On April 5, 2024, residents along the East Coast of the United States experienced an unusual earthquake. In Newark, New Jersey, three houses partially collapsed, while approximately 150 buildings in New York City reported minor damage. A sinkhole appeared in Long Island. Even people in Virginia, New Hampshire, and Maine felt the tremors. The United States Geological Survey (USGS) estimated that around 42 million people were affected by this earthquake.
However, in the epicenter of this 4.8 magnitude earthquake, residents in Tewksbury Township, New Jersey, hardly felt any shaking at all.
Won-Young Kim, a researcher at the Lamont-Doherty Earth Observatory of the Columbia Climate School and a co-author of the study, told FOX Weather that scientists were puzzled by why areas very distant from the epicenter could feel the earthquake while the epicenter itself experienced “little to no damage.” A study published in the journal “The Seismic Record” on September 18 provided an explanation.
In a “normal” earthquake, the shaking is most severe at the epicenter, which is the point on the Earth’s surface directly above the seismic source or hypocenter, where the earthquake starts within the constantly and slowly shifting Earth’s crust. The shaking typically radiates outward in a concentric pattern from the source.
Based on the magnitude of the April earthquake, its relatively shallow depth of 2.9 miles, and the geological model of the area, it was predicted that an area of about 6 miles around the epicenter should have felt strong shaking of intensity VII on the revised Modified Mercalli Intensity Scale, described as “very strong.”
However, the actual impact was different. Even though it was the largest earthquake in the region since 1884, hours later when the research team arrived to investigate the epicenter, the situation seemed unexpectedly calm.
Kim stated in a statement, “We expected some property damage such as fallen chimneys, cracked walls, or dislodged plaster, but there were no apparent signs.” He said, “We spoke with the police, but they were not too excited. It was like nothing had happened. This was surprising for an earthquake with a magnitude of 4.8 at the epicenter.”
To understand what had occurred, the team analyzed the Lg waves generated by the earthquake, which are low-frequency energy waves that bounce back and forth between the surface and the boundary where the Earth’s crust meets the mantle (known as the Moho). An unknown fault was discovered running north-south and dipping eastward at an angle of about 45 degrees.
Scientists concluded that the motion responsible for this earthquake was both rapid and complex, involving two types of movement: one called “strike-slip,” which is horizontal sliding on both sides of the fault; and the other called “thrust,” where one side moves over the other. Normally, the earthquake energy would move towards the areas of least resistance, such as reaching the surface at the epicenter, but this time it was different. According to the research team, the energy traveled down along the fault and bounced off the Moho, then collided with the surface in places like New York City, and kept bouncing up and down, hitting other distant areas.
Earthquakes on the East Coast are relatively rare, especially compared to the frequent seismic activity on the West Coast. However, due to geological differences, earthquakes on the East Coast are felt over a wider area. The East Coast is primarily composed of ancient, colder, and denser rocks that facilitate the propagation of seismic waves.
Christopher Carchedi, a seismologist at the Carnegie Science Institution in Washington (not involved in the study), told The Washington Post in April, “The rocks are warmer, so they are more active and have undergone recent activity, which weakens or absorbs some of the seismic energy as it passes through those rocks.”
The fault lines on the East Coast are also fewer compared to the West Coast, which could cut off seismic waves. The existing fault lines can be traced back to about two billion years ago when Europe and North America began to separate. Occasionally, geological activity still occurs along these fault lines.
Shortly after the earthquake in New Jersey, Carchedi suggested that such unusual East Coast seismic activity could be due to the ongoing reshaping of the Earth’s surface to accommodate the immense weight this region bore during the end of the last Ice Age when it was under glaciers.
In any case, this event serves as a reminder that earthquakes are not limited to known seismic zones. Kim’s team emphasized the need for further research to understand if similar unusual earthquakes could potentially occur in other densely populated areas in the future.
Kim stated in the release, “If the earthquake in April had been slightly stronger or closer to New York City, the impact would have been greater. We need to understand this phenomenon and its implications for ground motion predictions.”