Closer look at rupture in New Jersey earthquake may explain reports of tremors

The 4.8 magnitude Tewksbury earthquake surprised millions of people on the East Coast of the United States, who felt the tremors of this largest earthquake recorded by instruments in New Jersey since 1900.

But the researchers noted something else unusual about the quake: Why were so many people 40 miles away from New York City reporting strong shaking, while the damage was so close as the quake? epicenter Did it look minimal?

on a piece of paper It was published inside Seismic RecordingYoungHee Kim of Seoul National University and colleagues show how the earthquake’s rupture direction may have affected those who felt the strongest shaking on April 5.

Kim and 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 quake’s epicenter region just eight hours after the main shock.

“We expected some property damage, such as chimneys collapsing, 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 away from the reported epicenter calmly described the tremors caused by the main shock. It was a surprising reaction of the public and homes to the 4.8 magnitude earthquake that occurred in the area.”

“This contrasts with the broad and massive response from residents in and around New York City, approximately 40 miles from the epicenter,” they added.

The earthquake garnered more than 180,000 felt reports; This led to the U.S. Geological Survey’s “Did You Feel It?” It is the largest number his research has ever received for a single earthquake. According to a, app and website second article published in Seismic Recording By USGS seismologist Oliver Boyd and colleagues.

The earthquake was felt by an estimated 42 million people between Virginia and Maine, Boyd and 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 from the epicenter felt “light to moderate” shaking.

However, according to previous magnitude and earthquake intensity models developed for the eastern United States, a magnitude 4.8 earthquake would cause very strong shaking about 10 kilometers, or about six miles, from the epicenter.

With this model in mind, Kim and his colleagues wanted to take a closer look at the direction of the rupture in the earthquake. To model the rupture, they turned to a type of seismic wave called Lg waves, due to the lack of nearby seismic observations during the mainshock. Lg waves are shear waves that bounce back and forth within the crust between the Earth’s surface and the boundary between the crust and mantle.

The resulting model showed that the earthquake rupture propagated east-northeastward and downward on an east-dipping fault plane. Researchers concluded that the direction of the rupture could have caused the earthquake to shake from the epicenter toward the northeast.

In general, earthquakes in the northeastern United States occur as thrust faults along north-south trending faults. Kim and colleagues noted that the New Jersey earthquake was unusual because it appeared to be a combination of thrust and strike-slip mechanisms along a possible north-northeast-trending fault plane.

“Earthquakes in eastern North America generally occur along pre-existing weak zones, that is, existing faults,” the researchers explained. After the Tewksbury mainshock “A latent fault plane trending north-northeast and moderately dipping in the Tewksbury area can be mapped from the numerous minor aftershocks detected and located”.

Some damage was documented by an expedition team commissioned by the Geotechnical Extreme Events Exploration Association and the National Institute of Standards and Technology, Boyd and colleagues noted. Along with cracks in drywall and objects falling off shelves, the team documented the partial collapse of the stone facade of Taylor’s Mill, a pre-Revolutionary War structure near the town of Lebanon, New Jersey.

Researchers have not yet attributed the earthquake to a specific fault, but the locations of the mainshock and aftershocks indicate that the region’s well-known Ramapo fault system was not active at the time of the earthquake.

Boyd said the findings “may help us identify new earthquake sources and rethink how stress and strain are encountered in the eastern United States.”

He noted that some seismometers quickly deployed to the region by the USGS will remain in place for at least another five months.

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

“Good station coverage also means how earthquake Ground motions vary across the region as a function of magnitude, epicenter distance, and Earth structure. “And each of these examples can help us better assess potential seismic hazards.”