Villagers in northeastern Croatia feared their homes might be swallowed up when nearly 100 huge sinkholes appeared in a month. Now scientists are trying to figure out if the remaining land is safe.
It happened suddenly and without warning. Where the first shoots of potato seedlings were supposed to appear behind the orchard in the spacious garden of Nikola Boroevich, there was now a huge hole. At 30 m (98 ft) wide and 15 m (49 ft) deep, it quickly filled with water. And she was not the only one.
Over the course of several weeks, dozens of such holes opened around the village of Mechenciani and neighboring Borojevići in northeastern Croatia. A pit near Boroevich's house in Mechenchani appeared on January 5, just six days after an earthquake of magnitude 6, 4 that occurred in the vicinity of the nearby town of Petrinya. It was the worst earthquake in Croatia in more than four decades, killing seven people and destroying thousands of homes.
While it is known that landslides and sinkholes can be caused by earthquakes, along with other strange geological phenomena such as liquation - when solid ground begins to behave like liquid - the sheer number of holes that have appeared around the two villages surprised and puzzled experts. A month after the earthquake, on an area of 10 sq. km (3.8 sq. miles) nearly 100 sinkholes have been discovered, with new ones opening every week.
The hole in Boroevich's garden is now the largest in the area. When it first appeared, it was 10 m (33 ft) wide, but almost immediately began to grow.
“My wife was in the house all morning, occasionally looking out the window,” Boroevich says. "At about two o'clock in the afternoon, she noticed something strange in the garden. We went outside, and there was a huge hole in our orchard." Over the next three months, the hole tripled.
But the Boroevichs were lucky. Other sinkholes in the area opened just meters from people's doorsteps, and one appeared under the house, prompting officials to consider evacuating both villages. Others appeared in the surrounding forests and agricultural fields, where one of them, according to some local rumors, almost devoured a local farmer and his tractor.
The unusually large number of craters in one location has attracted the attention of local and foreign geologists seeking to understand how an earthquake could trigger ground collapses.
"Nobody expected so many craters to appear," says seismologist Josip Stipcevic from the Department of Geophysics at the Faculty of Natural Sciences in Zagreb.
Croatia is located in a seismically active zone where a small Adriatic plate collides with the Eurasian tectonic plate, causing a series of active faults, explains Stipcevic. Before the earthquake on December 29, 2020, since the beginning of the 20th century, nine earthquakes with a magnitude of more than 6 have occurred in the country since the beginning of the 20th century. The last major earthquake on the Pokupka-Petrinsky fault, along which the last earthquake occurred, occurred in 1909.
The 1909 earthquake struck just 23 km northwest of the epicenter of the late 2020 earthquake. This also attracted the attention of the leading seismologists of the time. The famous Croatian geophysicist Andrija Mohorovicić studied the seismograms of the 1909 earthquake in Pokupko and came to the conclusion that seismic waves propagate at different speeds, passing through different layers of the Earth. His findings led to the discovery of the boundary separating the earth's crust from the mantle, known today as the Mohorovicic rupture, or simply Moho.
Today, researchers are studying the same area in hopes of understanding how the earthquake led to the sudden appearance of so many craters.
Funnels are not the most common consequence of powerful seismic shocks, but they do occur, especially in areas with hidden underground cavities. After the devastating earthquake near the Italian city of L'Aquila in 2009, two sinkholes immediately opened on the roads in the old part of the city.
“The real anomaly in the case of Croatia is the very large number of sinkholes of significant size,” says Italian geologist Antonio Santo of the University of Naples Federico II.