- Vedrana Simičević
- BBC Future
It happened suddenly and without warning. Where the first potato shoots should have grown behind the orchard, in Nikola Borojević’s spacious garden, there was now a huge hole.
At 30 meters wide and 15 meters deep, it quickly filled with water. And he wasn’t the only one.
In the space of a few weeks, dozens of similar holes opened up around Mečenčani and Borojovići villages in northeastern Croatia.
The one outside Borojević’s home in Mečenčani appeared on January 5, just six days after a 6.4 magnitude earthquake struck the area around the nearby town of Petrinja.
It was the strongest earthquake to hit Croatia in more than four decades, killing seven people and destroying thousands of homes.
Although it is known that landslides and craters can be caused by earthquakes, as well as other strange geological phenomena such as liquefaction – when solid soil starts to behave like liquid – the large number of holes that appeared in villages surprised and intrigued people. experts.
A month after the earthquake, there were nearly 100 craters spread over an area of 10 km², with new holes opening every week.
The hole in Borojević’s garden is now the largest in the area. When it appeared, it was 10 meters wide, but it began to grow almost immediately.
“My wife stayed home all morning, looking out the window every now and then,” says Borojević. “Around 2:00 pm, she noticed something strange in the garden. We went out and there was a huge hole in our orchard.”
Over the next three months, the hole tripled in size. But the Borojevićs were lucky.
Other craters in the region have opened up a few meters from the entrance to residents’ homes, and one has appeared under a residence, prompting authorities to consider vacating both villages.
Other holes emerged in adjacent forests and farmland, where one, according to local rumors, nearly swallowed a farmer and his tractor.
The exceptionally high number of craters in a single location has caught the attention of local and foreign geologists, eager to understand how the earthquake might have led to the ground’s collapse.
“Nobody expected so many craters to appear,” says seismologist Josip Stipčević, from the Department of Geophysics at the Faculty of Science in Zagreb, the Croatian capital.
Intense seismic activity
Croatia is located in an area of intense seismic activity, where the small Adriatic tectonic plate collides with the Eurasian plate, causing a series of active faults, explains Stipčević.
Before the December 29, 2020 earthquake, the country had been hit by another nine tremors, with a magnitude greater than 6, since the beginning of the 20th century.
The last major earthquake that had happened on the Pokupsko-Petrinja tectonic fault—along which the most recent occurred—was in 1909.
The 1909 quake occurred just 23 km northwest of the epicenter of the earth-shattering one in late 2020. It also attracted the attention of seismologists at the time.
Renowned Croatian geophysicist Andrija Mohorovičić analyzed seismographs from the 1909 earthquake in Pokupsko and concluded that seismic waves travel at different speeds as they pass through different layers of the Earth.
His observation led to the discovery of an intermediate layer separating the earth’s crust from the mantle, known today as the Mohorovicic discontinuity, or simply Moho.
Currently, researchers are studying the same region in hopes of understanding how the earthquake caused so many holes to suddenly appear.
Craters are not the most common consequence of strong earthquakes, but they can arise, especially in areas with hidden underground cavities.
After the devastating earthquake near the Italian city of Aquila in 2009, two potholes immediately opened up in roads in the old town.
Experts at the time suspected that a previous excavation of vertical pits for a sewer channel had weakened the underground cavern cover, contributing to the collapse.
“The real anomaly in the case of Croatia is a very high number of craters with a significant dimension”, says Italian geologist Antonio Santo, from the University of Naples Federico II, in Italy.
The deep, wide holes that threaten the two Croatian villages often appear in areas where underground rock has been excavated by water to form cavities and caverns — and is covered by a thick layer of soil, sand or silt and, most importantly, clay.
Over time, water slowly carries surface material from its deeper layers into the cavernous subsoil.
If there were only sandy soil, this process could end up being detected at the surface.
But the presence of clay makes this surface material more resistant and consistent, so after a while a void forms below the ground but is practically undetectable above it.
As the surface layer becomes structurally weaker, it eventually collapses into the void below.
Typically, this process takes place over a long period of time, but can be accelerated by heavy rains, floods, or even human activities such as mining or aggressive groundwater pumping.
Strange events, complex factors
After analyzing data collected in the region around Mečenčani and Borojovići, Croatian geologists concluded that the strange events resulted from a complex combination of several different factors.
