Everything happened suddenly. In the vast garden behind Nikola Borovich's house, where the potato seedlings were supposed to sprout, a large hole had formed in the ground. It was about 30 meters wide and 15 meters deep. It flooded quickly. And it wasn't the only well.
Within a few weeks, dozens of such wells formed in the village of Michanchani, in northeastern Croatia, and in the neighboring village of Borovichi. The crater in front of Borovich's house in Michinchani appeared on January 5, six days after a 6.4 magnitude earthquake.
The epicenter was reported under the bottom of the Pacific Ocean. It was the strongest earthquake in Croatia in four decades, killing seven people and destroying thousands of homes.
Earthquakes often cause landslides and potholes, but at the same time strange geological events such as liquefaction, in which solid ground begins to turn into a liquid state, make the two peoples lose hope. The large number of wells nearby made geologists uncomfortable.
One month after the earthquake, about 100 wells were drilled in an area of 10 square kilometers (3.8 square miles) and new wells were drilled every week.
The hole in Borovich's garden is the largest in the area. When it first appeared, it was 10 meters (33 feet) wide, but soon after that it began to grow.
"My wife was always at home and sometimes she looked out the window for a while," says Borovich. Around two in the afternoon he saw something strange happening in the garden. We went out and there was a big hole in our garden. The hole had tripled in size over the next three months.
But the Borovich family was lucky. In the same area, some ditches formed a few meters from the doors of the locals' houses and turned into a ditch under a person's home, forcing the administration to evacuate both villages.
Several wells have appeared in the surrounding woods and farmland where, according to some local rumors, a well nearly engulfed a local farmer and his tractor.
Due to the unusually large number of wells in one location, it has attracted the attention not only of local but also foreign geologists. They want to know how earthquakes can cause landslides.
"Nobody expected so many holes to form," says Youssef Stapchivich, a meteorologist in the Department of Geophysics at the Faculty of Science in Zagreb, Croatia.
The risk of strong earthquakes is very high in Croatia, where small Adriatic plates collide with the Eurasian tectonic plate, causing a series of active "faults". Prior to the December 29, 2020 earthquake, there were nine other earthquakes in the country since the early 1900s, with a magnitude greater than six. The last earthquake that occurred was in Poke Upsko, Paternia, and the last in 1909.
The epicenter of the 1909 earthquake was located just 23 kilometers northwest of the epicenter of the late 2020 earthquake. It also attracted the attention of seismologists at the time. Famous Croatian physicist (geophysicist) Andrea Mohorovicic studied the accounts of the 1909 Poke Upsko earthquake and concluded that seismic waves travel at different speeds as they pass through different layers of the earth.
His expertise led to the discovery of a boundary separating the earth's crust from the veil above it, which today is known as 'Mohorovich's Closure' or simply 'Mold'.
Today, researchers are studying the same area according to the "Moho" principle of how such large craters appeared after a sudden earthquake.
It is not uncommon for such wells to form due to strong earthquakes, especially in areas hidden underground. After a devastating earthquake near the Italian city of Laquila in 2009, two holes immediately formed in the streets of the historic center.
Experts at the time suspected that the rear excavation of the vertical sewer trenches had weakened the ceiling of the underground cave and caused the rammed earth structure to collapse.
Antonio Santo, Federico, the Italian geologist of the University of Naples, affirms that "the uniqueness of Croatia is the formation of many large wells".
The deepest and deepest well that threatens two Croatian villages is known as the Cover Collapse Sinkholes. They usually appear in areas where underground rocks sink into the passage of water and create a void in which a thick layer of clay, sand or muddy soil is formed and the important thing is that it is a type of common ground.
The deepest and deepest well that threatens two Croatian villages is known as the Cover Collapse Sinkholes. They usually appear in areas where underground rocks sink into the passage of water and create a void in which a thick layer of clay, sand or muddy soil is formed and the important thing is that it is a type of common ground.
Over time, water slowly filters surface material from its deeper layers into deeper soil. If it were just sandy soil, this process could be seen at the final level. But the presence of soil makes this surface layer stronger and more stable, so after a while a void is formed in the lower soil layer, but this internal void is almost unrecognizable above the soil. When the surface layer weakened in terms of structure, it eventually fell into the space beneath the surface.
This process often takes time, but can also be accelerated by human activities such as heavy rain, flooding or mining or aggressive groundwater drainage.
