From a geological perspective, a multitude of processes occur beneath our feet: rock, metals, and chemical substances shift layer by layer. Since gaining direct access to the Earth’s interior is extremely difficult, scientists rely on a range of reasoned inferences to understand what lies beneath us.
Occasionally, genuine surprises and previously unknown discoveries emerge—such as those highlighted in a new study published in the Journal of Geophysical Research: Solid Earth, which analyzes a magnitude 4.8 earthquake that struck Tewksbury, New Jersey, in April 2024.
The most likely culprit for the earthquake appeared to be the Ramapo Fault, which stretches nearly 300 kilometers across the Northeast and Mid-Atlantic regions.
The only problem?
It is oriented at the wrong angle to have triggered the Tewksbury earthquake.
This prompted a team of researchers from Columbia University in the U.S. to conduct a more thorough investigation. After analyzing data on local topography, aftershock locations, and laboratory friction tests, the researchers concluded that there is another fault whose existence was previously unknown.
“The 2024 magnitude 4.8 earthquake in Tewksbury, New Jersey, occurred near the ~300-km-long Ramapo Fault; however, despite being the region’s largest fault, it is improperly oriented for slip under the current tectonic stress regime,” the researchers write. “The upward projection of aftershocks aligns with the topographic (LiDAR) trace of a previously unmapped fault zone located 3.5 kilometers west of the epicenter.” Dubbed the Mountainville Fault by the research team, it is classified as an “immature” basement fault—a fracture within ancient crystalline rock where individual cracks have not yet coalesced into a single, well-developed principal slip surface.
To identify the Mountainville Fault, researchers analyzed 3D seismic data from earthquake aftershocks to pinpoint the fault’s source. They combined this with LiDAR terrain mapping to detect surface ridges obscured by vegetation and conducted field surveys along the fault line to identify characteristic surface expressions of faulting.
Subsequent laboratory studies modeled the geometry, stress conditions, and composition of the Mountainville Fault to confirm whether it could indeed behave in the manner observed during the New Jersey earthquake.
“These structural characteristics align with previously studied earthquake sources in the region, suggesting that immature basement faults may pose a greater seismic hazard in intraplate settings than previously thought,” the researchers write.
However, while the earthquake described in the field records aligns well with the laboratory models, it is not an exact match. The researchers believe additional factors were at play, such as changes in water pressure or the removal of rock mass in a critical area.
“We hypothesize that a localized shallow stress perturbation may have triggered the main shallow earthquake,” the researchers write. “The heterogeneous rock composition of the Mountainville fault may have further exacerbated frictional instability during fault rupture by redistributing normal stresses and concentrating them primarily in quartz-rich zones.”
One interpretation proposed by the research team is that the “misorientation of the Ramapo fault relative to frictional failure” may concentrate stress in the surrounding crust rather than releasing it along the major fault itself. This could increase the likelihood of rupture on nearby, less conspicuous faults.
The researchers suggest that these smaller, obscure (or previously unknown) faults warrant further study—as is planned for the Mountainville fault, now that its existence is known.
“We propose that along the densely populated U.S. East Coast, seismically unstable, immature, and structurally complex faults like the Mountainville fault may represent significant earthquake sources and, consequently, pose previously unrecognized yet critical seismic hazards,” the researchers write.
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