Geologists explain how the direction of the earthquake’s rupture may have influenced the intensity of shaking felt by people on April 5.
The magnitude 4.8 Tewksbury earthquake took many by surprise, affecting millions along the U.S. East Coast and marking the largest instrumentally recorded earthquake in New Jersey since 1900.
Researchers observed another peculiarity regarding the earthquake: why did so many individuals in New York City, located 40 miles away, report feeling strong shaking, while near the epicenter, damage seemed minimal?
In a paper published in The Seismic Record, YoungHee Kim from Seoul National University and her team discuss how the direction of the earthquake’s rupture may have influenced who experienced the strongest shaking on April 5.
Kim and her co-author Won-Young Kim from the Lamont-Doherty Earth Observatory at Columbia University became intrigued by this unusual shaking pattern after visiting the area near the epicenter just eight hours post-mainshock.
“We anticipated some property damage—like fallen chimneys, cracked walls, or plaster on the ground—but there were no visible signs of damage,” the researchers noted in an email. “Police officers within a few kilometers from the epicenter calmly discussed the shaking from the mainshock. Their response was surprising for a magnitude 4.8 earthquake in this area.”
“This was in stark contrast to the widespread and intense reactions from residents in and around New York City, which is about 65 kilometers from the epicenter,” they added.
The earthquake generated over 180,000 felt reports—the highest number ever recorded for a single earthquake on the U.S. Geological Survey’s “Did You Feel It?” app and website, as mentioned in a second paper published in The Seismic Record by USGS seismologist Oliver Boyd and his team.
Boyd and his colleagues stated that an estimated 42 million individuals experienced the quake from Virginia to Maine.
Reports from individuals southwest of the epicenter, towards Washington, D.C., indicated “weak” shaking according to the USGS intensity scale, whereas those northeast of the epicenter perceived “light to moderate” shaking.
Yet, based on existing models for magnitude and intensity in the eastern U.S., a magnitude 4.8 earthquake should typically produce very strong shaking within about 10 kilometers (approximately six miles) from its epicenter.
Given this pattern, Kim and her colleagues aimed to further investigate the earthquake’s rupture directivity. They utilized a type of seismic wave known as Lg waves for modeling the rupture due to the lack of nearby seismic observations at the time of the mainshock. Lg waves are shear waves that travel back and forth within the Earth’s crust between the surface and the boundary separating the crust from the mantle.
Their model suggested that the rupture spread towards the east-northeast and downwards on an east-dipping fault plane. The researchers concluded that the rupture’s direction may have redirected the earthquake’s shaking away from the epicenter and toward the northeast.
Typically, earthquakes in the northeastern U.S. occur along north-south trending faults with thrust faulting. However, Kim and her team noted that the New Jersey earthquake was unusual as it seemed to be a combination of thrust and strike-slip mechanisms along a potentially north-northeast trending fault plane.
“Earthquakes in eastern North America normally happen along existing zones of weakness, meaning established faults,” the researchers clarified. “In the Tewksbury area, a concealed fault plane running north-northeast with a moderate dip can be inferred from the many small aftershocks recorded following the mainshock.”
Some damage was documented by a reconnaissance team from the Geotechnical Extreme Events Reconnaissance Association and the National Institute of Standards and Technology. Their findings included drywall cracks and items that fell from shelves, as well as partial collapse of the stone façade of Taylor’s Mill, a structure dating back to before the American Revolution, located near Lebanon, New Jersey.
The researchers haven’t directly linked the quake to a specific fault, but the positioning of the mainshock and aftershocks suggests that the nearby Ramapo fault system was inactive during the event.
These outcomes could assist in identifying new earthquake sources and reevaluating how stress and strain are handled in eastern United States, Boyd mentioned.
He also pointed out that some seismometers rapidly installed by the USGS in the area will stay in place for at least five months.
“This allows us to investigate, for instance, the mechanisms involved in how the crust reacts to the stress of a mainshock in the region, and how significant aftershock sequences can be in the eastern U.S.,” Boyd elaborated.
“Good station coverage will also enable us to observe the variation in earthquake ground motions across the region depending on magnitude, distance from the epicenter, and underlying Earth structure. Such insights can enhance our understanding of potential seismic hazards.”