After analyzing the data collected in the area around Michanchani and Borovichi, Croatian geologists concluded that the strange phenomena were the result of a complex combination of several factors.
Although the coastal part of Croatia belongs to the famous Denarik Karst (karst is called limestone zone, the bottom of which is drained by rocks to form drainage ditches and alleys), it is made of deep limestone. There are thousands of caves and hundreds of local caves made of limestone, but the chain of underground limestone formation also extends across the country under central Croatia.
The limestone, which forms in the form of cavernous rock between the Denark crust and the Pennon basin, was deposited during the Mycenaean period, when the area was underground and is now connected to the Mediterranean Sea.
"Although the karst dinark comes mainly from the Cretaceous and Jurassic eras, the karst we have here is younger and even more insecure and empty," said Youssef Terezich of the Croatian Geological Survey. It is limited to a few small areas around here and near the city of Zagreb.
Eleven million years ago, when the Pennin Basin was cut off from the Mediterranean by the relocation of the Penn Land Islands, it became a huge lake. Rivers gradually filled it with mud, sand and gravel, forming the vast expanse of modern times. As a result, a layer of mud, rock and clay lay on the rock about 10-15 meters below the villages of Michanchani and Borovichi.
This threat was difficult to detect. Previously, some wells appeared suddenly, but according to the locals they were very small and in small quantities.
"Obviously, the earthquakes accelerated part of the process already underway," says Youssef Terezich. In fact, after a 5.0 magnitude earthquake, large holes began to appear in the area. When the earthquake and its aftershocks shook the region, the ground shook more than 30 centimeters (12 inches) on one side. This landslide created uncertainty.
"Earthquakes caused enormous dynamic stresses on the earth and in places that had already reached critical levels, they suddenly collapsed," says Terezich.
Bruno Tomal Yanovic, a geophysicist at the University of Zagreb, a colleague of UCP Terezich, believes that earthquakes affected the movement of groundwater, causing it to rise and fall relative to high-pressure areas. Bruno Tomal Yanovic explains that this has accelerated the surface cutting process that occurs in underground hydrodynamics.
"Furthermore, there is a possibility that some landslides have caused further changes in hydrodynamics, leading the water to find new routes and perhaps more burrows," said Bruno Tomal Janovic. It became the cause.
George Wayne, a geologist at the National Cue and Karst Research Institute in New Mexico, says an unusually high number of tremors at the same time could contribute to many landslides. Wayne warns that the rate of human activity is increasing, which can cause the earth's crust to collapse.
A recent report by scientists from the University of Zagreb warns that the construction of irrigation systems in the areas of Michanchani and Borovichi may have accelerated the "karstification" process.
At present, scientists do not have enough data to analyze the relationship between the appearance of the fortresses and the strength and number of earthquakes.
"The situation in Croatia can be seen as a warning to earthquake-prone countries and areas that could be affected by the earthquake and are prone to landslides," he said.
Mario Paris, a geologist at the Aldo Moro University of Bari in Italy, says it is not easy to predict where such wells might form.
"So far, we can only rely on historical data and documents to identify the areas that have been most affected by these types of processes," he said, although some of the fortification threats in the past decade have been proposed models, but "construction The Fortress Alert System is an area where there is a lot of work to be done. "
Tomal Yanovich learned from Croatian strongholds that it is necessary to identify earthquake-prone areas, or "micro-zoning", to identify populated areas where dangerous earthquakes can occur. He and his colleagues are using electrical resistivity tomography and seismic refraction studies in the areas of Michanchani and Borovichi to identify areas where no sinks have formed but the danger persists.
But the threat of the emergence of new forts next year is on the minds of many residents. According to Stepchevich, changes in groundwater levels and failures throughout the year, as well as more aftershocks, could lead to more wells.
Meanwhile, a large water-filled well still exists in Boruwich Garden and is a major tourist attraction. Work to fill the wells is expected to begin soon, six months after the earthquake.
"This is a difficult task," said Dover Leubicich, geotechnical coordinator of a geotechnical task force created to protect citizens. Near these two villages there is a common source of water supply, 'Pino Verilo', as well as several private wells. Therefore, it is necessary to be very careful in choosing the material to close these strengths.
Using concrete or unsuitable materials to fill these wells can pollute drinking water, so engineers are thinking of filling them with large rocks and then filling the rest of Zagreb with rocks and gravel.
This job will not be cheap. Filling a hole in Borovich's garden can cost around 200,000.
"I can turn it into a pond," joked Borovich.
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