Frost quakes could become increasingly frequent in the north, with potentially significant consequences for buildings and infrastructure
In 2005, residents of the small town of Limington in Maine, USA, heard a series of loud booms that rattled the windows and shook the ground. One resident stepped outside to check whether his car had exploded, only to find a crack running through his paved driveway. According to the Maine Geological Survey, the damage was probably caused by cryoseisms – otherwise known as frost quakes.
Frost quakes aren’t a particularly rare occurrence. They happen when water in soil freezes and expands rapidly, causing the ground to crack. This sudden release of seismic energy can often be heard as loud snaps or booming sounds that are accompanied by small tremors. In Finland, Canada and the USA, members of the public have reported a number of frost-quake-like events over the past 20 years. Despite this, relatively little research has focused on the phenomenon – in part because they often occur in sparsely populated areas, such as the central and northern American Plains or sub-Arctic regions. New research from Finland, however, suggests that frost quakes could become a more common natural hazard in the north.
Early in 2016, a swarm of frost quakes shook the residents of Talvikangas, a suburban area of the city of Oulu in central Finland, rupturing the soil and creating cracks in roads and building foundations. Fourteen kilometres away, the University of Oulu’s Sodankylä Geophysical Observatory recorded a number of unusual seismic events, and a long-term environmental-monitoring study, which was just coming to an end, also happened to collect data on the meteorological conditions.
While a thick layer of snow would normally insulate the soil from any sudden change in temperature, on the day of the frost quake, the researchers noticed that there was little snow cover on the ground. An abrupt drop in temperature was recorded shortly before the tremors were felt; scientists have since determined that when the air temperature drops below –20°C at a rate of 1°C per hour, frost quakes are most likely to occur.
With this information, researchers realised they could recreate the conditions that had led to the frost quakes, to help better understand when they might occur in the future. However, locating exactly where the frost quakes had happened, was a more difficult matter. ‘The problem is that frost quakes are usually small events. It’s rarely possible to observe them with normal seismological observations,’ says Kari Moisio, a senior researcher at the University of Oulu. ‘You need to have instruments close to the areas you want to study.’
Ahead of the winter of 2022–23, the researchers installed seismic sensors at two locations: one on the outskirts of Oulu and one further north, above the Arctic Circle, in the town of Sodankylä. To their surprise, they found that most of the frost quakes in Talvikangas originated in the nearby wetlands and marshes, particularly those that have been furrowed with irrigation channels (until now, most frost quakes were thought to occur near snow-cleared roads). Half of the quakes recorded near Sodankylä were caused by ice fracturing on the Kitinen River, but here as well, many strong cryoseismic events were formed by water freezing in sodden marshland ground.
Finland is among the most wetland-rich countries in Europe; the results of Moisio and his colleagues’ research suggest that this could make it particularly susceptible to frost quakes. ‘Usually, of course, it doesn’t matter because most occur in uninhabited areas,’ says Moisio, but there have recently been a number of frost quakes in wetlands close to cities and other infrastructure. In January this year, Talvikangas was hit by another 26 frost quakes, all within the space of just seven hours. It’s thought to be the highest concentration of the phenomenon ever recorded.
These recent events have garnered a lot of public attention, with local residents’ reports shared on news websites and social media. This raises the question: is the number of frost quakes increasing, or is a greater awareness encouraging more people to report them? ‘It’s a really difficult question to answer with the limited amount of information we have now, so we can’t come to any strong conclusions yet,’ says Moisio. ‘It does seem that these events are more common than was previously thought.’
One concern is that as the planet continues to warm, we might see favourable conditions for frost quakes happening more often. Climate studies indicate that sub-Arctic areas will experience greater fluctuations in snow cover and temperatures; in parts of Finland, studies have already shown that snow cover has decreased at a rate of 2–4 centimetres every ten years since 1961.
And while most frost quakes are small, the strongest can produce ground movements strong enough to cause damage to urban infrastructure located several hundreds of metres away from wetlands. The next stage of the research is identifying any areas that might be at risk. ‘Just like earthquakes have hazard maps,’ explains Moisio, ‘we’d like to do the same thing for frost quakes.’