Forecasting of earthquake-induced floods – Eos

Today, Reelfoot Lake in northwest Tennessee is a quiet place and a haven for migratory birds. But the 15,000-acre water was born violently and surprisingly: After a series of earthquakes in 1811 and 1812 temporarily diverted the flow of the Mississippi River, water flooded the landscape and pooled to form the lake.

Historical examples of rivers suddenly changing course in response to surface-shocking earthquakes abound, and now a team of researchers has modeled these events to better assess the risks they pose. Their results were published in May in Science Advances.

sudden ataxia

Rivers change course all the time. Erosion and sediment deposition causes those course corrections – known as castoffs – gradually, over decades or even centuries. But the river inversions that Erin McEwan, a tectonic geomorphologist at the University of Canterbury in Christchurch, New Zealand, and colleagues recently studied, are starting to move much faster.

When a fault oscillates and shifts the Earth’s surface either horizontally or vertically, the resulting fault deflection can divert the river which occurs to cut through the area. “The fault collapse presents an immediate hurdle,” said Timothy Stahl, a geologist at the University of Canterbury and a member of the research team. Obviously, the river water still has to go somewhere, McEwan said. “It may pool against the fault of the rift and then flow out of its channel.”

Dozens of cases of river rifting collapse have been recorded over the past few hundreds of years. And the potential for future events is significant: Worldwide, there are more than 25,000 places where rivers flow over active faults. Adding people into the mix, McEwan said, makes the situation precarious. “Anywhere [there are] Very active faults, large populations, rivers covering those faults, there’s going to be a problem. “

Lake in farmland

McEwan and colleagues conducted a case study of an event that occurred in New Zealand after midnight on November 14, 2016, with the aim of predicting the characteristics of river contractions caused by rift rupture.

When a 7.8-magnitude earthquake struck the country’s South Island, the Papatia Fault suddenly moved about 7 meters vertically and 4 meters horizontally where the Wayau Tua/Clarence River intersected. As the river water – flowing at 200 cubic meters per second – encountered this crack, he had to change direction by about 45 degrees. It seeped from its current channel and tracked for more than a kilometer across farmland.

These floodwaters continued to form Lake Murray, which submerged over 80 acres for several months. Even today, more than 6 years later, parts of the Waiau Toa/Clarence River flow hundreds of yards from their ancient channels, McEwan said. “There was a real change of scenery.”

It is impossible to predict the exact parameters of an earthquake or how a river will flow at any given moment.

McEwan and colleagues modeled the Waiau Toa/Clarence River event. First, they tested whether they could accurately reproduce the river’s flow extent using river discharge data obtained just one hour before the event and a digital elevation model of the post-earthquake landscape compiled from lidar images. The researchers found that their 2D simulation was 94% accurate compared to real-world observations of the river’s fluctuations.

But the most useful test was whether the river collapse could be accurately reproduced using only pre-event data. To this end, the researchers created a digital elevation model for the landscape based on lidar data collected in 2012, and modified it using an artificial fault that was notified by the movement of the real fault. When they fed these inputs into their simulation, they reproduced ejaculations with 89% accuracy.

Finally, the researchers simulated a range of potential flood scenarios by looking at five different fault fault displacements and five different water discharge rates. The approach made sense, said Killian Dascher-Cousino, an earth scientist at the University of California, Berkeley, who was not involved in the research. He said it was impossible to predict the exact parameters of an earthquake or how a river would flow at any given moment. “You need to deal with this through a probabilistic process.”

revolution 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 course through the landscape. Not surprisingly, the team found, larger fault displacements and higher discharge rates lead to more flooding.

The researchers also found that in some cases, the displacement of the fault merely set the landscape for future fluctuation, which would occur later, when the river’s flow rate increased. Extraction does not necessarily follow an earthquake, Stahl said. “It can be immediate or delayed.”

McEwan said similar models could be used to assess the potential vulnerabilities of other earthquake-prone rivers. “That’s what we’ll do in places where this hasn’t happened yet and try to assess the risks.” McEwan said the findings could inform risk planning and legislation around mandatory construction setbacks from fault lines.

—Katherine Kornei, science writer

Citation: Kornei, K. (2023), Forecasting Earthquake-induced Floods, Eos, 104, https://doi.org/10.1029/2023EO230226. Posted on June 12, 2023. Text © 2023. Authors. CC BY-NC-ND 3.0 Unless otherwise noted, images are subject to copyright. Any reuse without the express permission of the copyright owner is prohibited.

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