Aftershocks from the New Jersey earthquake continue to shake the ground, helping researchers solve a major mystery

Giant underground rock layers in western New Jersey have been shifting quite a bit — sometimes once a day, sometimes several times a day — all because of a large April 5 earthquake that shook Hunterdon County, shaking homes, stores and office buildings. Hundreds of miles away.

Occasional tremors over the past two weeks were caused by smaller quakes, known as aftershocks, and their number has now risen to nearly 100, according to data from the US Geological Survey, the government agency that monitors earthquake activity around the world. .

While many of the aftershocks in New Jersey were so small that few people noticed them, each quake is closely monitored and provides vital clues to the USGS and a team of researchers from Rutgers University and several other universities.

The main mystery they hope to solve is: Where exactly did the April 5 earthquake occur?

Based on past history and preliminary data emerging from seismometers, researchers are fairly certain that the initial magnitude 4.8 quake originated near the Ramapo Fault, which runs diagonally from southeastern New York State through northern New Jersey into eastern Pennsylvania.

However, no one knows for sure whether the quake occurred directly along the main portion of the Ramapo Fault or on one of several fractures associated with the fault, according to USGS and Rutgers researchers.

“The Ramapo Fault actually splits into different segments that form the boundary between the New Jersey Uplands and the Newark Basin,” said Ken Miller, distinguished professor of geology in the Department of Earth and Planetary Sciences at Rutgers University. “Sometimes they are grouped together and called the Ramapo fault system or boundary fault.”

The main fault runs primarily from northeast to southwest, but some preliminary data indicated that the initial earthquake in early April was moving from north to south, suggesting that the tremor may have occurred on a “divergent,” one of the branches that separated from the main fault. Miller said.

The Ramapo Fault is the culprit on April 5 is not a lock, said Dara Goldberg, a research geophysicist with the USGS. She noted that the Ramapo is just one of several known faults — a fracture or zone of fractures between two masses of rock. Rock – in the Hunterdon County area. There are two other faults, the Tewksbury fault and the Flemington fault.

Dozens of seismic sensors have been installed in the Hunterdon County area of ​​New Jersey to help researchers investigate the source of the April 5 earthquake and several aftershocks. Pictured here is a seismograph that records the seismic activity of an earthquake. Getty Images/iStockphoto

Dozens of earthquake sensors

To help determine the source, strength and duration of aftershocks, the USGS recently installed five new seismometers within a mile or two of the town of Readington, the epicenter of the April 5 earthquake. In addition, researchers from Rutgers, Columbia University, Yale University and the University of Texas are installing up to 100 smaller seismic sensors in the same general area, Miller said.

Placed about 9 inches deep in the ground, these narrow, cylindrical-shaped devices can detect the location and size of even the smallest earthquakes.

All of these sensors should be in place within a few days, and most will remain in New Jersey for a few weeks, Miller said. The five USGS sensors are expected to operate for several months.

The agency noted that USGS monitoring devices, known as “aftershock groups,” were deployed a week and a half ago and are “recording strong movement and high frequencies.”

“The sensors will send real-time data to the USGS National Earthquake Information Center in Golden, Colorado, for analysis,” the agency said. “Some of the ‘sets’ will be self-contained and others will be co-located with flow gauges already in place and monitored by the USGS New Jersey Water Science Center.”

Each aftershock will provide clues about the specific fault or part of the fault that caused the initial quake, Miller and Goldberg said.

“Knowing the details about the earthquake’s source addresses many research questions,” Goldberg noted. “On the one hand, it helps us understand the stress state within the crust at this location. Identifying more aftershocks helps gain insight into the causative fault, as well as the structure of this fault at increasing depth.

She added: “Understanding where earthquakes have occurred in the past can help improve seismic hazard models and indicate where earthquakes could occur again in the future, whether through using seismographs or identifying fault zones.”

This map shows the location of more than 90 small earthquakes or aftershocks that occurred in New Jersey following the 4.8 magnitude earthquake on April 5, 2024. US Geological Survey

More aftershocks to shake New Jersey?

Current USGS forecasts estimate a 1% chance of a magnitude 4 or greater aftershock and a 9% chance of a magnitude 3 or greater aftershock occurring in the next month. Over the next year, there is a 3% chance of a strength of 4 or higher and a 21% chance of a strength of 3 or higher.

Goldberg said New Jersey's strongest aftershock to date — a magnitude 3.7 quake that struck in the early evening on the same day as the first quake — generated more than 12,000 reports from people who felt it. (This quake was originally reported to have a magnitude of 4.0, but its magnitude was later downgraded to 3.7 after new data was analyzed.)

But some smaller aftershocks were felt as well. “We received dozens to hundreds of DYFI (Did You Feel It) responses for several Magnitude 2 aftershocks as well,” Goldberg noted.

The DYFI system allows citizens to report whether they feel any shaking, how strong it is and whether any damage has occurred after an earthquake.

“There are many factors that determine whether a person will feel an earthquake,” Goldberg said. “One factor is size, of course. Another is how deep the quake is (shallow quakes are felt more strongly). It also depends on the geology under your feet and what you might be doing when the quake hits.”

The 4.8 magnitude quake on April 5 was the strongest quake centered in New Jersey since 1938, when an early morning quake of similar magnitude shook the Trenton area. The 1938 earthquake was felt as far north as Jersey City and as far south as Delaware.

Can't see the aftershock graph? click here.

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Len Melisurgo can be reached at [email protected] or X at @LensReality.

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