Natural hazards don’t disappear when the storm ends or the earthquake stops – they evolve

Hurricane Helene lasted only a few days in September 2024, but it altered the landscape of the Southeastern U.S. in profound ways that will affect the hazards local residents face far into the future.

Mudslides buried roads and reshaped river channels. Uprooted trees left soil on hillslopes exposed to the elements. Sediment that washed into rivers changed how water flows through the landscape, leaving some areas more prone to flooding and erosion.

Helene was a powerful reminder that natural hazards don’t disappear when the skies clear – they evolve.

These transformations are part of what scientists call cascading hazards. They occur when one natural event alters the landscape in ways that lead to future hazards. A landslide triggered by a storm might clog a river, leading to downstream flooding months or years later. A wildfire can alter the soil and vegetation, setting the stage for debris flows with the next rainstorm.

I study these disasters as a geomorphologist. In a new paper in the journal Science, I and a team of scientists from 18 universities and the U.S. Geological Survey explain why hazard models – used to help communities prepare for disasters – can’t just rely on the past. Instead, they need to be nimble enough to forecast how hazards evolve in real time.

The science behind cascading hazards

Cascading hazards aren’t random. They emerge from physical processes that operate continuously across the landscape – sediment movement, weathering, erosion. Together, the atmosphere, biosphere and the earth are constantly reshaping the conditions that cause natural disasters.

For instance, earthquakes fracture rock and shake loose soil. Even if landslides don’t occur during the quake itself, the ground may be weakened, leaving it primed for failure during later rainstorms.

That’s exactly what happened after the 2008 earthquake in Sichuan Province, China, which led to a surge in debris flows long after the initial seismic event.

Earth’s surface retains a “memory” of these events. Sediment disturbed in an earthquake, wildfire or severe storm will move downslope over years or even decades, reshaping the landscape as it goes.

The 1950 Assam earthquake in India is a striking example: It triggered thousands of landslides. The sediment from these landslides gradually moved through the river system, eventually causing flooding and changing river channels in Bangladesh some 20 years later.

An intensifying threat in a changing world

These risks present challenges for everything from emergency planning to home insurance. After repeated wildfire-mudslide combinations in California, some insurers pulled out of the state entirely, citing mounting risks and rising costs among the reasons.

Cascading hazards are not new, but their impact is intensifying.

Climate change is increasing the frequency and severity of wildfires, storms and extreme rainfall. At the same time, urban development continues to expand into steep, hazard-prone terrain, exposing more people and infrastructure to evolving risks.

The rising risk of interconnected climate disasters like these is overwhelming systems built for isolated events.

Yet climate change is only part of the equation. Earth processes – such as earthquakes and volcanic eruptions – also trigger cascading hazards, often with long-lasting effects.

Mount St. Helens is a powerful example: More than four decades after its eruption in 1980, the U.S. Army Corps of Engineers continues to manage ash and sediment from the eruption to keep it from filling river channels in ways that could increase the flood risk in downstream communities.

Rethinking risk and building resilience

Traditionally, insurance companies and disaster managers have estimated hazard risk by looking at past events.

But when the landscape has changed, the past may no longer be a reliable guide to the future. To address this, computer models based on the physics of how these events work are needed to help forecast hazard evolution in real time, much like weather models update with new atmospheric data.

Thanks to advances in Earth observation technology, such as satellite imagery, drone and lidar, which is similar to radar but uses light, scientists can now track how hillslopes, rivers and vegetation change after disasters. These observations can feed into geomorphic models that simulate how loosened sediment moves and where hazards are likely to emerge next.

Researchers are already coupling weather forecasts with post-wildfire debris flow models. Other models simulate how sediment pulses travel through river networks.

Cascading hazards reveal that Earth’s surface is not a passive backdrop, but an active, evolving system. Each event reshapes the stage for the next.

Understanding these connections is critical for building resilience so communities can withstand future storms, earthquakes and the problems created by debris flows. Better forecasts can inform building codes, guide infrastructure design and improve how risk is priced and managed. They can help communities anticipate long-term threats and adapt before the next disaster strikes.

Most importantly, they challenge everyone to think beyond the immediate aftermath of a disaster – and to recognize the slow, quiet transformations that build toward the next.

This article is republished from The Conversation, a nonprofit, independent news organization bringing you facts and trustworthy analysis to help you make sense of our complex world. It was written by: Brian J. Yanites, Indiana University

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Brian J. Yanites receives funding from the National Science Foundation.