A 4.8 magnitude earthquake hits the east coast of the United States

On April 5, residents of a wide swath of New York, New Jersey, and Pennsylvania experienced what was perhaps their first — but perhaps not last — earthquake.

Written by Rebecca Owen, Science Writer (@beccapox)

Citation: Owen, R., 2024, 4.8 Magnitude Earthquake Shakes US East Coast, Temblor, http://doi.org/10.32858/temblor.339

On Friday, April 5, at 10:23 a.m. local time, residents across a wide swath of the East Coast experienced the unexpected: a 4.8-magnitude earthquake. New Yorkers, no strangers to loud, near-constant noise, were startled.

“My first thought was that it could be a boiler explosion or a construction accident,” Brooklyn resident Jimmy Canton told me. “My neighborhood is mostly old brownstones, and many of them are being remodeled on the block I live in. As it went on, I realized it might be an earthquake.

“I felt something like a subway train passing underneath me,” said Lisa Bruno, who also lives in Brooklyn.

Topographic map showing location of New Jersey earthquake. Credit: Tumbler, CC BY-NC-ND 4.0

And in Wappingers Falls, 80 miles north of New York City, Valerie Whitney felt and heard what sounded like a large truck dumping a pallet of bricks onto her driveway. “I never thought it was an earthquake,” she told me.

On the West Coast, the possibility of an earthquake is part of everyday life. But in the densely populated areas surrounding the epicenter of the April 5 quake — 6.5 kilometers (4 miles) from Whitehouse Station in New Jersey — earthquakes are not common but possible. Between 1950 and today, 40 other earthquakes larger than magnitude 3 have occurred within a 250 km (155 mi) radius, although the April 5 event was the largest.

No doubt many East Coast residents who were surprised by Friday's shaking now know they live near active faults. Although there are few reports of injuries and damage to buildings and infrastructure this time, people living in this area now realize that there is another danger to watch out for. Additionally, more can be done to build a more complete understanding of the disadvantages that exist beneath the population centers of major East Coast cities.

West Coast vs. East Coast

Over the past 30 million years, the western United States has been subjected to major deformations and faults due to mountain building—think of the Rocky Mountains, says Andy Newman, a geophysicist at Georgia Tech. Newman points to another example, which is the expansion and thinning of the Earth's crust in the basin and range region located between Mexico and Utah. There was lateral shear associated with plate boundaries, as evidenced by movement along the San Andreas Fault as well as other eastern faults and active volcanoes throughout the Intermountain West.

All this geological unrest means that earthquakes look different on the West Coast. “When seismic waves pass through these deformed and recently fractured rocks, the faults and microfractures heat up due to friction, stealing energy away from the wave's continued propagation away,” Newman says. “In stark contrast, the older, thicker eastern crust of the United States has far fewer large and small fractures, allowing waves to travel much farther.”

In California, a 4.8 magnitude earthquake may not be felt across state lines or even in the next big city. In contrast, earthquakes along the East Coast tend to be felt more widely, because the foundation is stronger and older, says Judith Hubbard, a geologist at Cornell University and co-author of Earthquake Insights. Bedrock can transmit seismic energy more efficiently, resulting in a wider area in which people are exposed to the earthquake.

In 1886, a moderate-sized earthquake occurred in Charleston, South Carolina, and was felt as far away as Milwaukee, Wisconsin, Newman says. This is a distance of more than 1,300 kilometers (800 miles). Across the country, more than 100 years later, California's 1989 magnitude 6.9 Loma Prieta earthquake south of San Francisco was felt only as far away as Los Angeles, 550 kilometers (340 miles) to the south. “Seismic waves traveling through the continental crust beneath eastern North America have a very easy time traveling through relatively pristine, well-healed ancient rocks,” he says.

Find the error

News reports immediately after the tremors linked Friday's quake to the Ramapo fault. This fault system extends approximately 300 kilometers (185 mi) across New York, New Jersey, and Pennsylvania, and was active millions of years ago when the Appalachian Mountains formed. Despite their presence and potential to unleash earthquakes, they likely weren't the cause of this particular event, Hubbard says.

Aerial view of New York City from the Empire State Building. New Yorkers felt the shaking from the New Jersey earthquake. Image source: Dietmar Rabic via Wikimedia Commons, CC BY-SA 4.0

In fact, the location of the epicenter indicates a different source fault when you consider what happened in three dimensions. The epicenter was located northwest of the Ramapo Fault, which dips to the southeast. At depth, this suggests that the epicenter of the explosion would not be aligned with the Ramapo Fault, says John Ebel, a seismologist at Boston College. “Although this earthquake was near the Ramapo Fault (especially in map view), the focal earthquake mechanism suggests that the Ramapo Fault did not slip.”

It is almost impossible for most East Coast earthquakes to be attributed to a specific fault. The deep bedrock, lack of surface ruptures, low earthquake strength, and unresolved questions about East Coast earthquake behaviors are all obstacles for researchers to build a fuller picture. “Many of these earthquakes cannot be explained by mapped faults,” Newman says. Instead, many earthquakes can only be explained by where they occur compared to other small earthquakes in the area that may indicate a fault at depth.

The next earthquake

In the hours and days following the earthquake, several aftershocks were recorded, with the strongest aftershocks occurring later in the afternoon of April 5. The US Geological Survey indicates an 11% chance of at least one aftershock of magnitude 3 occurring next week. The USGS also cites a 0.2% chance that the April 5 quake was actually a harbinger of a larger quake. These estimates are current as of April 10.

“Possible, [the USGS forecast is] A combination of time that has already passed — the rate of aftershocks decreases rapidly in the hours and days after the earthquake — and new data from sequences of aftershocks that have already occurred. “This is good news,” Hubbard says. Additionally, precise location details about aftershocks may reveal more information about the fault responsible.

Although this latest earthquake was relatively mild, with no major reports of injuries and damage, many New Yorkers in particular were concerned about the lack of emergency text notifications. In fact, these alerts came long after the earthquake had ended. In such a densely populated area, having more knowledge of fault systems and their behavior is crucial.

Besides being a major population centre, many parts of the major infrastructure in this region could be affected in the event of a moderate-sized earthquake. Hubbard explains that the Flemington Fault extends under several dams connected to the Round Valley Reservoir, New Jersey's source of drinking water. This fault – which is younger, younger and located to the west of the Ramapo Fault – closely matches the type and direction of the fault that slid on April 5. Hubbard hypothesizes that the Flemington Fault is the most likely cause of the recent earthquake. This fault was discovered in the 1980s by a USGS research team that was studying the geology and potential hazards at potential sites for nuclear reactors.

“This event may have been too small and too far away to cause damage, but a surface earthquake closer to the levees could be more dangerous. Of course, these levees are only a small part of the infrastructure,” Hubbard says.

In the Pacific Northwest, much is known about the frequency of monster earthquakes in the Cascadia Subduction Zone from tsunami records and seismic signatures on the ground, Newman says. This is in contrast to the patchy earthquake history on the East Coast.

“We are pretty much at a loss to know where (or how big) the next big seismic event will be [will be]”,” he says. “What we can say is that the eastern United States actually shakes once every 100 to 200 years.” [with quakes that exceed] “At approximately 6.5 degrees, we are not prepared for that.”

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