In 2016, an earthquake occurred in Oulu, Finland, causing cracks in building foundations and reports of shaking and loud noises.
It wasn’t an earthquake.
Rather, it was a cryoquake, a shaking caused by the rapid freezing of water underground. Freeze earthquakes belong to a group of seismic phenomena called cryoquakes, which all involve freezing. They have been reported throughout the polar regions and even as far south as Chicago and Connecticut. Scientists are still working to understand exactly what factors contribute to the frequency of freeze quakes and what risks they pose.
In a new study, researchers found that most frost heave earthquakes in Finland occur in areas with high water tables and standing water, such as swamps, wetlands, drainage canals, and paved areas. “It was a big surprise for us to discover events from these completely different types of environments,” said preprint author Kari Moisio, a geophysicist at the University of Oulu.
They also quantified ground shaking caused by frost quakes and determined that some caused shaking comparable to earthquakes and infrastructure damage.
rolling wetlands
To capture the reality of cryoquakes, researchers installed seismic arrays and ground temperature sensors at two locations in Finland. One is in the subarctic Oulu, the other in the arctic town of Sodankyla. The device recorded data from November 2022 to April 2023.
The hundreds of seismic events identified by the researchers were divided into two categories: frostquakes, which produce waveforms similar to tectonic movements, and frostheave tremors, which produce more irregular waveforms. . Rowan Romaine, a geophysicist at a Norwegian research institute called Nofima who studied freezing phenomena during his doctoral course, said that “frost tremors” are similar to waves generated by freezing phenomena that occur in sea and lake ice. He said there is. Romijn was not involved in the new study.
The research team compared frost quakes to known earthquakes using the Modified Mercalli Intensity (MMI) scale, which ranks the impact and damage of observable seismic events. The most powerful cryoquake swarm the researchers measured in Oulu occurred on April 1, 2023, causing vertical and horizontal ground motions comparable to MMI V, with moderate shaking and light damage. Residents reported shaking the ground, unusual noises and new cracks in the roads.
Researchers said the quake’s shaking was about 10 times stronger than that caused by a passing freight train. Quantifying the ground movement caused by freeze-earthquakes is an important step in freeze-earthquake research, Romijn said.
“Although frost heave earthquakes are very small, if they occur near something, they can cause quite large shaking.” [there]”
“Although frost heave earthquakes are very small, if they occur near something, they can cause quite large shaking.” [there]” Romijn said.
The earthquake occurred during a very rapid drop in temperature of -9.6°C (14.7°F) to -21°C (-5.8°F) over an 8-hour period, approximately 1.4°C (2.5°F) per hour. It occurred on. Decrease. Moisio said most cryoquakes occur when temperatures are well below freezing, dropping at least 1°C (1.8°F) per hour.
Besides temperature, other factors such as soil moisture also contribute to the frequency of frost heave earthquakes. “When there’s a lot of water, there’s usually a lot of events,” Moisio said. His team is currently studying how soil type contributes to freeze earthquakes.
Most of the frost quakes observed in Oulu occurred in irrigated wetlands, while about half of the events in Sodankyla were caused by ice breaking on the Kittinen River, which runs through the town. The researchers say their study is the first to detect frost earthquakes occurring in wetlands. The phenomenon that did not occur in wetlands occurred on roads and other paved surfaces.
Snow, rain, and earthquake risks
Frost heave earthquakes could become more common as northern latitudes begin to receive rain instead of snow and saturate the ground more frequently, Moisio said.
Ice freezing on snow, another phenomenon that will increase with climate change, and a decrease in overall snowfall could also increase the likelihood of frost heave earthquakes, Romin said. That’s because snow tends to protect the ground from sudden temperature changes, while ice transmits temperature changes to the ground more quickly. “If there’s a lot of ice on the ground instead of snow, there should be more.” [frost quakes],” he said. A 2016 paper analyzing the spike in frost heave earthquakes in Canada also suggested that an increase in winter days above freezing is contributing to the frequency of frost heave earthquakes.
As climate change progresses, frost heave earthquakes “should be considered” as potentially damaging events in seismic hazard assessments, the authors write. But the climate link could be more complex, Romijn said, especially if climate change makes subzero temperatures rarer in the polar regions.
“This research always brings new questions.”
A lack of long-term data on frost heave earthquakes means scientists are still unsure whether their frequency has actually increased. Moisio said his team hopes to expand observations over multiple years and over a wider area to further focus on cryoquakes. “This research always brings new questions,” he said.
Romijn agreed with the authors that frost heave earthquakes should be considered in seismic risk assessments, and that buildings in areas with a high risk of frost heave earthquakes may need to spend more on maintenance. Stated. However, the likelihood of a catastrophic freeze quake that would kill people or destroy homes is extremely low.
Romijn said it was also important to figure out whether freezing events were a major risk factor in the area. Other geotechnical phenomena related to freezing can also damage infrastructure, for example, permafrost thaw and subsequent ground subsidence.
—Grace Van Dieren (@GVD__), staff writer