Earthquake research deemed problematic by Kiwi experts

The AM Show 10/10/2019

Kiwi experts are pouring cold water on a new system intended to determine whether a massive earthquake is the main event - or if it could be followed by an even larger shake

On Thursday, researchers from Switzerland announced a new "traffic light classification system" they believe could be used to indicate the probability of an aftershock striking with a larger magnitude than the earthquake it is following. 

They believe this can be done by looking at the ratio of small to large shakes following an earthquake with a magnitude 6 or higher. From the seismic activity of the aftershock sequence, the scientists derived what they call a "bvalue".

If only smaller earthquakes occur after the first shake, the bvalue increases over time, but if the bvalue decreases, they say a larger earthquake is still to come. They based this upon data from earthquakes in Italy and Japan in 2016.

They applied their theory to 58 historical earthquake sequences and were able to predict whether the shakes were the main event or aftershocks 95 percent of the time.

The system would look at the bvalue following quakes in real time to determine if a larger one is coming. 

However, the researchers admitted more data was needed to understand the full implications of the study and the value of the system.

GNS Seismologist John Ristau agreed, saying the system wasn't based on enough data and couldn't immediately be used in New Zealand.

The problem that we have is that it is based only on two previous studies.

"So that is a very small data set to be using," he told Newshub.

"The reality is that large magnitude earthquakes are fairly rare so if you were waiting to have the data set of earthquakes like this in New Zealand, you'd be waiting a long time."

John Townend, a geophysics professor from Victoria University, said the retrospective nature of how the Swiss scientists came to their conclusion was also troublesome.

"Following every big earthquake, there are inevitably lots of aftershocks. The question that these researchers have been addressing is can we tell that we have had the biggest earthquake? Is there is anything larger to come?" he told The AM Show.

"That they have done by looking at earthquake sequences around the world, but they have done it retrospectively. At the moment, I think it would be very difficult to put this into practice on a routine basis because it needs to be customised to the different areas."

He said the public shouldn't rely on scientists to provide a warning before a big quake comes. 

"What scientists can't currently do is predict individual earthquakes. While they can talk about what is going to happen on average, on aggregate, they can talk about the sizes that are likely to come and which areas are likely to be affected, it is not yet possible and it may never be possible to talk specifically about individual earthquakes," Townend said.

Earthquakes will be a surprise when they occur.

"We can't hang our preparedness on the ability of scientists to tell us what is going to come."

"We can make assessments once an earthquake has occurred and we can tell you what is likely in terms of aftershocks and so on. But at the moment it would be a real fool's game to try and predict the exact location and the impact of an earthquake anywhere in the country."

Townend said it was also important to note that even if scientists could predict an upcoming earthquake's magnitude, that isn't the only important factor to a quake. The impact an earthquake can have is also affected by its depth and location relative to large populations. 

The full study was published in Nature.

The AM Show / Newshub reporter Jamie Ensor