More than 100 people attended a talk by AF8 scientists Dr Caroline Orchiston and Professor Mark Stirling in Wanaka last Thursday (March 28).
“Knowledge is power,” was the message from Orchiston at the talk which was one of 12 in the AF8 Roadshow aimed at delivering Alpine Fault science and hazard impact information to the South Island communities most likely to be affected by an AF8 earthquake.
The 800km-long Alpine Fault is part of the active boundary between the Pacific and Australian tectonic plates where they intersect on land. Scientists have established that the fault has ruptured 27 times over the last 8000 years: that’s an average of every 300 years and Orchiston said, “there’s no reason for that to stop happening”.
The last significant quake on the Alpine Fault was in 1717, which means the next severe earthquake on the Alpine Fault is likely to occur within the lifetime of most of us, or our children.
Wanaka is just 75km from the Alpine Fault. “In terms of an intensity footprint, you are right on the cusp of the green and the yellow: looking at intensities of five and six on the Modified Mercalli Intensity Scale (MM). So that means you’ll have some direct impact in terms of the ground shaking and some of those secondary hazards,” Orchiston (pictured) said.
MM5 is described as moderate and “felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop”, while MM6 is described as strong and “felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.”
Orchiston said the indirect impact, such as damage to the state highway, communications and electricity networks, would possibly be more significant.
AF8 scientists have prepared a number of scenarios to illustrate the impact of a magnitude 8 earthquake along the Alpine Fault. Orchiston stressed each scenario gives its own intensity footprint, so the impact on Wanaka could be more or less severe depending on where the earthquake originates along the fault line.
The issues facing Wanaka in the event of an AF8 include landslide risks, likely liquefaction beside the lakes, damage to heritage buildings, seiche waves and rockfall tsunamis.
“If you feel a long, strong earthquake you should move away from the lake margin,” Orchiston said.
An AF8 could also trigger earthquakes on other faults in the region.
Earthquake Early Warning alert
New Zealanders could be warned of an Alpine Fault earthquake in advance through an Earthquake Early Warning (EEW) alert.
The warning may be possible because an Alpine Fault rupture could take several minutes to travel the length of the fault.
Massey University and GNS Science are currently investigating whether an EEW alert in relation to an Alpine Fault earthquake would be of benefit.
An EEW is an alert you might get about an earthquake that has occurred (via mobile phone, TV or radio, for example) shortly before shaking begins in your location. If you are very close to the centre of an earthquake, you will not get a warning before shaking starts; however, if you are further away you might receive an alert a few seconds to a few minutes before shaking begins.
To give feedback on how an EEW could work in New Zealand, there's a short survey you can fill in at https://www.surveymonkey.com/r/EEWNZ.