Fault line discovery in New Jersey: Explaining New York City area's unusual shaking from April earthquake

An expert explains the earthquake felt in New York City

Steve Holler, associate professor of physics and engineering at Fordham University, explains what New Yorkers should know about a 4.8-magnitude earthquake.

NEW JERSEY – The discovery of a new fault line may explain unusually strong tremors from April's 4.8 magnitude earthquake that struck in the heart of New Jersey, yet was felt by people miles from the epicenter, a new research study shows.

The earthquake occurred late in the morning of April 5, and was centered in the town of Tewksbury, but the quake sent shockwaves across New York City 40-50 miles away and caused some damage in Brooklyn and Newark.

According to US Geological Survey estimates, this was the strongest earthquake in the region since 1884, shaking about 42 million people. The US Geological Survey said 184,000 people reported feeling the quake, a record number to fill out a report.

New Jersey earthquakes and aftershocks from the main magnitude 4.8 earthquake on April 5, 2024. (FOX Weather)

However, researchers who went to the epicenter were puzzled when they found that there was only minimal shaking and no noticeable damage reported.

“We expected some property damage — chimneys knocked down, walls cracked or plaster falling off, but there were no obvious signs,” Wong Young Kim, a study co-author from Columbia College’s Lamont-Doherty Earth Observatory, said in a press release announcing the study’s findings. . “We talked to police officers, but they weren't very enthusiastic about it. It was as if nothing had happened. It was a sudden response to a 4.8 magnitude earthquake.”

USGS aftershock intensity map from a 4.8 magnitude earthquake that struck New Jersey on April 5, 2024. (FOX Weather)

Kim's study said that based on current models, the quake should have caused significant damage at the epicenter. Kim said surface motion caused by earthquakes is measured on the 10-point Modified Mercalli intensity scale, and this quake should have generated level 7 or “very strong” shaking 6 miles from the epicenter.

However, no one in or around the epicenter reported shaking intensity 7 or anything close to it, according to the statement. The damage was limited to minor cracks in some of the drywall and some items falling off the shelves.

“So, there's a lot of confusion about why so many people felt it in the Northeast, (but) much less felt it south to Philadelphia and south to Virginia — much less than people in the Northeast.”

– Wong Young Kim

Instead, three rows of homes were damaged in Newark, New York City residents reported tremors equal to level 4 on the Mercalli scale, and about 150 buildings reported minor damage. Even New Hampshire reported Level 3 shaking about 280 miles away, the statement said.

“So, there's a lot of confusion as to why so many people felt it in the Northeast, (but) much less felt it south to Philadelphia and south to Virginia — much less than people in the Northeast,” Kim told FOX Weather on Friday. “As well as very little damage around the epicenter area.”

The earthquake waves went in a different direction

A typical earthquake would send much of its energy directly to the surface, making the epicenter usually the most dangerous place, Kim says.

But in this case, the energy was sent down until it reached the boundary between the Earth's crust and mantle about 20 miles underground. There the energy wave bounced back, and appeared under the New York City area to cause the big tremor there.

After another downward cycle, the wave reappeared a little weaker in New England, and the cycle repeated until the energy dissipated.

Kim's team's analysis indicates that the quake occurred on a previously unidentified fault running from south to north. But unlike traditional faults that run vertically, this one dips eastward into the Earth at about a 45-degree angle, sending energy in unusual directions.

“In this case, on the 45-degree tilt of the fault, the energy was directed to the northeast and deeper,” Kim told FOX Weather.

How do Northeast earthquakes differ from California earthquakes?

Unlike earthquakes that rock California and the West Coast with relative frequency and are caused by the movement of tectonic plates, the Northeastern quakes are remnants of ancient fault zones that extend back 200 million years, the researchers said.

The study said that was the time when what is now known as Europe separated from North America. Some areas are still settling and adapting after all these years, occasionally causing earthquakes.

“if [the April] “If the earthquake had been a little stronger, or a little closer to New York City, the impact would have been much greater.”

– Wong Young Kim

Kim's colleagues estimate that an earthquake as strong as April occurs about once every 100 years, but they point out that the region could see quakes up to magnitude 6 every 700 years, and magnitude 7 every 3,400 years.

“No one knows whether such earthquakes have or could have occurred in human time,” wrote Kevin Krajicek, author of the press release. “But if that happened, it would be disastrous.”

In the quake's aftermath, researchers placed a temporary network of dozens of seismometers near the quake's epicenter to measure dozens of subsequent aftershocks, hoping to better map the region's faults.

Why are there earthquakes in New Jersey?

FOX 5 NY's Aubrey Puente looks at the Ramapo Fault Line, the largest system of fissures in the Northeast.

The data may lead to the need to reevaluate how future earthquakes will affect the country's densely populated northeast.

“Some (earthquakes) that are not that strong may focus energy toward population centers,” Kim said. “if [the April] If the earthquake had been a little stronger, or a little closer to New York City, the impact would have been much greater. “We need to understand this phenomenon and its implications for ground motion prediction.”

FOX Weather helped contribute to this report.

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