When a magnitude 4.8 earthquake hit Tewksbury Township in northern New Jersey on April 5, it raised significant concern, marking the largest quake in the area since 1884. Current models anticipated that this earthquake would cause significant destruction at its epicenter; however, that did not occur. In contrast, the more distant New York City experienced stronger shaking than expected, resulting in some minor damage. Tremors were felt as far south as Virginia and as far north as Maine. A recent study provides a potential explanation for these unusual events, challenging previous assumptions about the regional risks of earthquakes.
The April 5 magnitude 4.8 earthquake in northern New Jersey’s Tewksbury Township alarmed many residents. While small tremors are not uncommon, this was the largest quake since 1884 when an estimated magnitude 5 quake struck under the seabed near Brooklyn, leading to damaged structures.
Expectations based on existing models indicated that the earthquake should have left significant damage in the surrounding area. Surprisingly, this was not the case; however, New York City, situated farther away, experienced unexpected shaking, which, though minor, did result in some damage. The shaking was felt in an unusually wide area, reaching locations from Virginia to Maine. A new study sheds light on these atypical outcomes, questioning some established ideas about the area’s susceptibility to seismic activity.
According to Won-Young Kim, coauthor of the study from Columbia Climate School’s Lamont-Doherty Earth Observatory, “There was some peculiar behavior concerning the earthquake.” While a 4.8 magnitude quake is not considered particularly strong on a global scale, the dense population of the U.S. Northeast is unaccustomed to such seismic events. The U.S. Geological Survey (USGS) reported that the quake was felt by around 42 million people, and a USGS online platform that collects reports received nearly 184,000 submissions, marking it the highest number for any U.S. quake, as noted in a related study recently published in the journal The Seismic Record.
After the quake, Kim and his team visited the epicenter to assess the effects. “We anticipated seeing property damage, like fallen chimneys and cracked walls, but there were no visible signs of destruction,” Kim remarked. Discussions with local police revealed a surprising lack of excitement, as they described little to no impact from the magnitude 4.8 earthquake.
The shaking caused by earthquakes is measured using the Modified Mercalli Intensity Scale. Considering the quake’s magnitude, its depth of about 5 kilometers (2.9 miles), and the area’s geological characteristics, existing models predicted that a 10-kilometer radius around the epicenter would register intensity VII shaking on this scale, referred to as “very strong.” Buildings built to good standards should withstand this kind of shaking with minimal damage, but older or poorly constructed structures, especially unreinforced masonry, could experience severe damage.
However, reports from the epicenter indicated no signs of intensity VII shaking or anything close to it. Damage was limited to some minor drywall cracking and a few items displaced from shelves. The only notable exception was an ancient grist mill from the 1760s, which was already in poor condition; part of its facade collapsed about 3.5 miles from the epicenter.
Generally, the shaking from an earthquake decreases in a circular pattern from the source, but that did not occur in this case. Instead, unexpected stronger shaking radiated primarily to the northeast and, to a lesser extent, in other directions.
In Newark, approximately 20 miles from the epicenter, three row houses partially collapsed, necessitating the evacuation of dozens of residents. People in New York City, located 40 to 50 miles away, reported experiencing intensity IV shaking characterized by vibrations in windows, doors, and walls. Over 150 structures reported minor damage, mainly superficial cracks in brickwork. Additionally, inspectors mandated two buildings in the Bronx to set up protective shed structures when cracks appeared, and a Brooklyn school had to close its gym for repairs due to significant vertical cracks in one of its walls. Gas and water lines experienced leaks as far away as lower Hudson Valley, and on Long Island, a sinkhole unexpectedly opened under a Jeep. Reports of intensity III shaking, likened to a large truck passing by, even came from parts of New Hampshire, nearly 280 miles away.
To delve into the reasons behind this phenomenon, Kim and his colleagues at Seoul National University analyzed low-frequency Lg waves, which oscillate between the Earth’s surface and the Moho—this boundary lies approximately 35 kilometers deep in this region. Their findings indicated that the earthquake likely occurred along an unmapped fault that extends north-south, which is not vertical but dips at about a 45-degree angle into the Earth.
The analysis unveiled a rapid and complex movement, where both sides of the fault shifted horizontally (known as strike-slip motion) while one side also moved upwards over the other (thrust motion). Initially, the rupture spread horizontally to the north. Typically, a considerable portion of a quake’s energy travels upwards to the surface, making the epicenter particularly hazardous.
In this instance, however, much of the energy traveled downwards along the dip of the fault until it met the Moho. It then reflected back upwards, emerging beneath New York City, which lay directly in its path. Following this, the wave descended yet again and resurfaced further into New England, albeit with reduced intensity, and continued until it dissipated. The strength of the distant echoes may be attributed to the region’s dense, sturdy rock formations, which effectively conduct energy—similar to how a bell resonates.
The region from Philadelphia to southwestern Connecticut has recorded around 500 known earthquakes from the 1600s onward, with many more likely unreported before modern seismic technology existed. Most of these quakes are so minor that they go unnoticed, and the vast majority have been harmless. Nevertheless, recent studies, including one by Lamont-Doherty seismologist Lynn Sykes, suggest that the seismic threat may be greater than assumed.
These earthquakes do not result from the active movement of massive tectonic plates like those in high-risk regions such as California. Instead, they originate from ancient fault lines created up to 200 million years ago, when Europe separated from North America, resulting in enormous seismic activity. Some of these fractured areas continue to settle and occasionally shift, causing jolts.
Based on historical data, earthquakes of the size observed in April occur roughly every century. However, according to Sykes and his team’s analysis of the known fault sizes, the region might experience a magnitude 6 quake every 700 years and a magnitude 7 every 3,400 years. The magnitude scale is logarithmic, meaning a magnitude 6 quake is ten times stronger than a magnitude 5, while a magnitude 7 quake is 100 times stronger than a magnitude 5. The occurrence of such strong quakes within human history remains uncertain, but their potential impact would be devastating.
The April 5 event has sparked increased research efforts. In coordination with the USGS and other researchers, Kim has helped set up a temporary network of seismometers near the epicenter to monitor aftershocks that have continued for several weeks. This data is being utilized to create a more detailed map of the quake’s impact and the region’s fault lines.
Structural geologist Folarin Kolawole from Lamont-Doherty and his team are investigating several bedrock fractures near the epicenter, remnants of previous earthquakes whose ages are uncertain. Although these fractures could be millions of years old, they might also indicate currently unmapped weaknesses lurking beneath the surface.
Meanwhile, geologist William Menke is researching evidence of possible prehistoric earthquakes. Harriman State Park in New York, adjacent to New Jersey, is filled with numerous sizable boulders deposited by receding glaciers from the last ice age, around 15,000 to 20,000 years ago. Many of these boulders rest precariously in their current positions. Menke hypothesizes that by analyzing the forces needed to disturb these boulders, he can eliminate the possibility of an earthquake of that magnitude occurring during that era.
Kim noted that the findings from this study indicate the necessity to reassess how seismic activity from future significant earthquakes may be distributed throughout the region. He cautioned that even moderately sized quakes could potentially focus energy into urban centers. “If [the April] earthquake had been slightly stronger or closer to New York City, the consequences could have been much more severe,” he explained. “We need to grasp this phenomenon and its implications for forecasting ground movement.”
The study’s primary author is YoungHee Kim, with co-authors Sangwoo Han, Jun Yong Park, and Min-Seong Seo, all affiliated with Seoul National University.
