Today, Reelfoot Lake in northwestern Tennessee is a tranquil place and a haven for migratory birds. But the 15,000-acre body of water was birthed violently and abruptly: After a series of earthquakes in 1811 and 1812 temporarily diverted the flow of the Mississippi River, water inundated the landscape and pooled, forming the lake.
Historical examples abound of rivers suddenly changing course in response to surface-rupturing earthquakes, and now a team of researchers has modeled these events to better assess the hazards they pose. Their results were published in May in Science Advances.
A Sudden Lurch
Rivers change course all the time. Erosion and sediment deposition drive those course corrections—known as avulsions—gradually, over decades or even centuries. But the river avulsions that Erin McEwan, a tectonic geomorphologist at the University of Canterbury in Christchurch, New Zealand, and her colleagues recently studied were set in motion much, much faster.
When a fault lurches and displaces Earth’s surface either horizontally or vertically, the resulting fault scarp can divert a river that happens to crosscut the region. “The fault scarp presents an immediate obstacle,” said Timothy Stahl, a geologist also at the University of Canterbury and a member of the research team. A river’s water obviously still has to go somewhere, McEwan said. “It may pool up against the fault scarp and then flow outside of its channel.”
Tens of fault rupture–induced river avulsions have been recorded over the past few hundred years. And the potential for future events is significant: Worldwide, there are more than 25,000 places where rivers flow over active faults. Add people to the mix, and that makes for a risky situation, McEwan said. “Anywhere [there are] highly active faults, large populations, and rivers overlaying those faults, there’s going to be an issue.”
A Lake in Farmland
With the goal of forecasting the characteristics of fault rupture–induced river avulsions, McEwan and her colleagues conducted a case study of an event that occurred in New Zealand just after midnight on 14 November 2016.
When a magnitude 7.8 earthquake struck the country’s South Island, the Papatea fault suddenly moved roughly 7 meters vertically and 4 meters horizontally where it intersected the Waiau Toa/Clarence River. As the river’s water—flowing at nearly 200 cubic meters per second—encountered that fault scarp, it was forced to change direction by about 45°. It spilled out of its existing channel and coursed for more than a kilometer across farmland.
Those floodwaters went on to form Lake Murray, which inundated more than 80 acres for several months. Even today, more than 6 years later, parts of the Waiau Toa/Clarence River are flowing hundreds of meters from their old channels, McEwan said. “There’s been a real change in the landscape.”
McEwan and her colleagues modeled the Waiau Toa/Clarence River event. First, they tested whether they could accurately reproduce the extent of the river avulsion using river discharge data obtained just an hour before the event and a digital elevation model of the postearthquake landscape assembled from lidar imagery. The researchers found that their 2D simulation was 94% accurate compared with real-world observations of the river avulsion.
But a more useful test was whether the river avulsion could be accurately reproduced using only pre-event data. To that end, the researchers created a digital elevation model of the landscape based on lidar data collected in 2012 and modified it using a synthetic fault scarp informed by the real fault’s movement. When they fed those inputs into their simulation, they reproduced the avulsion with 89% accuracy.
Finally, the researchers simulated a range of possible flooding scenarios by considering five different fault scarp displacements and five different water discharge rates. That approach made sense, said Kélian Dascher-Cousineau, an Earth scientist at the University of California, Berkeley who was not involved in the research; it’s impossible to prognosticate the exact parameters of an earthquake or how a river will be flowing at any instant, he said. “You need to approach this from a probabilistic process.”
An Avulsion in the Future
Of the 25 modeled scenarios the researchers produced, in seven cases the Waiau Toa/Clarence River completely abandoned its parent channel and carved a new path through the landscape. Not surprisingly, larger fault displacements and higher discharge rates tended to produce more inundation, the team found.
The researchers also found that in some instances, fault displacement merely primed the landscape for a future avulsion, which would occur later, when the river’s flow rate increased. An avulsion doesn’t necessarily follow in lockstep with an earthquake, Stahl said. “It could be instantaneous or delayed.”
Similar modeling could be used to assess the potential vulnerabilities of other earthquake-prone rivers, McEwan said. “That’s what we’d do in places where this hasn’t occurred yet and we’re trying to assess the risk.” The results could inform hazard planning and legislation around mandatory building setbacks from fault lines, McEwan explained.
—Katherine Kornei (@KatherineKornei), Science Writer