New research reveals the chances of the South Island’s Alpine Fault generating a damaging earthquake in the next 50 years are much higher than previously thought.
A study led by Te Herenga Waka—Victoria University of Wellington senior lecturer Dr Jamie Howarth shows the probability of that earthquake occurring before 2068 is about 75 percent. Until now, it had been thought to be about 30 percent, based on sequences of sediment deposited adjacent to the Alpine Fault in northern Fiordland.
Scientists from Victoria University of Wellington, the University of Otago, GNS Science, the University of California, and the United States Geological Survey also calculated there is about an 82 percent chance the earthquake will be of magnitude 8 or higher.
The study, funded by EQC and the Rutherford Foundation, has now been published in Nature Geoscience. EQC funds a range of research on natural hazards that helps to better understand New Zealand’s risks.
The researchers studied evidence of 20 previous Alpine Fault earthquakes recorded in sediments in four West Coast lakes (Kaniere, Mapourika, Paringa and Ellery) and two swamps over the past 4,000 years to build one of the most complete earthquake records of its kind in the world.
“Detailed analysis and careful dating of the sediments in lakes that lie adjacent to the Alpine Fault have led to new insights into the distribution of earthquakes along the fault,” says Professor Sean Fitzsimons, a member of the research team from the University of Otago.
One of the researchers’ findings is that a curious “earthquake gate” on the fault south of Jackson Bay, near Martyr River, appears to determine how large an Alpine Fault earthquake gets. Some ruptures stop at the gate, producing “major” earthquakes in the magnitude 7 range; but the ruptures the gate lets through grow into “great” earthquakes of magnitude 8 or more. These passing or stopping ruptures tend to occur in sequences, producing phases of major or great earthquakes through time.
The Alpine Fault is a more than 850km-long split in the Earth’s crust marking where the Australian and Pacific crustal plates meet and grind against each other, forcing up the Southern Alps.
Dr Howarth, from Victoria University of Wellington’s School of Geography, Environment and Earth Sciences, says this earthquake gate may help with forecasting the size of future Alpine Fault earthquakes.
“From the record of past earthquakes, we can determine that the probability of a magnitude 7 or higher event is about 75 percent in the next 50 years. So we now know the chances of seeing a large Alpine Fault earthquake in our lifetime are better than a coin toss. That is a really significant result but we can’t forecast the magnitude of the next event from these data alone.”
Dr Howarth says the Alpine Fault earthquake record shows the past three earthquakes ruptured through the gate, producing great (magnitude 8 or higher) earthquakes. “Our modelling shows that if you’ve had a run of three passing ruptures, then the next one will also likely pass through the gate. We are therefore expecting the next earthquake to be similar to the last one in 1717, an estimated magnitude 8.1, which ruptured about 380 kilometres of the fault.”
The best forecast is there is an 82 percent probability the next such earthquake will pass through the gate and cause an earthquake of magnitude 8 or higher.
“This finding doesn’t change the fact the Alpine Fault has always been hazardous,” says Dr Howarth. “But now we can say the next earthquake will likely happen in most of our lifetimes. We need to move beyond planning the immediate response to the next event, which has been done well through the AF8 programme, to thinking about how we make decisions about future investment to improve our infrastructure and community preparedness.”
From space, the fault appears like a straight line on the western side of the Southern Alps. But in reality it has variations in geometry and slip rate and is split up into different segments, says Assistant Professor Nicolas Barth, one of the researchers from the University of California, Riverside. The earthquake gate at Martyr River appears to be due to a slight change in direction of the fault and the angle it is dipping below the surface, he says.
The researchers took a physics-based model of how earthquakes behave and applied it to the Alpine Fault, testing it against the paleoseismic data.
“If you just run the model on a fault with no geometric complexity, then you just get through-going earthquakes, so they are just magnitude 8 all the time,” says Assistant Professor Barth. “But when you actually include all the geometric complexity that occurs here, then you get the behaviour we have observed, with phases of passing and terminating ruptures.”
“The fact that, as a science, we might be getting to the point where we can actually use physics-based models to do forecasting is really interesting,” says Dr Howarth. “This is the first time there has been a paleoseismic data set that spans multiple large earthquakes/seismic cycles of sufficient quality to allow us to evaluate how such models behave.”