A closer look at the New Jersey earthquake rupture can explain the shaking reports

The 4.8 magnitude Tewksbury earthquake surprised millions of people on the East Coast of the United States who felt the shaking from the largest earthquake recorded in New Jersey since 1900.

But researchers noticed something else unusual about the quake: Why did so many people 40 miles away in New York City report strong tremors, while damage near the epicenter seemed minimal?

In research published in The Seismic Record, Young-Hee Kim and colleagues from Seoul National University show how the direction of the earthquake's rupture may have affected who felt the strongest tremors on April 5.

Kim and her colleague and co-author Won Young Kim of Columbia University's Lamont-Doherty Earth Observatory became curious about the strange pattern of shaking after visiting the epicenter area just eight hours after the main shock.

“We expected some property damage — chimneys being knocked down, walls cracking or plaster falling to the ground — but there were no obvious signs of property damage,” the researchers said in an email. “Police officers a few kilometers from the epicenter of the reported earthquake spoke calmly about the shaking from the main shock. It was a surprise reaction by people and homes to a 4.8-magnitude earthquake in the area.”

They added that this “contrasts with the widespread and massive response from residents in and around the New York City area, about 65 kilometers from the epicenter.”

The earthquake received more than 180,000 hair reports — the largest number ever for a single earthquake received by the USGS in its “Did You Feel It?” The app and website, according to a second paper published in The Seismic Record by USGS seismologist Oliver Boyd and colleagues.

An estimated 42 million people felt the quake between Virginia and Maine, Boyd and his colleagues said.

Reports from people southwest of the epicenter, toward Washington, D.C., indicated “weak” shaking on the scale the USGS uses to measure earthquake intensity, while people reporting northeast of the epicenter felt “light to moderate” shaking.

However, based on previous models of earthquake magnitude and intensity developed in the eastern United States, a 4.8 magnitude earthquake should produce very strong shaking about 10 kilometers, or about six miles, from the epicenter.

With this pattern in mind, Kim and his colleagues wanted to look more closely at the direction of the earthquake's rupture. To model the rupture, they turned to a type of seismic waves called Lg waves, due to the lack of nearby seismic monitoring at the time of the main shock. Lg waves are shear waves that bounce back and forth within the crust between the Earth's surface and the crust-mantle boundary.

The resulting model indicated that the earthquake rupture propagated east-northeastward and down the eastward-tilting fault plane. The researchers concluded that the direction of the rupture may have directed the earthquake's shaking away from its epicenter toward the northeast.

In general, earthquakes in the northeastern United States occur as thrust faults along faults that trend north to south. Kim and his colleagues note that the New Jersey quake was unusual because it appeared to be a combination of a thrust and slip mechanism along a likely north-northeast-trending fault plane.

“Earthquakes in eastern North America typically occur along a pre-existing zone of weakness, that is, existing faults,” the researchers explained. “In the Tewksbury area, a hidden fault plane that trends north-northeast and dips moderately can be mapped from the numerous small aftershocks that were detected and located” after the Tewksbury main shock.

Boyd and his colleagues noted that some of the damage was documented by a survey team published by the Extreme Geotechnical Events Survey Society and the National Institute of Standards and Technology. Besides cracks in the drywall and items falling from shelves, the team documented the partial collapse of the stone facade of Taylor's Mill, a pre-Revolutionary War building near Lebanon Township, New Jersey.

Researchers have not yet attributed the quake to a specific fault, but the locations of the mainshocks and aftershocks indicate that the well-known Ramapo fault system in the area was not active during the quake.

The findings could “help us identify the sources of new earthquakes and rethink how stress and pressure are accommodated in the eastern United States,” Boyd said.

He noted that some of the seismometers that the USGS quickly deployed in the area will remain in place for at least five months.

“This could help us study, for example, the mechanisms of how the crust responds to stress from a major tremor in the region, and how aftershock sequences could be productive in the eastern United States,” Boyd explained.

“Good station coverage can also allow us to observe how seismic ground motions vary across a region as a function of size, central distance, and ground structure. Each of these examples can help us better estimate potential seismic hazards.”

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