthquakes.”

Holden said the high density of instruments near the fault would provide information in unprecedented detail.

“We will also be able to record the actual seismic paths between the fault and the urban centers – we usually have to calculate them by simplifying the geology,” she said.

“This dataset will not be taken for granted. Each installation in remote and challenging mountainous environments will take approximately three hours by a team of three.”

Each seismometer will be buried in a hole up to one meter deep and placed on a layer of clean sand and paving stone, and attached to a data logger, GPS sensor and solar power system.

Equipment installed on farmland will be fenced to prevent curious animals from interfering with or damaging it.

Wire mesh will also be used, as will plastic conduits for non-buried wiring, so kea and other wildlife do not interfere with the equipment.

The study comes after new evidence gleaned about the behavior of past earthquakes led scientists to revise the odds of a major Alpine fault rupture over the next 50 years from 30% to 75%.

They also calculated an 82% chance that the resulting quake would be greater than 8.0 – large enough to cause widespread damage and disturbance.

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