Ice quakes, frost quakes and exploding trees: You might hear them in the Philadelphia area this weekend

This has been a winter full of surprises. The Philadelphia area has seen the biggest snowstorm in 10 years and one of the longest below-freezing streaks in our history.

Nearly every day since Thanksgiving has registered below-average temperatures, and many of those days dipped below freezing.

Now the coldest air of the season is set to slide over our region this weekend. Temperatures will plummet from highs Friday in the low to mid 30s to low single digits by Saturday night. That's an extremely fast and large drop of 20 to 30 degrees in less than 36 hours.

Rapid drops in temperature like these can lead to frost quakes, ice quakes and lake quakes.

Frost and ice quakes

Following late January's snowstorm, the temperature plummeted across the eastern half of the country. There were frost quakes with shaking and booming reported from Tennessee to Pennsylvania.

The scientific name for frost quakes, ice quakes and lake quakes is cryoseisms. Non-tectonic seismic events can occur during extreme cold from the rapid freezing of water-saturated soil, rock and lake ice.

These mini-seismic events are not considered true earthquakes because there is no movement along faults or tectonic plate boundaries.

So, exactly how does a frost or ice quake happen in our area?

• A much colder-than-normal Arctic air mass arrives. We are expecting the coldest air of the season this weekend.
• They usually follow some thawing or rain prior to the Arctic air arrival. Just like the thawing we had the past few days, with temperatures above freezing.
• Up to 6 inches of snow on the ground. That is what remains for much of the area away from the snow piles created by plows.
• A rapid temperature drop from just above freezing to single digits within 24 to 48 hours. Which is what we will experience between Friday afternoon and Sunday morning.
• As temperatures rapidly fall, the water quickly freezes in small cracks and areas of underground water.
• The newly formed ice expands and puts pressure on the adjacent soil, bedrock and existing lake ice.
• This rapid expansion can cause loud booms, vibrations and weak shaking similar to a weak earthquake.
• The most likely hours for these quakes are late night through sunrise, which is typically the coldest time each day.

Smaller version of the quakes at home

Think of a plastic water bottle filled to the rim with the cap firmly screwed on.

Then put that bottle outside in the below-freezing temperatures or in the freezer with a pan underneath to prevent leaking.

Leave it untouched for 6 to 8 hours. The water in the bottle will freeze and expand. Since there is no extra room in the bottle, the pressure from the expanding ice will stretch and burst the bottle. You may even hear a bang or a pop when the bottle bursts.

Many of us have accidentally left a water bottle in the car and had this happen.

Unfortunately, the same process can burst water pipes in homes and city streets.

Lake ice quakes

Lake ice quakes form in a similar way, with ice on already frozen lakes contracting and pulling apart from the rapid drop in temperature. This causes cracks and booming sounds. 

Exploding trees

Also known as "frost cracking." The trees don't really explode, instead the dry bark on the outside of a tree cools more rapidly than the soft, moist cambium inside the tree.

This combination shrinks the outer bark while the inner cambium freezes and expands.

The combination of shrinking outside and expanding inside will crack the tree and create a popping or gunshot sound.

Typically, the tree won't die, but the cracking may be severe enough to splint the tree with branches falling off.

If the split happens in the middle of the tree trunk, what's left looks hollow and shredded, resembling an explosion.