What you need to know during and after an earthquake in Philadelphia

A 4.8 magnitude earth quake rumbled through New Jersey on Friday morning, sending seismic waves that were felt 100 miles away in Philadelphia.

At the time of publication, no casualties or severe injuries have been reported, although widespread accounts indicate that people in the region experienced the tremors for less than a minute.

Here are answers to your most urgent questions about the New Jersey earthquake.

The first layer beneath the Earth’s surface is called the “crust,” which is made up of seven major tectonic plates along with smaller ones. These plates are constantly shifting and readjusting, moving on average a half-inch every year, according to David R. Wunsch, a state geologist for the Delaware Geological Survey at the University of Delaware.

“The crust is under a lot of stress because tectonic plates are constantly moving, partly due to the weight of glacial ice that presses down on Earth, especially in the Northeast U.S.,” Wunsch said. “That ice has melted and retreated thousands of years ago, but it takes a long time for the Earth to respond to the release of stress during adjustment.”

The epicenter is directly above the point in the Earth’s crust where energy is released from the shifting tectonic plates, Wunsch noted. The epicenter of this earthquake was in Tewksbury, New Jersey, nearly 100 miles from Philadelphia.

When an initial earthquake happens, smaller ones can happen in its wake days or even years afterward. These are called “after shocks” and occur as the tectonic plates make minor adjustments returning to regular activity.

“Usually the larger the earthquake the more after shocks you may have,” said Wunsch. “Since today’s earthquake was not a very large one as far as earthquake’s go — it’s kind of in the small to moderate range — there may be after shocks that we won’t even feel, but the seismic instruments can pick them up.”

While there haven’t been any wide spread reports of aftershocks since the initial earthquake this morning, and Wunsch doesn’t believe there will be any more felt in the region, the Federal Emergency Management Agency, advises people to expect them anyway.

In the U.S., earthquakes are more common on the West Coast, particularly in California, which lies between two major tectonic plates that shifts and releases pressure much more frequently, according to the California Department of Conservation. On the flip side, the East Coast sits more centrally on a tectonic plate, offering more stability.

Across the world, earthquakes commonly happen around the edges of the Pacific Ocean, according to the British Geological Survey.

What’s interesting about East Coast earthquakes is that the crust tends to be older and colder, which means seismic energy can travel farther, allowing earthquakes to be felt hundreds of miles away, Wunsch added.

Earthquakes can happen in any location at any time, according to the U.S. Geological Survey, but they’rerarer in the Northeast U.S., said Wunsch. The most recent event similar to Friday’s was a 4.1 magnitude earthquake in Delaware in 2017. Before that, Virginia experienced a 5.8 magnitude quake in 2011.

For context,California records up to 20 earthquakes per year with a magnitude greater than 4.0, per the USGS.

With a magnitude of 4.8, Friday’s earthquake is considered small to moderate in intensity, Wunsch said. This means that near the epicenter, residents might notice picture frames falling and feel their houses rumble, but severe damage to infrastructure or people is unlikely.

“In California, they tell you to hide under something sturdy and things of that nature, but that’s generally not gonna happen around here,” he said.

On the East Coast, the approach is to “experience” an earthquake, Wunsch said, as they are not as intense as those in other parts of the U.S. Generally, it’s best to stay calm and wait out the tremors, as significant damage to property or people is not expected.

No casualties or severe injuries have been reported as of yet.