Just after 2:30 p.m. on Monday, people in Perth Road Village north of Kingston, Ont., heard a loud bang and felt the ground shake. They thought there'd been an explosion, a crash or even an earthquake.
"Some people said their dogs were startled and started to act weird," said Alex Braun, a geophysicist at Queen's University in Kingston, on Thursday.
"Others went outside and thought there was a car pileup, an accident. Some even mentioned a plane may have crashed in the nearby forest. Others thought there was a blasting event in a nearby quarry."
What they'd actually heard and felt, as a couple residents correctly guessed on the Perth Road Villagers Facebook page, was none of those things. What they'd witnessed was a frost quake.
Frost quakes or ice quakes are much less powerful than earthquakes, but can still be "quite loud," Braun said.
"It depends how close you were to this event, but frost quakes exhibit a pretty significant sound as opposed to earthquakes, which often happen in deeper layers of the earth, so the sound emissions are not as strong."
So, how do frost quakes occur? You need three ingredients:
- Precipitation. Rain, freezing rain or an ice storm is the first step.
- Water accumulation in soil. That precipitation then has to sink into the soil column, filling up all the space between soil particles with water.
- Sudden temperature drop. And finally, the temperature has to drop significantly in a short period of time, freezing all the water in the soil and causing it to expand, thereby increasing pressure.
"And [that pressure] can't go anywhere, because the surface of the soil is frozen ... and the surface is sealed, so there's no way to release this pressure," Braun explained.
"And suddenly the pressure reaches a certain level, which the soil cannot take anymore, and you have this burst where something ruptures."
'A pretty decent frost quake'
Frost quake ruptures can leave cracks in ice about two-and-a-half centimetres wide, and in this case, the energy released north of Kingston was picked up by a seismometer and a gravimeter at Queen's.
"It was a pretty decent frost quake because it was recognized by many people in the area, which normally does not happen in unless it's in a populated area," Braun said.
"In a rural area like around Perth Road, it's not common that people notice in these numbers."
You can listen to the entire interview with Braun below.