Increased Alpine Fault monitoring of 'micro-quakes' called for

Better monitoring of earthquakes around the West Coast and Otago is needed ahead of a major earthquake on the Alpine Fault, new research concludes.

Addressing the situation, a new research paper from GNS Science suggests monitoring of earthquake behaviour along the Alpine Fault should be improved before the forthcoming quake.

The research finds while much of the North Island is well covered, sparse station spacing and shallow seismicity resulted in low-quality monitoring locations in the West Coast of the South Island, Otago and Northland - mostly due to monitoring stations being too far from the earthquake being monitored.

“High seismic hazard regions, such as Fiordland and the West Coast, have very sparse station spacing, meaning adequate monitoring of seismicity there is very challenging,” the research paper states.

“These regions should be prioritised for future network expansion.”

The paper highlights that at present, there is a sparse monitoring network, and despite much needed and ongoing upgrades by GeoNet, it is “insufficient to closely monitor the fault’s behaviour ahead of the next major earthquake”.

Lead author of the  study Dr Emily Warren-Smith, a senior seismologist at GNS Science says improved monitoring infrastructure would allow scientists to identify changes in the fault’s behaviour over time and enable better recording of aftershocks following a major earthquake.

“Monitoring of small, micro-earthquakes and aftershocks is a very important way that we keep tabs on how our faults are behaving.

“The addition of new micro-earthquake monitoring sites along the Alpine Fault is fantastic news for our ability to keep the fault under surveillance. But it remains not as well instrumented as other regions, despite us knowing it will produce a very large earthquake in the future. 

“We should be learning as much as we can about the fault now, to allow us to better prepare for that next big earthquake.”

Graphic by GNS Science: The Alpine Fault shown clearly.

The Alpine Fault runs for about 600km up the spine of the South Island, on the "on-land" boundary of the Pacific and Australian Plates. At one point, it runs directly underneath the Franz Josef township.

GNS studies show the fault has ruptured four times in the past 1,100 years, each time producing an earthquake of about magnitude 8 and there is 75 percent chance of it rupturing in the next 50 years.

Geologist Robert Langridge, who is part of GNS Science’s AF 8 project preparing for the next major Alpine Fault event agrees more stations are needed in the southern South Island to consider the hazard there and for preparedness for the next Alpine Fault earthquake.

“Better monitoring of small earthquakes can help us understand the overall behaviour of the fault better,” he said.

“It also allows for better aftershock locations following a moderate to large earthquake across the region.

“A more extensive GeoNet network can provide a better response to the high probability Alpine Fault earthquake when it occurs."