Exceptional rainfall saturates the subsoil: what could the perfect storm affecting Andalusia mean?

BarcelonaStorms and heavy rain continue to batter the Iberian Peninsula this winter, and this week Andalusia is the epicenter of the storms. Provinces like Seville, Cádiz, and Córdoba are the hardest hit by the abundant rainfall caused by Storm [name missing]. Leonardowith exceptional rainfall totals. The worst affected area was the municipality of Grazalema, in Cádiz, where almost 600 liters per square meter fell in just 24 hours on Wednesday. This record figure for the area is very similar to the rainfall recorded during the historic and tragic DANA storm of October 2014 in Valencia. And all that water is also having consequences for the subsoil. Grazalema is located at an altitude of over 800 meters in the Sierra de Cádiz mountains, a geographically exposed area that is very rainy and receives nearly 2,000 liters per square meter annually. The problem is that this amount of rain, which usually falls over an entire year, has already been exceeded between January and the first days of February. This exceptional rainfall has completely saturated the subsoil. The ground can no longer absorb any more water, which is gushing out uncontrollably everywhere, even through electrical outlets, toilets, and walls of houses. And a phenomenon little known until now has occurred: hydroseisms.

A hydroseism is a small earthquake caused by the pressure of a large amount of water accumulated in the ground. It's worth remembering that faults run through Andalusia, making seismic activity in the area more frequent. According to the National Geographic Institute (IGN), several earthquakes occurred in the area during the recent heavy rains, none reaching magnitude 2.5 on the Richter scale. "The overpressure from so much water caused by the rains can be channeled through the fault in the area, causing it to shift and triggering these small earthquakes of just over magnitude 2," explained Roberto Espínola, a member of the College of Geologists of Catalonia, in statements to ARA.

"Hydroseism is the consequence of a massive influx of water into the subsoil, which alters and disrupts the most common natural balances. All of this generates overpressure that must be released, and one way to do so is by inducing the movement of these small faults," the expert states.

But why does this phenomenon occur precisely in this area? According to Espínola, beyond the faults that run through the subsoil of Andalusia, it's worth remembering that Grazalema is located in a massif formed by many fragile calcareous materials, which results in high porosity, with cracks and fractures in the rocks beneath the surface. Under normal conditions, this allows water to be absorbed more easily. The problem is that so much rain causes saturation and pressure in the ground. "It's like a kind of sponge that can't absorb any more water, which literally comes out wherever it can, using any hole as a drain," he explains.

According to Espínola, a column of water about ten meters high generates a pressure of one kilogram per square centimeter. "Imagine an area becoming saturated with a lot of water and generating so much pressure in the subsoil; this can destabilize the ground and cause many problems in infrastructure and buildings," he says. In this regard, Espínola points out that hydroseisms themselves are small and it is very unlikely that they will cause problems on their own. It is the saturation of so much water that can cause landslides or damage to the foundations of houses and bridges, among other things.

Could this happen in Catalonia?

Catalonia has many limestone massifs similar to the Sierra de Cádiz, such as the Garraf Massif, and even the Pyrenees, among others. Therefore, in the event of an exceptional rainstorm like this one, water would also gush out everywhere, and the possibility of a hydroseismic event could not be ruled out, although seismic activity here is not as pronounced as in Andalusia. "If we were to have episodes of such unusually heavy rainfall as in Grazalema, theoretically, a landslide like this one in Catalonia cannot be ruled out," admits Espínola.

More rain is coming

What is happening in Andalusia is yet another extreme weather event affecting the entire Mediterranean region lately due to the climate crisis. "These episodes, occurring with such unusual intensity and frequency, are linked to climate change and the disruption of natural balances; nature responds, and now we are suffering the consequences," Espínola states. The expert affirms that droughts as extreme as those of recent years also make the ground more impermeable than normal, making it harder for it to absorb water in the event of sudden heavy rainfall. "The water can rise to the surface more quickly, causing flash floods," he concludes.

The weather forecast for the coming days is not very promising for the area. Another high-impact storm is expected to arrive in the Iberian Peninsula over the weekend. This is Storm... MartenThe storm will bring more heavy rain, especially to Andalusia on Saturday. While the totals shouldn't be as extraordinary as in recent days, it's still a case of adding insult to injury. This weather pattern is expected to continue into next week.