An unusual earthquake beneath New Jersey back in April this year is shaking up our understanding of seismic hazards in the New York tristate area.
At magnitude 4.8, the quake was hardly major in global terms—but it was unusually strong for the region, being felt by an estimated 42 million people and logging a record-breaking 184,000 first-person American reports with the U.S. Geological Survey.
The seismic activity should have caused substantial damage at its epicenter, but barely any was seen. Instead, the impact was felt in New York City—where buildings suffered minor damage—and as far away as Virginia and Maine.
"There was some peculiar behavior," said paper author and seismologist professor Won-Young Kim of Columbia University's Lamont-Doherty Earth Observatory.
"Shaking was relatively intense around New York City, about 45 miles northeast from the epicenter," he told Newsweek.
The reason, the team found, is that the quake was focused downward by the fault, allowing the seismic waves to bounce off the base of the crust and come up further afield than if they'd traveled directly to the surface.
More research is needed, the team said, to see if similar, oddly behaving quakes in the future could put population centers such as New York City at risk.
The quake struck the New Jersey township of Tewksbury on April 5. While small tremors are not uncommon in this part of the world, this was the strongest in 140 years.
That previous episode—a magnitude 5 earthquake with a focus under the seabed, off the coast of Brooklyn—caused walls to crack and chimneys to topple.
The aftermath of the recent quake, however, was very different.
"We expected some property damage—chimneys knocked down, walls cracked or plaster fallen—but there were no obvious signs," said Kim,
"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."
Normally, when an earthquake strikes, tremors fade out in a more-or-less-symmetrical, bull's-eye pattern around the "epicenter," the point on the surface of the earth above the "focus" where the quake originated.
In the New Jersey quake, however, a quite different pattern was observed—with strong shaking reaching far out mainly to the northeast, and diminished activity in other directions.
Twenty miles from the epicenter, In Newark, three rows of houses were damaged by the quake, forcing dozens of people to be evacuated; while 40 to 50 miles out, in New York City, buildings shook and more than 150 reported cracks and other minor forms of damage.
Gas and water lines developed leaks as far away from the epicenter as the lower Hudson Valley, while a sinkhole opened up on Long Island.
To try to explain the pattern of the earthquake's impacts, in their study Kim and colleagues analyzed the paths of so-called Lg waves generated by the fault movement.
These are a type of low-frequency seismic wave that bounces up down between the surface of the earth and the "Mohorovičić discontinuity" (Moho, for short)—the boundary between the crust and the underlying mantle, which lies some 35 kilometers deep under New Jersey.
The team determined that the quake was caused by movement along a previously unknown fault, running in a north-south direction, which is titled eastward in the Earth at an angle of around 45 degrees.
According to the researchers' analysis, the fault's movement, which saw it spread northwards, was both rapid and complex—with a combination of both horizontal (or "strike-slip") and vertical (thrust) relative motion.
Ordinarily, the lion's share of the energy released in a quake shoots straight up to the surface of the Earth; it is this that makes the epicenter the most dangerous place to be during an earthquake.
In the New Jersey quake, however, it appears that the opposite happened. Most of the energy released when the fault slipped headed downward, along the dip of the fault, and carried on until it hit the Moho.
From here, it bounced back up toward the Earth's surface—to be felt in various places around New York City—and down and up again to arrive, weaker, further afield in New England, and so on until it finally faded out.
Extrapolating from the historical record, quakes the size of April's or slightly larger are expected to hit the region around once a century.
Previous research factoring in the sizes of known geological faults, however, has indicated that the area could be hit by magnitude 6 quakes (more than 10 times more powerful than April's movement) around every 700 years, and magnitude 7 quakes (10 times more powerful again) every 3,400 years.
It is unclear whether quakes of such catastrophic severity have occurred in the area in human history, or whether they will in the future.
Regardless, the findings of the new study highlight the need to reconsider how future quakes might be expected to impact the region, Kim said.
"Some that are not even that big could maybe focus energy toward population centers," he warned.
Had the April earthquake been just a little stronger, or a little closer to New York City, he said, its effect could easily have been much greater.
He concluded: "We need to understand this phenomenon and its implications for ground motion prediction."
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References
Sykes, L. R., Armbruster, J. G., Kim, W.-Y., & Seeber, L. (2008). Observations and Tectonic Setting of Historic and Instrumentally Located 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., & Kim, Y. (2024). Rupture Model of the 5 April 2024 Tewksbury, New Jersey, Earthquake Based on Regional Lg‐Wave Data. The Seismic Record, 4(3), 214–222. https://doi.org/10.1785/0320240020
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