Tremors from April's New Jersey quake have exceeded a freak scale, raising questions about regional risks – State of the Planet

Peak ground velocity from a 4.8 magnitude earthquake beneath the town of Tewksbury, New Jersey, (Gold Star) on April 5, 2024. Shaking was relatively weak at the epicenter, but unexpectedly spread far away, mostly to the northeast. (Courtesy of Won Young Kim)

When a 4.8-magnitude earthquake struck the northern New Jersey town of Tewksbury on April 5, it sparked widespread alarm. Small tremors occur intermittently in the area, but this was the largest since 1884, when a roughly 5.0 magnitude earthquake struck beneath the seafloor off Brooklyn, cracking walls and toppling chimneys.

Based on current models, the earthquake should have caused significant damage at its epicenter, but it did not. Meanwhile, relatively distant New York City shook more strongly than expected, causing damage, albeit minor. Huge tremors extended all the way into Virginia and Maine. A new study suggests why this happens, calling into question some assumptions about regional earthquake risk.

“There was some strange behavior,” said study co-author Won Young Kim of Columbia Climatology School's Lamont-Doherty Earth Observatory.

While a magnitude 4.8 is not considered a major earthquake in global terms, people in the densely populated northeastern United States are not accustomed to anything that strong. The United States Geological Survey (USGS) estimates that about 42 million people felt the quake. The USGS's crowd-sourced online portal for first-person shaking reports received nearly 184,000 entries — the most ever of any U.S. earthquake, according to an accompanying paper about the event. Both papers have just appeared in the journal The Seismic Record.

New damage to John Taylor's 1764 mill in Readington, New Jersey, following the April 5 earthquake. It was built of unreinforced stone and was a mere shell, the only visible casualty being near the epicenter. (gear-nest)

Hours after the earthquake, Kim and his colleagues went to the epicenter to inspect the situation. “We expected some property damage, such as chimneys falling, walls cracking or plaster falling off, but there were no obvious signs,” Kim said. “We spoke to police officers, but they were not very enthusiastic about it. It was as if nothing had happened. It was a sudden response to a 4.8 magnitude earthquake.”

Surface motion caused by earthquakes is measured on the Modified Mercalli intensity scale. Depending on its strength, the depth of the quake (a fairly shallow 5 kilometers, or 2.9 miles) and the geology of the area, current models assume that a 10-kilometre area around the epicenter should experience a magnitude VII tremor on that scale, described as “very strong.” Most well-designed and built structures will likely collapse without significant damage, but other structures of lesser design or materials can collapse, especially masonry walls and unreinforced chimneys.

However, no one in or around the epicenter reported the intensity of the shaking or anything close to it. Damage was limited to minor cracks in some drywall and some items falling off shelves. The only exception: an already dilapidated gristmill built in the 1760s of unreinforced stone, which is already largely a ruin. About 3.5 miles from the epicenter, part of the factory facade fell.

Earthquakes recorded in the Philadelphia, New York, and southwestern Connecticut area, 1683-2008. Only three of them are believed to have equaled or exceeded the April 5 earthquake, in 1737, 1783 and 1884. (From Sykes et al., Bull. Seism. Soc. of America, 2008).

Normally, earthquake shaking fades into a fairly symmetrical bull's-eye pattern from the source. But that didn't happen either; Stronger than expected tremors extended farther afield, mainly to the northeast, and to a lesser extent in other directions.

In Newark, New Jersey, about 20 miles from the epicenter, three homes partially collapsed, and dozens of people had to be evacuated. Residents of New York City, 40 or 50 miles away, reported intense IV motion, with constant shaking of windows, doors, and walls. More than 150 buildings reported minor damage, mostly superficial cracks in the masonry. However, inspectors ordered two buildings in the Bronx to erect protective sheds on sidewalks when cracks appeared in their facades, and a public school in Brooklyn had to close its gymnasium for repairs due to vertical cracks in the form of steps along an interior wall. Gas and water line leaks occurred as far away as the lower Hudson Valley, and on Long Island, the front of someone's Jeep suddenly fell into an open ditch. Even people in parts of New Hampshire, about 280 miles away, reported high-intensity shaking, similar to a large truck passing by.

To understand what happened, Kim and his colleagues at Seoul National University in South Korea analyzed so-called Lg waves. These are a type of low-frequency energy waves that bounce back and forth between the Earth's surface and the Moho, the boundary between the Earth's crust and the mantle, which in this region is located about 35 kilometers down. Analysis indicates that the earthquake occurred on a previously unidentified fault that extends from south to north. The fault is not vertical, but dips eastward into the Earth at about a 45 degree angle.

