Villagers in north-east Croatia feared their homes might be swallowed when more than 100 enormous sinkholes appeared in a month. Now scientists are trying to understand if the land that is left is safe.
It happened suddenly and without warning. Where there should have been the emerging first shoots of potato seedlings behind the orchard in Nikola Borojević’s spacious garden, there was now huge hole. Measuring 30m (98ft) wide and 15m (49ft) deep, it quickly filled with water. And it wasn’t the only one.
Within the space of a few weeks, dozens of similar holes had opened up around the village of Mečenčani and neighbouring Borojovići in north-east Croatia. The one outside Borojević’s home in Mečenčani appeared on 5 January, just six days after a 6.4 magnitude earthquake struck the area around the nearby city of Petrinja. It was the strongest earthquake to have hit Croatia for more than four decades, killing seven people and destroying thousands of homes.
While landslides and sinkholes are known to be triggered by earthquakes, along with other strange geological phenomena such as liquefactions – where the solid ground begins to behave like a liquid – the sheer number of holes appearing around the two villages surprised and baffled experts. A month after the earthquake, there were almost 100 sinkholes spread over a 10 sq km (3.8 sq miles) area, with new ones opening every week.
The hole in Borojević’s garden is now the largest in the area. When it first appeared it was 10m (33ft) wide, but started to grow almost immediately.
“My wife was in the house the whole morning, looking occasionally through the window,” says Borojević. “Around 2pm she noticed something strange in the garden. We went outside and there was this huge hole in our orchard.” Over the following three months, the hole tripled in size.
But the Borojevićs were lucky. Other sinkholes in the area opened up just a few metres from the doorsteps of people’s homes and one appeared beneath a house, prompting officials to consider evacuating both villages. Others appeared in the surrounding woods and agricultural fields, where one of them, according to some local rumours, almost swallowed the local farmer and his tractor.
The unusually large number of sinkholes in one spot has drawn the attention of local and foreign geologists, keen to understand how the earthquake may have triggered the ground collapses.
“Nobody expected the appearance of so many sinkholes,” says seismologist Josip Stipčević from the Department of geophysics at the Faculty of Science in Zagreb.
Croatia sits on a highly seismically active area, where the small Adriatic plate is colliding with the Eurasian tectonic plate, causing a number of active faults, explains Stipčević. Before the earthquake on 29 December 2020, the country had experienced nine others with a magnitude over 6 since the beginning of the 20th Century. The last major earthquake to have occurred on the Pokupsko-Petrinja fault – along which the most recent one occurred – was in 1909.
The 1909 earthquake occurred only 23km north west from the epicentre of the one that shook at the end of 2020. That too attracted the attention of leading seismologists at the time. The famous Croatian geophysicist Andrija Mohorovičić studied seismographs from the 1909 Pokupsko earthquake and concluded that seismic waves travel with different speeds as they pass through different layers of the Earth. His insight led to the discovery of a boundary that separates Earth’s crust from its mantle, known today as the Mohorovicic discontinuity, or simply the Moho.
The real anomaly in Croatia’s case is a very high number of sinkholes with significant dimension – Antonio Santo
Today, researchers are studying the same area in the hope of understanding how the earthquake led to so many holes suddenly appearing.
Sinkholes are not the most common consequence of powerful seismic shocks but they do occur, especially in the areas with hidden underground cavities. After the devastating earthquake near the Italian city L’Aquila in 2009, two sinkholes immediately opened on roads in the old part of the city. Experts at the time suspected that a previous excavation of vertical trenches for a sewage conduit weakened the conglomerate roof of the underground cave, contributing to the collapse.
“The real anomaly in Croatia’s case is a very high number of sinkholes with significant dimension,” says Italian geologist Antonio Santo at University of Naples Federico II.