By Cameron Kovach

This year’s unseasonably warm winter weather mixed with waves of bitterly cold air has brought a rare and often frightening natural phenomenon back to Vermont—Cryoseisms. A cryoseism, or frost quake, occurs after a sudden deep freezing of the ground, when the pressure created from water expanding into ice abruptly causes stress fractures in underground soil and rocks. The result of these fractures can produce localized ground shaking and noises similar to an earthquake. While usually alarming, frost quakes typically pose little physical harm beyond occasionally causing a small crack in the ground where the frost quake occurred.

On the night of January 8, 2016 and morning of January 12, 2016, several Shelburne residents in the vicinity of the La Platte River near Falls Road reported hearing mysterious sounds that many equated to an earthquake. One resident, despite not living near a highway, described the event as similar to the rumblings caused by a large semi-truck driving nearby, while another on Front Porch Forum likened her experience to a large boulder rolling towards her house. In each case, the startled residents described the sound as very loud and different from thunder, but no one at the time could explain exactly what caused the earthquake-like tremors.

After speaking with state geologist, Marjorie Gale, and confirming that there was no seismic activity on either of those days, she explained that in 1955, geologists mistakenly reported frost quakes in Burlington as minor earthquakes in the state’s earthquake catalog, and that local frost quakes could have caused the mysterious rumblings in Shelburne. However, given their relative infrequency and generally minor effects, the Vermont Geological Survey does not track frost quakes. As a result, there is little historical scientific data regarding the geographic locations, cause, or frequency of frost quakes in the state.

According to Weatherwise magazine, for a frost quake event to occur, the ground must be saturated with water, have minimal snow accumulation, and experience a large temperature drop from around freezing to near zero degrees Fahrenheit over the course of 16 to 48 hours. The last reported frost quakes to happen in our area occurred following an ice storm in December 2013. After a high temperature of 42°F and rain on December 29, 2013, the temperatures plummeted to a low of 3°F on December 30, 2013, resulting in several reported frost quakes throughout southern Canada, Vermont, and upstate New York.

Similarly, during both of the suspected frost quake events this year, the high temperature measured 39°F before dropping into the low teens at night. Furthermore, on January 10, 2016, the high temperature reached 53°F and we experienced over a half inch of rain that saturated the ground and removed any potentially insulating snow cover. Therefore, within a 48 hour period, temperatures dropped over 36°F exposing bare, saturated soil to an arctic blast of cold air; near perfect conditions for a frost quake. The same winter system that brought frost quakes to Shelburne also reportedly caused frost quakes in Dodge County, Wisconsin.

So, if the wild temperature swings of this winter continue, and you find yourself startled by mysterious rumblings or booms in the middle of the night, you may be in the epicenter of a frightening but benign frost quake.

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