Geologists Finally Explain New Jersey’s Strange Earthquake That Rocked the Northeast in April

On April 5, 2024, people across the East Coast of the United States experienced an unusual earthquake. In Newark, New Jersey, three houses partially toppled over. About 150 buildings in New York City reported minor damage, a sinkhole opened on Long Island and people noticed the vibrations as far away as Virginia, New Hampshire and Maine. The U.S. Geological Survey (USGS) estimates the quake was felt by around 42 million people.

But in New Jersey’s Tewksbury township—the epicenter of the magnitude 4.8 quake—residents barely felt any shaking at all.

Scientists were doing “lots of puzzling” over why people experienced the earthquake at such distant places, while they saw “very little damage around the epicenter area,” study co-author Won-Young Kim, ​a researcher at the Columbia Climate School’s Lamont-Doherty Earth Observatory, tells FOX Weather’s Scott Sistek. Now, a recent study published this fall in The Seismic Record offers an explanation.

In “normal” earthquakes, the shaking is felt most severely at the epicenter, the point on the Earth’s surface directly above the focus, or hypocenter, which is where the earthquake starts within the planet’s ever-shifting crust. The shaking usually radiates outward from the focus in a bull’s eye pattern.

Based on the April earthquake’s magnitude, its fairly shallow depth of 2.9 miles and the geology of the area, models suggest that a roughly six-mile area around the epicenter should have felt intensity VII shaking on the Modified Mercalli Intensity Scale—a level described as “very strong.”

That, however, is not what happened. Although the region was struck with its biggest earthquake since 1884, it didn’t seem that way hours later, when the research team went to survey the epicenter.

“We expected some property damage―chimneys knocked down, walls cracked or plaster fallen, but there were no obvious signs,” Kim says in a statement. “We talked to police officers, but they were not very excited about it. Like nothing happened. It was a surprising response for a magnitude 4.8 earthquake.”

To figure out what had happened, the team analyzed Lg waves caused by the quake—low-frequency waves of energy that bounce back and forth between the Earth’s surface and the boundary between our planet’s crust and the mantle, called the Moho. The analysis found a previously unknown fault that runs south to north and is tilted eastward at around a 45-degree angle, per the statement.

The scientists concluded that the movement that caused the earthquake was fast and complex, combining two motions—a horizontal sliding of the two sides of the fault called a “strike-slip,” as well as one side going over the other in a movement called a “thrust.” While the energy from earthquakes normally moves toward the areas of least pressure—such as up toward the surface at the epicenter—this time it was different. The energy followed the fault downward and bounced off the Moho to then strike the surface at places like New York City, before rebounding down and up again to hit other, more distant regions, the team suggests.

Earthquakes in the East Coast are rare, especially in comparison to the much more seismically active West Coast. The former, however, are more widely felt because of geological differences between the regions. The East Coast is mostly made up of old, cold and dense rocks that are more conducive to seismic waves.

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,” Christopher Carchedi, a seismologist at the Carnegie Institution for Science in Washington who wasn’t involved in the study, told the Washington Post’s Carolyn Y. Johnson back in April.

The East Coast is also less broken up by fault lines, which could interrupt seismic waves, than the West Coast is. The fault lines that do exist date back to about 200 million years ago, when Europe split from North America, and they sometimes still see activity, per the statement.

Shortly after the New Jersey earthquake, Carchedi hypothesized that the uncommon movement of East Coast quakes could also be due to the Earth’s surface still readjusting to the end of the last ice age, when the area’s geology was subjected to significantly more weight under the glaciers, per the Washington Post.

Either way, the occurrence is a reminder that earthquakes aren’t just limited to known seismic areas. Kim’s team also stresses that more research is needed to understand if similar unusual quakes could target other large population centers in the future.

“If [the April] earthquake was just a little stronger, or a little closer to New York City, the effect would be much greater,” Kim says in the statement. “We need to understand this phenomenon and its implications for ground motion prediction.”

Margherita Bassi | READ MORE

Margherita Bassi is a freelance journalist and trilingual storyteller. Her work has appeared in publications including BBC Travel, Discover magazine, Live Science, Atlas Obscura and Hidden Compass.