By Nick Fontaine
In the midst of the brutal cold brought about by the polar vortex that engulfed the Midwest a few weeks ago, people reported hearing loud cracking sounds at night. Some assumed a transformer exploded, others thought it may have been gunshots— but the real cause was much stranger. These booming sounds where actually cryoseisms, also known as ice quakes or frost quakes. Ice quakes were first reported in 1819 by geologist Edward Hitchcock. Since then, they have been observed in many northern climates, including here in Vermont as recently as 2016.
So, what causes this odd event that is equal parts geological and meteorological? An examination of recent ice quakes in the US and southeast Canada using police and social media reports of the sounds found four common characteristics that tend to cause cryoseisms: seasonal frost, rapid saturation of the soil/bedrock, little or no snow cover, and a rapid drop of temperature to near or below zero degrees Fahrenheit (Battaglia & Changnon, 2016).
These commonalities make sense when you think about it. If the ground is saturated with water (such as after rain or a thaw) and the temperature drops rapidly, said water freezes very quickly. Since water expands as it freezes the frozen ground around it gives way and cracks— producing a loud noise and minor seismic activity. The lack of snow helps in that a deep sheet of snow would insulate the ground from the cold air temperature. In an example used by Battaglia & Changnon (2016)-- from January 6 to the 7 of 2014, Toronto saw the temperature drop from 34°F to -7°F in just 28 hours with 2 to 4 inches of snow on the ground. This was the perfect combination and lead to reports of frost quakes.
While it is possible to tell when an icequake is likely to occur, they are very isolated events (only heard for a few hundred yards at the most) and there is currently no way to predict precisely where one will occur. However, prediction is not really vital, as cryoseisms pose little or no threat— the worst that can happen is being startled by a loud noise. The extreme cold outside would be much more dangerous.
Ice quakes do not threaten our wellbeing or even our property. However, they are fascinating examples of the science that is all around us and how if you look into something you are curious about you will often find a simple yet elegant explanation. Also the term ice quake just sounds cool. So next time you feel like you’re barely holding yourself together during the cold Vermont winter, remember the ground feels the same.
References:
Battaglia, S., & Changnon, D. (2016). Frost Quakes: Forecasting the Unanticipated Clatter. Weatherwise, 69(1), 20-27.
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