Quake psychology: Why some of us fear big aftershocks more

Doug Avery in his earthquake-damaged bedroom in Grassmere, near Seddon, on August 17, 2013. Photo / Tim Cuff

Twelve per cent.

That was the probability GNS Science experts gave, hours after the 7.8 Kaikōura Earthquake rattled the country, of a similar-sized aftershock striking within that same day.

It might've seemed an oddly specific number, yet GeoNet has been calculating and issuing probabilistic forecasts for future quakes since the 1990s.

It was only after the Canterbury earthquakes that our interest in them began to grow.

For some, however, hearing these figures can be enough to trigger anxiety, particularly when it comes to our gender – and now scientists have explored how public messaging can be improved.

"No one can predict earthquakes, but we can give probabilistic forecasts for some types of hazards," explained Dr Sally Potter, a social scientist at GNS Science.

"Probabilistic forecasts are the chance of an event occurring, such as an earthquake. The forecasts are relevant for an event with a defined range of severity, and for a particular area and time frame."

Soon after the Kaikōura quake on November 14, 2016, scientists issued a series of scenarios, with various probabilities of their happening.

While there was a 12 per cent chance of a quake measuring bigger than 7.0 striking in an area spanning from Christchurch to Wellington within 24 hours after, there was a much higher chance - 93 per cent – of a smaller quake between 6.0 and 6.9 hitting within the week.

That came to pass with a mid-afternoon 6.3 aftershock that day.

Following the Canterbury quakes, which began five years earlier, Potter and colleagues found some people didn't like getting the forecasts, instead just wanting to get on with life and take things as they came.

Aftershock probabilities that were issued immediately after the Kaikoura Earthquake on November 14, 2016. Image / GNS Science

"But for others, the more information there is on what is happening and what to expect, the better," she said.

"It's particularly important for response and recovery agencies to help inform their decisions. So it's important that GNS Science communicates these forecasts.

"In order to make these forecasts as useful and effective as possible, we conduct social science research on how people perceive and use the information."

In a new study, scientists looked at people's experiences amid a rare "doublet" of quakes that rocked the Cook Strait area in 2013 – the 6.5 Seddon Earthquake of July 21, and the 6.6 Lake Grassmere of August 16 – which together caused more than $7 million in damage.

It was after those quakes that scientists added something called an anchoring window to their forecasts, to give people more certainty.

Whereas the overall forecast after the July 21 shake estimated a 30 per cent chance of another quake greater than 6.0 occurring within a week, its anchoring window was 24 hours, when there was a 20 per cent chance of an aftershock occurring in that size range.

"Often, people use these probability forecasts to make decisions, and to work out when to make these decisions, such as getting an earthquake kit, or securing their house," said the study's lead author, Dr Emma Hudson-Doyle, of Massey University and GNS Science's Joint Centre for Disaster Research.

"We noticed in other research, that even if you said the aftershock could happen within the next week, people would see the chance of it happening 'today' or 'tomorrow' as less than the end of the week, even though that isn't what we are trying to communicate.

"People 'discount' the risk in the nearest times and delay getting prepared or taking actions."

By surveying around 200 people who were in the south Wellington area at the time, the researchers found the added anchoring window helped people see the risk "today" or "tomorrow" more accurately.

"We still need to do more research to look at how best to write and use these anchoring windows, but these initial findings are very promising and they could help encourage people to take prompt preparedness actions."

Kurt Flowerday picks through rubble in Seddon near Blenheim following the M6.5 earthquake that struck near the Cook Strait, July 21, 2013. Photo / Tim Cuff

Their investigations also turned up fascinating insights into how people's personal experiences of quakes can skew their own aftershock expectations.

Those who felt the shaking of the first event, or who responded with fear or nervousness, were more likely to expect another one.

Gender was also found to play a part - women were more concerned over the issued forecasts, and similarly saw higher chances of aftershocks – although this trend wasn't so clear in global studies.

"Our previous research found [females] took more preparedness action after these earthquakes," Hudson-Doyle said.

"There are a few reasons in the literature as to why this might be happening. One is that females often have larger care roles in families, and so the responsibilities associated with that role means they are more concerned about the risk and taking actions.

"Other reasons are that there may be a gender difference in what risks are perceived as concerning."

Yet another possible explanation was that, as men have had more privileged positions, they could have lower expectations of bad outcomes.

"Given how family structures have changed over recent decades, I think it would be interesting to instead ask people more specifically about their care roles and responsibilities when we ask them about how likely they think an earthquake is, and when - or if they'll prepare," Hudson-Doyle said.

"We might then be able to see if people with more care roles have higher risk perceptions."

The researchers said, however, that the big take-away of their study was for agencies and journalists to take care in reporting aftershock forecasts.

That included using consistent language – a statement reading "30 per cent chance within the next week" could be perceived differently if "within" was swapped out for just "in" – and linking back to the source information.

"Our results contribute towards our understanding of how to best frame our forecasts at GNS Science and for similar agencies worldwide," Potter said.

"Whenever possible during significant geohazard events, we will put forecasts and other information relating to the event on the GeoNet website for everyone to see."