Geologists finally explain the strange New Jersey earthquake that rocked the Northeast in April

Seismologist Won Young Kim examines signals from a network of seismometers that monitor earthquakes in New York, New Jersey and New England. Kevin Krajicek/Lamont-Doherty Earth Observatory

On April 5, 2024, people across the East Coast of the United States experienced an unusual earthquake. In Newark, New Jersey, three homes partially collapsed. About 150 buildings in New York City reported minor damage, a sinkhole opened up on Long Island and people noticed shaking as far away as Virginia, New Hampshire and Maine. The US Geological Survey (USGS) estimates that about 42 million people felt the quake.

But in Tewksbury, New Jersey, the epicenter of the 4.8-magnitude quake, residents barely felt any tremors at all.

Scientists were “puzzling a lot” about why people would experience the quake in such remote places, while they saw “very little damage around the epicenter area,” said study co-author Won Young Kim, a Lamont researcher at Columbia University's climate school. – Doherty Earth Observatory, says FOX Weather's Scott Sestek. Now, a recent study published this fall in The Seismic Record offers an explanation.

Map showing the maximum ground speed of the April 5 earthquake around the epicenter in Tewksbury, New Jersey, which is marked with a gold star. The vibration unexpectedly spread far away. Won Young Kim/Lamont-Doherty Earth Observatory

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

Based on the size of the April quake, its fairly shallow depth of 2.9 miles and the geology of the area, models suggest that an area about six miles around the epicenter should feel magnitude 7 shaking on the Modified Mercalli intensity scale — a level described as “very strong.” “

But that's not what happened. Although the area was hit by the largest earthquake since 1884, that did not appear to be the case hours later, when the research team went to survey the epicenter.

“We were expecting some property damage, such as chimneys falling, walls cracking or plaster falling off, but there were no obvious signs,” Kim said in a statement. “We spoke to police officers, but they were not very enthusiastic about it. It was as if nothing had happened. It was a sudden response to a 4.8 magnitude earthquake.”

Map of earthquakes recorded around Philadelphia, New York, and southwestern Connecticut from 1683 to 2008. Only three of them are believed to equal or exceed the April 5 quake: in 1737, 1783, and 1884. From Sykes et al., Paul. Earthquakes. SOC. America, 2008

To find out what happened, the team analyzed the earthquake's Lg waves, which are low-frequency waves of energy that bounce back and forth between Earth's surface and the boundary between our planet's crust and mantle, called the Moho. The analysis found a previously unknown fault running south to north and tilting east at an angle of approximately 45 degrees, according to the statement.

Scientists concluded that the movement that caused the earthquake was fast and complex, and combined two movements: a horizontal sliding of both sides of the fault called “strike slip,” in addition to the passage of one side over the other in a movement called “fault slip.” “Orientation”. While energy from earthquakes usually moves toward areas of lower pressure — such as higher toward the surface at the epicenter — this time was different. The team suggests that the energy followed the fault downward and bounced off the Moho to hit the surface in places like New York City, before bouncing back down and up again to hit other, more distant areas.

Earthquakes on the East Coast are rare, especially compared to the more seismically active West Coast. However, the former is felt more widely due to the geological differences between the regions. The east coast consists mostly of old, cold, dense rocks that are more suitable for seismic waves.

On the West Coast, “the rocks are warmer, so they're more active, and they've seen activity recently, so they're warmer and will attenuate or absorb some of the seismic energy as it passes through that rock,” said Christopher Carchedi, a seismologist at the Carnegie Institution for Science in Washington. who was not involved in the study, she told The Washington Post's Carolyn Johnson in April.

The East Coast is also less riven by fault lines, which can interrupt seismic waves, than the West Coast. The existing fault lines date back to about 200 million years ago, when Europe separated from North America, and they still occasionally see activity, according to the statement.

Shortly after the New Jersey earthquake, Carchedi hypothesized that the unusual motion of the East Coast earthquakes could also be because the Earth's surface was still adjusting at the end of the last ice age, when the region's geology was subjected to much greater weight beneath the glaciers. According to the Washington Post.

Either way, this event serves as a reminder that earthquakes are not limited only to known seismic zones. Kim's team also stresses that more research is needed to understand whether similar unusual earthquakes could target other large population centers in the future.

“if [the April] “If the quake had been a little stronger, or a little closer to New York City, the impact would have been much greater,” Kim says in the statement. “We need to understand this phenomenon and its implications for ground motion prediction.”

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Filed under: Earth's crust, Earth's mantle, Earth's surface, earthquakes, geology, new research, New York City

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