When buildings throughout Northwest Philadelphia started to rattle last Friday at 10:23 a.m., it took most people more than a minute to catch on to what was happening. 

“Could there be a big truck going by?” people thought, or “I didn’t know there was construction happening near here!”

But they got their answer fast. A rare East Coast earthquake had just rumbled through the region – a 4.8 tremblor that emanated from New Jersey and was felt from Baltimore to Boston. 

At 4.8 on the Richter scale, which is the 1-10 scale scientists use to measure the intensity of an earthquake, Friday’s quake was a relatively minor shake-up. According to the U.S. Geological Survey, earthquakes measure at least 5.5 on the scale before they do damage to buildings, and you can’t even feel one that’s below 2.5. The strongest quake ever recorded was the 9.5 magnitude Great Chilean Earthquake, which hit a town called Valdivia on May 22, 1960.

But still – earthquakes in these parts are rare and people were rattled. 

“It was as if someone was using a jackhammer under the house,” said Kristin Herman, who lives in Mt. Airy near Carpenter’s Woods. “Initially, I thought there might be an explosion about to occur.”

Once the literal and proverbial dust settled, many wondered about the underlying science that explains it all. Earthquakes typically occur at boundaries between tectonic plates, which are the large, rocky pieces of the Earth’s surface or crust. But Philadelphia is nowhere near a plate boundary, so why are we experiencing earthquakes? 

For starters, it’s important to understand how an earthquake occurs. An earthquake is what happens when two parts of the Earth’s surface move past each other. That creates energy, which builds up and is suddenly released – and we feel it as a trembling underneath our feet. The aftershocks are smaller earthquakes that occur as the crust adjusts to its new positioning.

While we’re not located atop a tectonic plate boundary, the North American plate that we live on is slowly being pushed, and stressed, by ocean-floor spreading. That stress builds – and sometimes pushes through old fractures or other weak spots in the plate. 

Hence our quake. 

And while rare, quakes like these are not unheard of. This area’s last significant earthquake, which originated in Delaware and registered 4.1 on the Richter scale, happened in 2017. And more than a decade ago in 2011, buildings throughout Center City were evacuated as a 5.8 quake, courtesy of Virginia, rumbled through. 

Another oddity about the earthquakes here in our region is that they travel much, much farther than those on the West Coast, where the rocks are younger and more fractured. This is because the rocks underlying much of the East Coast are older and denser – in some cases, hundreds of millions of years older. This has allowed them more time to heal previous fractures, which in turn allows seismic waves to travel through them more efficiently. 

And for the next time, whenever that might be, it is always good to know what to do. According to the CDC, remember the saying: Drop! Cover! Hold On! In other words, get down on your hands and knees; crawl under a table or desk or next to low-lying furniture, cover your head and neck and hold onto whatever piece of furniture you are ducking under. 

Virginia Friedman is a science teacher at Springside Chestnut Hill Academy, and Landon Ramer is one of her sixth-grade students.