The straw that doesn't break the camel's back
Victoria University of Wellington Scientist trawls through 10 years of seismic data to gain insight into how earthquakes develop.
Postdoctoral research fellow Dr Calum Chamberlain from Victoria’s School of Geography, Environment and Earth Sciences is trying to find out whether the speed at which a fault is loaded by tectonic stresses has any bearing on when and how earthquakes start.
“Some laboratory experiments have shown that the rate at which a fault is loaded has some effect on whether a large earthquake develops,” he explains. “I want to apply those laboratory findings to the real Earth, but until now it’s been difficult to measure how fast each fault is being loaded because they’re buried deep in the earth and we can’t access them.”
Dr Chamberlain will look at data from what are known as repeating earthquakes, which are the recurring ruptures of a sticky patch on the surface of a fault that is otherwise stably creeping.
“When the patch ruptures, the stress around it is released and the loading process begins again. The frequency at which the ruptures repeat and how big the earthquakes are tells us how fast the fault around them is sliding—if you can work that out then you should also be able to establish how quickly the part of the fault that might generate a large earthquake is being loaded.”
Dr Chamberlain says New Zealand has excellent earthquake data, which will allow him to calculate slip-rates and stressing-rates for faults that have not recently failed in large earthquakes.
“You need many years of data to be able to really look at changes in these kinds of repeating earthquakes,” he says. “Using super-computing facilities, I’ll be scouring more than a decade’s worth of seismic data—New Zealand has a fantastic seismic network thanks to GeoNet, which has been recording for long enough now that we can do this kind of study.”
Dr Chamberlain and his colleagues—Professor John Townend of Victoria and Dr Amanda Thomas of the University of Oregon—will incorporate the data in numerical models, which Dr Chamberlain says will allow the team to better understand the relationship between small earthquakes and large, locked patches of faults.
“Before the magnitude 9 earthquake in Japan in 2011—as well as some other recent large earthquakes in other parts of the world—changes were observed in repeating earthquakes which are thought to represent changes in slip-rate and stress loading-rate in the vicinity of the main earthquake,” he says. “That means we know these changes can be associated with large earthquakes, but we’ve seen similar changes in New Zealand and they haven’t been linked to large earthquakes. So I’ll be trying to work out what the difference is between stress changes that result in earthquakes and those that don’t.
“The main goal of this research is to try and work out what the lower limit on stressing rates affecting New Zealand’s faults is that doesn’t cause an earthquake—a bit like the straw that doesn’t quite break the camel’s back. I also hope this research will give us a better understanding of the physical processes that control the nucleation of earthquakes.”