Common rocks may hold the secret to forecasting future earthquakes

Our ability to forecast the likelihood of future earthquakes in New Zealand is becoming more accurate.

The secrets lie in a rock many of us have in our backyard - which may make you never look at these rocks in quite the same way again.

Greywacke is everywhere in New Zealand and most of us wouldn't take a second glance.

But for Victoria University geography lecturer Dr Carolyn Boulton, it's a rock that has important secrets to reveal. 

"It's these rocks that start the earthquakes that we've been feeling lately," Dr Boulton says.

That's because an earthquake is caused by a slip on a fault, and some of its impact is determined by the types of rocks in the fault.

"Earthquakes in greywacke tend to start on a single point at depth but as they come up to the surface they tend to branch out and use lots of pre-existing plains of weakness in the rock," Dr Boulton says.

It's those weaknesses that caused a record 22 different faults to rupture at the surface during the 7.8 magnitude Kaikoura earthquake in 2016.

"It's kind of grey, it's kind of monotonous and it's also very strongly deformed, meaning it's hard to work with," Dr Boulton says.

It's made up of a mixture of sandstone and shale and for years people have ignored it - until now.

The Earthquake Commission (EQC) has funded research to correct the gap in knowledge. 

"It's got some real underpinning science and info that will inform us how faults behave and occur," EQC research manager Natalie Balfour says.

Researchers put greywacke into machines to replicate earthquakes. 

"I measure things like how strong the rocks are, what temperature the earthquakes happen at and actually how fast the rocks move," Dr Boulton says.

This helps to explain why New Zealand has multi-fault earthquakes, and how a quake in one region can have a knock-on effect. 

"When one fault ruptures it places an added stress on another fault and we need to understand how those stress interactions accelerate the timing of the next earthquake or decelerate," Dr Boulton says.

Dr Boulton's research has helped validate other data we already have of Wellington quakes, helping scientists to understand why they're happening and where.

We will never be able to predict exactly when earthquakes will happen, but this information will help us better plan for them including for people who live near faults, land use and better buildings.  

So, look again at that boring, grey rock - it's an important piece of the puzzle that makes up New Zealand's complex fault lines.