First, although the coastal part of Croatia belongs to the Dinaric Karst, which is home to thousands of deep limestone caves and hundreds of endemic cave species, underground limestone formations also extend inland beneath the country’s center.
The limestone that forms the rocky caves on the border between the Dinaric Karst and the Pannonian basin was deposited in the Miocene period, when this area was submerged and connected to what is today the Mediterranean Sea.
“Although the Dinaric karst is mainly from the Cretaceous and Jurassic periods, the karst we find here is younger and even more porous and hollow,” says Josip Terzić, hydrogeologist at the Croatian Geological Survey.
“It’s limited to a few small areas around here and near the city of Zagreb.”
When the Pannonian basin disconnected from the Mediterranean due to landmasses shifting about 11 million years ago, it became a large lake.
The rivers slowly filled it with silt, sand and gravel, forming the vast plains of the present day.
Consequently, about 10 to 15 meters of earth, stones and clay are deposited on top of the porous rock below the villages of Mečenčani and Borojevići.
The threat, however, was difficult to detect. Some sporadic craters have appeared before, but according to locals, very rarely.
“It is obvious that the earthquakes have accelerated some processes that are already underway,” says Terzić.
In fact, the first hole discovered began to open after a magnitude 5 quake hit the area a day before the biggest one.
As the stronger earthquake and the aftershocks that accompanied it shook the area, it caused the ground to shift more than a foot. This displacement destabilized the precarious situation.
“The earthquakes caused enormous dynamic stress to these lands and places that were already in a borderline equilibrium and suddenly collapsed,” says Terzić.
His colleague Bruno Tomljenović, a geophysicist at the University of Zagreb, believes the earthquakes affected the movement of water underground, causing it to rise towards the surface, moving from areas of high pressure to low pressure.
This increased hydrodynamics in the underground passages accelerated the collapse of surface material, explains Tomljenović.
“Furthermore, there is a chance that some craters that collapsed ’caused additional changes in hydrodynamics, causing the water to seek new passages and possibly causing more sinks,” says Tomljenović.
An exceptionally large number of quakes may also have contributed to so many cave-ins at the same time, says geologist George Veni, director of the National Caves and Cartes Research Institute in New Mexico, an area also known for crater problems, often caused by for activities related to industrial wells.
Human influence also increases the rate of sudden crater formation, warns Veni.
A recent report by scientists at the University of Zagreb states that irrigation systems built in the Mečenčani and Borojevići areas likely accelerated the karstification process.
For now, scientists don’t have enough data to analyze the connection between the intensity and number of earthquakes and the appearance of sinks.
“The situation in Croatia can be considered a warning of what can happen in countries with earthquakes and areas that are prone to sudden craters,” says Veni.
But predicting where holes like these might open is far from easy, says geologist Mario Parise, an expert on threats in karst environments at the University of Bari Aldo Moro in Italy.
“For the time being, we can only rely on historical data and documents to know the areas most susceptible to this type of process,” he says.
Although some sink risk models have been proposed in the last decade, “the development of a crater warning system is a field in which a lot of work needs to be done,” he adds.
For Tomljenović, one of the lessons of Croatian craters is the need for more intensive seismic microzoning to detect areas within populated regions that are especially vulnerable to the dangerous consequences of earthquakes.
He and his colleagues are trying to do this using electrical resistivity tomography and seismic refraction survey in the Mečenčani and Borojevići area, hoping to identify sites that can be considered safe, and those that are still subject to craters.
But the threat of new craters appearing next year is still on the minds of many residents.
Changes in the water table throughout the year, combined with new tremors as the tectonic fault settles, could lead to more collapses, according to Stipčević.
Meanwhile, the huge water-filled hole is still in Borojević’s garden — and has even become a local tourist attraction.
Six months after the earthquake, efforts to plug the craters are expected to begin soon.
“It’s also a tricky business,” says geotechnical Davor Ljubičić, who is coordinating the civil defense crisis unit’s geotechnical working group.
“Very close to these two villages is the communal water source Pašino vrelo, as well as a number of private wells. So you have to be very careful when choosing the material to cover the holes.”
Using cement or the wrong material to plug holes can pollute local drinking water sources, so engineers plan to cover them with large stones and then fill in the rest with smaller stones and gravel, explains Mario Bačić, civil engineer of the University of Zagreb.
And it won’t be cheap. Plugging the hole in Borojević’s backyard could cost around 200,000 euros ($1.2 million at current prices). “I could turn it into a fish pond,” jokes the resident.
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