On April 5, residents across a wide swath of New York, New Jersey, and Pennsylvania experienced what may have been their first — and possibly not last — earthquake.
By Rebecca Owen, Science Writer (@beccapox)
Citation: Owen, R., 2024, Magnitude 4.8 earthquake shakes the U.S. 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 magnitude 4.8 earthquake. New Yorkers, no strangers to loud and near-constant noise, were flummoxed.
“My first thought was that it might be a boiler explosion or construction accident,” Jamie Canton, a Brooklyn resident, told me. “My neighborhood is mostly old brownstones, and several are being remodeled on my block. As it continued, I realized it may be an earthquake.”
“I felt something like a subway train going by underneath me,” said Lisa Bruno, who also lives in Brooklyn.
And in Wappingers Falls, 80 miles north of New York City, Valerie Whitney felt and heard what seemed like a big truck dumping a pallet of bricks onto her driveway. “It didn’t occur to me that it was an earthquake,” she told me.
On the West Coast, the possibility of an earthquake is part of daily life. But in the highly populated areas surrounding the epicenter of the April 5 quake — 6.5 kilometers (4 miles) from the New Jersey town of Whitehouse Station — earthquakes are not common but they are possible. Between 1950 and today, 40 other earthquakes larger than magnitude 3 have occurred within a 250-kilometer (155-mile) radius, though the April 5 event was the largest.
Many East Coast residents who were surprised by Friday’s shaking have no doubt learned by now that they are living near active faults. Though there were few reports of injuries and damage to buildings and infrastructure this time around, people living in this region are now aware of another hazard to pay attention to. Additionally, much more can be done to build a more complete understanding of the faults existing below the population centers of major East Coast cities.
West Coast versus East Coast
Over the last 30 million years, the western United States has undergone significant deformation and faulting from mountain building — think of the Rockies, says Andy Newman, a geophysicist from Georgia Tech. Another example is the extension and crustal thinning in the Basin and Range Province between Mexico and Utah, Newman notes. There’s been plate boundary-related lateral shear, as evidenced by movement along the San Andreas Fault as well as other more easterly faults and active volcanoes throughout the Intermountain West.
All of this geologic upheaval means that earthquakes feel different on the West Coast. “When seismic waves pass through these recently highly deformed and faulted rocks, the faults and microfractures heat up due to friction, stealing energy away from the continued propagation of the wave farther away,” says Newman. “In stark contrast, the old and thick eastern U.S. crust has far fewer macro- and microfractures, allowing for waves to travel much, much farther.”
In California, a magnitude 4.8 earthquake might not be felt across state borders or even in the next big city. In contrast, earthquakes along the East Coast tend to be felt more widely because the bedrock 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, leading to a wider area in which people experience the earthquake.
In 1886, a moderate-sized earthquake in Charleston, South Carolina was felt as far away as Milwaukee, Wisconsin, says Newman. That’s a distance of more than 1,300 kilometers (800 miles). Across the country and more than 100 years later, California’s 1989 magnitude 6.9 Loma Prieta earthquake south of San Francisco was only felt as far away as Los Angeles, 550 kilometers (340 miles) to the south. “Seismic waves that travel through the continental crust below eastern North America have a pretty easy time traveling through relatively pristine and well-healed old rock,” he says.
Finding the fault
News reports that came out immediately after the shaking connected Friday’s quake to the Ramapo Fault. Spanning almost 300 kilometers (185 miles) through New York, New Jersey, and Pennsylvania, this fault system was active millions of years ago as the Appalachian Mountains formed. Despite its presence and possibility of unleashing earthquakes, it was likely not the culprit for this particular event, Hubbard says.
Credit: Dietmar Rabich via Wikimedia Commons, CC BY-SA 4.0
In fact, the location of the earthquake’s epicenter points to a different source fault when you think about what happened in three dimensions. The epicenter falls northwest of the Ramapo Fault, which dips to the southeast. At depth, this implies that the hypocenter would not align with the Ramapo Fault, says John Ebel, a seismologist at Boston College. “Even though this earthquake was near the Ramapo Fault (especially in map view), the earthquake focal mechanism indicates that the Ramapo Fault did not slip.”
Most East Coast earthquakes are nearly impossible to attribute to a particular fault. Deep bedrock, a lack of surface ruptures, low earthquake magnitudes, and unresolved questions about East Coast earthquake behaviors are all hindrances to researchers creating a fuller picture. “Many such earthquakes cannot be explained by mapped faults,” says Newman. Instead, many earthquakes can only be explained by where they occur relative to other small earthquakes in the region that might hint at a fault plane at depth.
The next earthquake
In the hours and days following the earthquake, several aftershocks have been recorded, the strongest occurring later in the afternoon on April 5. The USGS suggests an 11% likelihood of at least one magnitude 3 aftershock in the next week. The USGS also suggests a 0.2% chance that the April 5 earthquake was actually a foreshock to a larger earthquake. Those estimates are current as of April 10.
“Presumably, [the USGS forecast is] a combination of the time that has already elapsed — the rate of aftershocks decreases quickly in the hours and days after the earthquake — and new data from the aftershock sequence that has already occurred. So, that’s good news,” Hubbard says. Plus, accurate location details about the aftershocks may reveal more information about which fault is responsible.
Even though this most recent earthquake was relatively mild, with no major reports of injuries and damage, many New Yorkers in particular felt on edge about the lack of emergency text notifications. In fact, those alerts came long after the earthquake had ended. In such a densely populated region, more knowledge of the fault systems and their behavior is crucial.
Besides being a major population center, several key pieces of infrastructure in this region could be affected in the case of even a moderately sized earthquake. The Flemington Fault, Hubbard explains, runs underneath several dams with connections to the Round Valley Reservoir, the source of New Jersey’s drinking water. That fault — which is smaller, younger and to the west of the Ramapo Fault — more closely aligns with the type and direction of the fault that slipped on April 5. Hubbard posits that the Flemington Fault is a more likely culprit for the recent earthquake. That fault was discovered in the 1980s by a USGS research team that was studying the geology and potential hazards in possible locations for nuclear reactors.
“This event was probably too small and too far away to cause damage, but a shallow earthquake closer to the dams might be more serious. And, of course, those dams are only a fraction of the infrastructure,” Hubbard says.
In the Pacific Northwest, much is known about the recurrence of monstrous Cascadia Subduction Zone earthquakes through tsunami records and seismic signatures on the land, Newman says. This is in contrast to the incomplete earthquake histories on the East Coast.
“We are largely at a loss to know where (or how big) our next major seismic event [will be],” he says. “What we can say is that the eastern U.S. is really shaken up about once every 100 to 200 years [with quakes that exceed] about a magnitude 6.5, and we are just not ready for it.”
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