One of my favorite musical artists is Prince. He had a song on the brilliant "Sign O' The Times" CD called Housequake. Prince lived in Minneapolis, Minnesota. I wonder if he ever heard of a frost quake. The Polar Vortex is dominating the headlines from so many angles. Here is another one for you. It is being reported that frost quakes are happening in Chicago as a result of the Polar Vortex. It is not that I am psychic, but I know that many of you are asking, "What in the world is a frost quake?" Here is the answer.
The scientific term for a "frost quake" is cryoseism. My Forbes colleague Robin Andrews wrote about frost quakes recently, but I wanted to dive more into the science. The root words give you a clue. According to Merriam-Webster's online dictionary, the word "seism" is a less common word for the "earthquake." The word "cryo" is associated with cold or extreme cold. With this foundation, you may be putting some things together, but let's do it formally.
During winter or very cold outbreaks, the temperatures can cool rapidly from above 32 degrees F to below freezing (graphic above). With the most recent Polar Vortex infusion of cold air, temperatures have been well into minus values in the Midwest and wind chills were in the -60 to -70 degree F range. By the way, what does a minus temperature mean anyhow? You never see length given as -10 kilometers or mass as -50 grams. That is meaningless. The Kelvin scale is a temperature scale that has no negative values - but I digress. Let's get back to frost quakes.
When the temperatures plunge rapidly below freezing, subsurface water can freeze and expand. According to a 2015 paper by Steven Battaglia and David Shannon in Weatherwise magazine,
The subsurface ice continues to expand as the temperature cools because water molecules, upon cooling, arrange themselves into a structure that takes up more volume than when the molecules were arranged in a liquid. This continuous expansion imposes a large amount of stress on the local surrounding frozen soil and rock around the ice. The surrounding frozen surface can withstand the increase in pressure from the expansion of ice for a while, but not for as long at very low temperatures and high pressures. Once the pressure is too great, it is relieved violently—a fracture on the surface forms and a loud, explosive noise echoes within the proximity of the breakage!
That's what a frost quake is. You may also see the term ice quake used to describe a cryoseism. Ironically, like the Polar Vortex, these terms are not new. You may not have heard of them until now. My colleague Dr. Edward Colon, an atmospheric physicist in Maryland, alerted me that ice quakes have also been identified in non-seismic activity. Scientists have observed ice quakes during the process of glacial calving.
By the way, some people often report seeing flashes of light with cryoseisms. What in the world is that about? John Ebel addressed this question during an interview with the The Weather Notebook many years ago:
there are electrical changes that happen in rocks when they get squeezed. And of course earthquakes are really due to squeezing and then release of that pressure as the rock cracks and moves. And that can explain the earthquake light and lightning type effects that we see."
While cryoseisms are somewhat uncommon in the popular literature (until now), people that live in very cold climates may be more familiar with them. A recent paper in the Canadian Journal of Earth Sciences documented some of the larger lake-associated ice quakes (ML 2.0). Yep, cryoseisms can be measured using the same scale referenced in more common earthquakes. Most people are familiar with the "Richter scale," but there is a bit more to the discussion. According to the U.S. Geological Survey (USGS) website,
The magnitude is a number that characterizes the relative size of an earthquake. Magnitude is based on measurement of the maximum motion recorded by a seismograph. Several scales have been defined, but the most commonly used are (1) local magnitude (ML), commonly referred to as "Richter magnitude", (2) surface-wave magnitude (Ms), (3) body-wave magnitude (Mb), and (4) moment magnitude (Mw).
Since we are throwing around new terms today, another term that you may not be familiar with is frost heave. This is a process in which ground moves upward in response to the seasonal freeze and thaw of the ground. The picture below is an example discussed in a report by the state of Maryland.