According to the analysis, the movement was fast and complex—a circular combination of two sides of the fault sliding horizontally against each other (known as strike-slip motion) and one side also pushing itself up and over the other (known as thrust-slip motion). Once the rupture began, it spread horizontally to the north. A large portion of the energy from such an earthquake usually takes the path of least resistance, directly to the surface, where the pressure on the rock is less. This makes the epicenter the most dangerous place.

Seismologist Won Young Kim examines signals from a network of seismometers monitoring the New York, New Jersey and New England area for earthquakes. (Kevin Krajicek)

Researchers say that was not the case here. Instead, most of the energy was directed downward, along the slope of the fault, and continued until it collided with the Moho. Then it bounced back, emerging among other places in New York City that were on the right track. Then the wave bounced back down and reappeared farther in New England, somewhat weaker, and so on until it died out. The long-range echoes are likely enhanced by the fact that most of the rocks beneath this area are solid and dense, transmitting energy efficiently, like the ringing of a bell.

The region from Philadelphia to southwestern Connecticut has experienced about 500 known earthquakes from the 17th century to the present, but many more would almost certainly have gone unnoticed before the advent of modern seismic instruments. Most are very faint, felt by few people, if any, and the vast majority of other earthquakes are harmless. But the threat may be greater than previously thought, according to a previous research paper led by Lamont-Doherty seismologist Lynn Sykes.

Structural geologist Folarin Kolawole has cataloged fractures in the rock created by ancient earthquakes. This photo was taken near the epicenter of the April 5 Tewksbury earthquake shortly after it occurred. Photo: Kevin Krajicek

These earthquakes are not caused by the constant movements of giant tectonic plates like those found in more dangerous places like California. Rather, they emerge from ancient fault zones dating back two hundred million years, when what is now known as Europe separated from what is now known as North America, causing the Earth's interior to crack due to massive earthquakes. Some of these fragmented areas are still settling and adjusting themselves, and sometimes parts of them move with a tremor.

Based on the short historical record, earthquakes with a magnitude of the April earthquake or slightly larger occur about every 100 years. But based on known error sizes and other calculations, Sykes et al. They suggested that the region could experience a magnitude of 6 every 700 years, and a magnitude of 7 every 3,400 years. The magnitude scale is exponential, so a magnitude 6 is ten times stronger than a 5, while a magnitude 7 is 100 times stronger than 5. No one knows whether such earthquakes have occurred in human time or could occur, but if they did, it would It's catastrophic.

In New York's Harriman State Park, geologist William Menke (right) and a student study a precariously balanced rock that has remained in place for more than the past 15,000 years. It could hold evidence of the maximum magnitude of earthquakes during that period. Photo: Kevin Krajicek

The April 5 earthquake sparked a surge in new research. Working with the US Geological Survey and other researchers, Kim helped place a temporary network of dozens of seismometers near the epicenter to monitor aftershocks that continued for weeks. These signals are used to better map the various details of the earthquake and the faults of the area.

Structural geologist Lamont Doherty Folarin Kolawole and his colleagues have mapped several foundation fractures near the epicenter caused by past earthquakes of undetermined ages. They could be millions of years old, Kolawole says, but they could also indicate current, unidentified areas of weakness underlying them.

Meanwhile, Lamont-Doherty geologist William Menke is working to document possible prehistoric earthquakes in the recent past. Harriman State Park in New York, on the border with New Jersey, is filled with giant boulders that fell to the surface when glaciers from the last ice age, about 15,000 to 20,000 years ago, melted. Many are precariously balanced in their original positions. Menke's Hypothesis: If he could calculate the magnitude of the earthquake required to overturn the rocks, he could rule out an earthquake of that size occurring, at least during that time period.

Kim said the new study points to the need to re-evaluate how tremors from any future major earthquake will be distributed across the region. “Some companies that aren't that big can maybe focus their energy toward population centers. If [the April] “If the earthquake had been a little stronger, or a little closer to New York City, the impact would have been much greater,” he said. “We need to understand this phenomenon and its implications for ground motion prediction.”

The study's lead author is Young Hee Kim. Other co-authors are Sangwoo Han, Jun Young Park, and Min Seung Seo, all of Seoul National University.

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