Seismic velocity changes associated with a flash flood crisis as a proxy for slope instability

The ground beneath our feet is rarely silent. Even when no earthquake is shaking the surface, the Earth hums with a constant, low-level vibration caused by the ocean waves crashing against coastlines, the wind rustling through trees, and the daily rhythm of human activity. For decades, seismologists have listened to this background hum, known as ambient noise, to map the deep structure of the planet. More recently, scientists have realized that this hum can also act as a sensitive thermometer for the Earth's crust, revealing subtle changes in how fast seismic waves travel through the soil and rock. When the ground becomes wetter, more fractured, or stressed, the speed of these waves shifts slightly. By tracking these tiny speed changes over time, researchers can potentially "listen" to the health of a slope, detecting when water is saturating the earth and altering the mechanical state of the ground.

In September 2022, a catastrophic storm struck the Marche region in central Italy, dumping an immense amount of rain over a short period and triggering thousands of landslides and severe flash floods. This disaster provided a rare, real-world laboratory for a team of researchers led by Andrea Albano and Mauro Palo at the University of Naples Federico II. They wanted to see if the subtle shifts in seismic wave speed could serve as a proxy for hydro-geological crises. By analyzing the continuous background noise recorded by a network of seismometers across the region, the team was able to track how the ground responded to the deluge, offering a new way to monitor slope stability that does not require drilling holes or installing sensors directly on dangerous hillsides.

The researchers focused their attention on the period surrounding the storm, which began on September 15, 2022. They used a technique called seismic interferometry, which essentially treats the constant background noise as a signal to measure the travel time of waves between different seismometer stations. By comparing these travel times day by day, they could calculate the relative change in seismic velocity, a value that indicates whether the ground has become softer or more stressed. The study covered a specific area of about 550 square kilometers that had been identified as the zone with the highest density of landslides following the storm. This area was centered on the mountainous terrain where the rainfall was most intense, particularly in the basins of the Metauro, Cesano, Misa, and Esino rivers.

The results revealed a clear and dramatic pattern. Roughly two days after the peak of the rainfall, the seismic waves traveling through the landslide-affected area began to slow down significantly. The velocity dropped by about 0.2 percent, a change that extended down to a depth of approximately one kilometer. This slowdown was not immediate; it took time for the water to infiltrate the ground and alter the pressure within the pores of the rock and soil. The anomaly was most pronounced in the central part of the affected zone, right where the rainfall had been heaviest and where the majority of the landslides occurred. Over the next five days, this area of slowed seismic waves expanded, eventually covering nearly 40 percent of the landslide-prone region, before gradually returning to normal levels as the ground stabilized.

This spatio-temporal behavior provided strong evidence that the velocity drop was driven by the movement of water deep underground. The researchers calculated that the water pressure was diffusing through the ground at a rate consistent with the local geology, moving downward and outward from the surface. Crucially, the team ruled out other common causes for such changes. They analyzed atmospheric pressure and temperature fluctuations, which are known to sometimes influence seismic speeds, but found that these factors were too small to explain the magnitude of the observed drop. They also noted that the velocity changes were localized to the area of heavy rain and landslides, whereas atmospheric pressure changes were uniform across the entire region. This confirmed that the signal was a direct response to the hydro-geological crisis, not a regional weather effect.

The study suggests that monitoring these relative velocity changes could be a powerful tool for assessing slope instability. Unlike traditional methods that rely on a limited number of rain gauges or piezometers, which only measure conditions at specific points, seismic velocity analysis offers a continuous, region-wide view of the ground's mechanical state. The researchers found that the fraction of the area experiencing a significant velocity drop could serve as a proxy for the overall fragility of the slope. In this specific event, the velocity drop emerged roughly two days after the rain peaked and persisted for several days, correlating with the period of intense saturation and instability.

While the study does not claim that seismic velocity changes can predict the exact moment a specific landslide will occur, nor does it imply a direct causal link between the velocity changes and individual landslide events, it demonstrates that these measurements provide a valuable, non-invasive window into the subsurface conditions that lead to failure. The method relies entirely on existing seismic networks, which are often already in place for earthquake monitoring, making it a cost-effective and sustainable addition to current hazard assessment frameworks. By listening to the subtle slowing of the Earth's hum, scientists may soon be able to detect the signs of slope instability, offering communities more time to prepare for the dangers of flash floods and landslides in an era of increasingly intense weather events.