Compound earthquakes-floods produce non‑additive urban recovery driven by financing bottlenecks

When a city faces a natural disaster, the clock starts ticking on its recovery. For decades, planners and engineers have tried to predict how long it would take for a community to bounce back after an earthquake or a flood. The traditional way of thinking was to treat these events as separate incidents. If a city gets hit by a flood, you calculate the damage and the time needed to fix it. If it gets hit by an earthquake later, you do the same calculation again. The total recovery time was assumed to be simply the sum of the two separate events. This approach works well when disasters happen far apart in time, allowing a city to reset its resources, finish its paperwork, and hire its workers before the next crisis strikes. However, the world is changing. Climate change and rapid urbanization are making it more common for different types of disasters to strike the same place in quick succession. When a flood follows an earthquake, or vice versa, the situation becomes far more complex than just adding two numbers together. The real challenge lies not just in the physical damage, but in the human and financial systems that try to repair it.

A team of researchers has developed a new way to look at this problem, focusing on what happens when two distinct hazards hit a city in rapid succession. They studied the city of Pompei in Italy, a place known for its history but also for its exposure to both earthquakes and floods. The researchers built a detailed computer simulation of the city's 3,282 residential buildings, categorizing them by their construction materials, age, and height. They then ran thousands of scenarios where an earthquake was followed by a flood, or a flood was followed by an earthquake, with varying amounts of time between the two events. The goal was to see how the recovery process for the first disaster interfered with the recovery for the second. They found that the old method of simply adding up the recovery times was dangerously optimistic. When the two events happened close together, the city took significantly longer to recover than the sum of the two individual disasters would suggest.

The key to this delay is not always the physical damage itself, but the bottlenecks that form in the recovery process. After a disaster, a building cannot be repaired immediately. There are several steps that must happen first: inspectors must visit the site, permits must be issued, and most importantly, funding must be secured. In Italy, this funding often comes from government loans and grants. The researchers discovered that when a second disaster strikes before the first recovery is finished, these administrative and financial steps get clogged. Imagine a city where thousands of homeowners are already waiting in line for government money to fix their earthquake damage. If a flood hits six months later, those same homeowners now need more money for flood damage, and the insurance companies and government agencies are already overwhelmed. The demand for funds spikes, and the waiting time for everyone stretches out. This creates a domino effect where the delay for the second disaster is much longer than it would have been on its own.

In their simulations, the researchers tested different time gaps between the disasters. When the gap was short, such as six months, the interaction between the two recovery processes was severe. They found that ignoring these interactions led to an underestimation of the total recovery time by up to 50 percent. In some scenarios, the city took nearly twice as long to recover as the simple math predicted. The most significant driver of this delay was the financing bottleneck. While the availability of construction workers also played a role, the researchers found that the struggle to get money was the dominant factor. Even if the physical damage from the second event was different from the first—for example, an earthquake cracking walls while floodwater ruins floors—the financial systems treating them were the same. The system could not handle the double load, and the recovery stalled.

The study also looked at what happens if the time between disasters is longer. When the researchers extended the gap to one year, the negative effects of the interaction dropped significantly. This is because, after a year, most of the buildings damaged by the first event had already secured their funding and were well into the repair process. The financial system had cleared its backlog, so the second disaster did not face the same level of congestion. This suggests that the timing of disasters is just as critical as their severity. A city might be resilient enough to handle a flood and an earthquake separately, but if they arrive too close together, the administrative machinery breaks down. The researchers emphasized that their findings are based on simulations using data from past Italian disasters, such as the 2009 L'Aquila earthquake and the 2023 Emilia-Romagna floods. While the specific numbers might vary in other cities, the mechanism of the delay is likely universal.

The implications of this work are profound for how cities plan for the future. Current risk models often treat multi-hazard events as simple superpositions, assuming that the total impact is just the sum of the parts. This new research shows that this assumption is flawed. By failing to account for the interactions between recovery processes, planners are systematically underestimating how long it will take for a community to return to normal. The study suggests that to build true resilience, cities need to prepare for these compound events by strengthening their financial and administrative systems. This could mean creating faster ways to disburse emergency funds or setting aside specific resources for scenarios where disasters overlap. The researchers hope that their framework will help decision-makers incorporate these complex interactions into their planning, ensuring that when the next double disaster strikes, the city is not just ready to fix the damage, but ready to navigate the bottleneck that follows.

The study focused on Pompei, but the lessons apply to any urban area facing multiple risks. The researchers did not claim to have solved the problem of disaster recovery, but they provided a clearer map of the obstacles. They showed that the path to recovery is not a straight line, but a winding road that can be blocked by the very systems meant to help. By understanding that financing delays are the primary driver of these extended recovery times, cities can begin to design better solutions. The work serves as a reminder that in the face of compound disasters, the physical damage is only half the story. The other half is the human system trying to repair it, and that system has limits. When those limits are tested by two disasters in quick succession, the recovery takes much longer than anyone expected.