Geologists highlight 'strange behaviour' of New Jersey earthquake

An unusual earthquake that occurred beneath New Jersey in April of this year shook our understanding of seismic hazards in the New York tri-state area.

The 4.8-magnitude quake was not globally significant, but it was unusually strong for the region, felt by an estimated 42 million people and generating a record 184,000 first-person American reports with the US Geological Survey.

Seismic activity was supposed to have caused severe damage at the epicenter, but barely anything was seen. Instead, the impact was felt in New York City — where buildings sustained minimal damage — and as far away as Virginia and Maine.

“There was some strange behavior,” said study author and seismology professor Won Young Kim of Columbia University's Lamont-Doherty Earth Observatory.

“The shaking was relatively intense around New York City, about 45 miles northeast of the epicenter,” he told Newsweek.

The reason, the team discovered, was that the quake was focused downward by the fault, allowing seismic waves to bounce off the base of the Earth's crust and ascend farther than if they had traveled directly to the surface.

The team said more research is needed to see if similar earthquakes with strange behavior in the future could put population centers like New York City at risk.

Seismologist Won Young Kim examines a seismograph. An April 5, 4.8-magnitude earthquake struck New Jersey, but it caused surprisingly little damage near the epicenter. Seismologist Won Young Kim examines a seismograph. An April 5, 4.8-magnitude earthquake struck New Jersey, but it caused surprisingly little damage near the epicenter. Kevin Krajicek/Lamont-Doherty Earth Observatory

The earthquake struck the town of Tewksbury, New Jersey, on April 5. Although small tremors are not uncommon in this part of the world, these were the strongest in 140 years.

That previous episode — a magnitude 5 earthquake whose epicenter was beneath the seafloor, off the coast of Brooklyn — caused walls to crack and chimneys to collapse.

But the effects of the recent earthquake were completely different.

“We expected some property damage, such as chimneys falling, walls cracking or plaster falling off, but there were no obvious signs,” Kim said.

“We talked to police officers, but they weren't very enthusiastic about it. Nothing happened. It was a sudden response to a 4.8 magnitude earthquake.”

New damage to John Taylor's mill in 1764 in Redington Township, New Jersey. This structure – built of unreinforced stone, which was really just a shell – was the only casualty of the April 5 earthquake actually located near… New damage to John Taylor's 1764 mill in Readington Township, New Jersey. This structure – built of unreinforced masonry, which was merely a shell – was the only victim of the April 5 earthquake located close to the epicenter. More from Gear-Nest

Normally, when an earthquake strikes, the tremors fade in a fairly central pattern around the “epicenter,” the point on the Earth's surface above the “focus” where the earthquake originated.

But in the New Jersey earthquake, a completely different pattern was observed, with strong tremors extending mainly to the northeast, and activity diminishing in other directions.

Twenty miles from the epicenter, in Newark, three rows of homes were damaged by the quake, forcing dozens of people to evacuate. While buildings 40 to 50 miles away in New York City shook and more than 150 cracks and other minor forms of damage were reported.

Gas and water lines caused leaks as far away from the epicenter as the lower Hudson Valley, while a crater opened up on Long Island.

To try to explain the pattern of earthquake effects, Kim and his colleagues in their study analyzed the paths of so-called Lg waves generated by fault movement.

These are a type of low-frequency seismic waves that bounce up and down between the Earth's surface and the “Mohorovich discontinuity” (Moho, for short) – the boundary between the Earth's crust and the underlying mantle, which lies about 35 kilometers underground. New Jersey.

The team determined that the quake was caused by movement along a previously unknown fault, running in a north-south direction, which dips east into the ground at an angle of about 45 degrees.

According to the researchers' analysis, the movement of the fault, which saw it propagate northwards, was both rapid and complex – with a mixture of horizontal (or 'striking-slip') and vertical (thrusting) relative movement.

Normally, the lion's share of the energy released in an earthquake goes directly to the Earth's surface; This is what makes the epicenter the most dangerous place an earthquake can be.

But in the New Jersey earthquake, the opposite appears to have happened. Most of the energy was released as the fault slid downward, along the fault's dip, and continued until it hit the Moho.

From there, it bounced back toward the Earth's surface — to be felt in various places around New York City — and down and up again to reach, weaker, farther into New England, and so on until it finally faded away.

Map showing peak ground velocities after the Tewksbury earthquake. Weak around the epicenter, the shaking unexpectedly spread far in the northeast direction. Map showing peak ground velocities after the Tewksbury earthquake. Weak around the epicenter, the shaking unexpectedly spread far in the northeast direction. Won Young Kim/Lamont-Doherty Earth Observatory

Based on the historical record, earthquakes the size of the April quake or slightly larger are expected to hit the region about once every century.

However, previous research that took into account known geological fault sizes has indicated that the region could experience magnitude 6 earthquakes (more than 10 times stronger than April's) approximately every 700 years, and magnitude 7 earthquakes (10 times stronger) about once. Other) every 3400 years.

It is unclear whether earthquakes of this catastrophic intensity have occurred in the region in human history, or whether they will occur in the future.

Regardless, Kim said, the results of the new study highlight the need to reconsider how future earthquakes will affect the region.

“Some areas that are not that large could perhaps focus their energy toward population centers,” he warned.

If the April quake had been a little stronger, or a little closer to New York City, its impact could have been much greater, he said.

He concluded: “We need to understand this phenomenon and its implications for ground motion prediction.”

Do you have advice on a science story Newsweek should cover? Do you have a question about earthquakes? Let us know at [email protected].

References

Sykes, L. R., Armbruster, J. J., Kim, W.-Y., and Sieber, L. (2008). Observations and tectonic settings of historical and effectively mapped earthquakes in the greater New York City–Philadelphia area. Bulletin of the Seismological Society of America, 98(4). https://doi.org/10.1785/0120070167

Han, S., Kim, W., Park, J.Y., Seo, M., and Kim, Y. (2024). Rupture model for the April 5, 2024 earthquake in Tewksbury, New Jersey, based on regional LG wave data. Seismic Record, 4(3), 214-222. https://doi.org/10.1785/0320240020

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