Why the New Jersey earthquake was felt several hundred miles away

Wendy Bohon, an earthquake geologist, was reading a budget report Friday morning at her home in Chesapeake Beach, Md., when she felt the unmistakable light shaking of an earthquake. The cat didn’t wake up. The dog looked around. A plant swayed.

The tremors traveled about 160 miles to Bohon’s home from a 4.8-magnitude earthquake near Whitehouse Station, N.J. It was a moderate earthquake from a geological point of view, with similar-sized quakes happening frequently all over the globe. But it marked a relatively infrequent event on the East Coast — one that jostled people across the Mid-Atlantic and Northeast from their daily routines.

More than 150,000 people reported feeling the New Jersey earthquake, some from several hundred miles away, according to the United States Geological Survey, which collects reports of shaking. While the number of reports reflects the population density of the area, it also highlights a fundamental geological difference between the tectonically active West Coast and the East Coast, which is covered by old faults that occasionally get reactivated.

The underlying rock in the East Coast is old, cold and dense, and the faults have had time to heal, meaning that seismic waves travel farther than on the West Coast, where the crust is broken up by faults.

A 4.8-magnitude earthquake “is not generally big enough to cause damage, but big enough to be widely felt,” said Susan Hough, a seismologist with USGS. “Once an earthquake happens, the waves travel more efficiently in the east than in the west — the crust is older, colder and less broken up — that’s something we’ve seen over and over. You put a [4.8] in California, and it won’t be felt nearly as far as this one.”

East Coast earthquakes

Several geologists said that while an earthquake greater than 4-magnitude was not a frequent event on the East Coast, such events aren’t unexpected. In 2017, a 4.1-magnitude earthquake struck near Dover, Del. A 2011 5.8-magnitude earthquake near Mineral, Va., was believed to be felt by more people than any earthquake in U.S. history and resulted in significant damage, including to the Washington Monument and Washington National Cathedral in D.C. A 4.6-magnitude earthquake struck near Reading, Pa., in 1994.

The New Jersey earthquake, which struck at 10:23 a.m. Friday, was relatively shallow, just three miles below the surface. The investigation into the quake will continue, but Christopher Carchedi, a seismologist at the Carnegie Institution for Science in D.C., said a common cause of such earthquakes on the East Coast is the shifting of Earth’s surface after being weighed down by ice sheets from the last Ice Age.

“It’s otherwise an inactive fault, readjusting to the loss of the ice, most likely,” Carchedi said.

Hough said the region is generally considered a “passive margin” between North America and the Atlantic Ocean, meaning there’s no active fault or plate boundary. But she notes that one of her colleagues calls it a “passive-aggressive margin — because it will occasionally bite you.”

“Moderate earthquakes in the East are always uncommon enough to be interesting,” Hough said, noting that Friday’s will be intensely studied.

Shaking travels far

As soon as she felt the tremors, Bohon began counting down. Earthquakes unleash multiple kinds of seismic waves that travel through the Earth at different speeds. Measuring the time between the first shaking and the next set of waves can give a rough estimate of how close it was — similar to measuring the time between a flash of lightning and the rumble of thunder.

Bohon got to 8 seconds, so she knew that the epicenter of the quake wasn’t very close. Then, she began looking up local seismic reports to make sure she hadn’t imagined it.

It has long been known that earthquake shaking travels farther on the East Coast than on the West Coast. The USGS says earthquakes can be felt over an area 10 times larger on the East Coast than the West.

On the West Coast, “the rocks are warmer, so they are more active, they’ve experienced activity more recently — so they’re warmer and will attenuate or absorb some of the seismic energy as they pass through that rock,” Carchedi said.

On the East Coast, in contrast, the rocks are cold and brittle, and transfer seismic energy much better than the West Coast.

The quake is a reminder, Bohon said, that while people think of earthquakes as a West Coast phenomenon, they can happen anywhere.