Explaining the unusual shaking in New York City from the April earthquake

NEW JERSEY – A new research study shows that the discovery of a new fault line may explain the unusually strong and displaced shaking from April's magnitude 4.8 earthquake, which shook the heart of New Jersey but was felt most strongly miles from the epicenter.

The quake occurred late morning on April 5, centered in Tewksbury Township, but the quake sent shock waves through New York City, 40-50 miles away, and caused some damage in Brooklyn and Newark.

According to estimates by the US Geological Survey, it was the strongest quake in the region since 1884 and shook around 42 million people. According to the USGS, 184,000 people said they felt the quake, a record number for the report.

However, researchers who visited the epicenter were puzzled to find that there was very little shaking and no apparent damage was reported.

“We expected some property damage – cracked chimneys, cracked walls or fallen plaster, but there were no obvious signs,” study co-author Wong-Young Kim of Columbia Climate School's Lamont-Doherty Earth Observatory said in a statement Press release announcing the study results. “We spoke to police officers but they weren't very keen on it. As if nothing had happened. This was a surprising response for a magnitude 4.8 earthquake.”

According to Kim's study, based on existing models, the quake should have caused significant damage at the epicenter. Surface movements generated by quakes are measured on the 10-point modified Mercalli intensity scale, and this quake should have produced level 7 quakes or “very strong” tremors within 6 miles of the epicenter, Kim said.

However, according to the press release, no one reported an intensity 7 tremor or anything similar at or near the epicenter. Damage was limited to minor cracks in some drywall and a few items thrown off shelves.

“So it's very puzzling why many people felt it in the Northeast, but many fewer people felt it south to Philadelphia and further south to Virginia – much fewer than people in the Northeast.” Wong Young Kim

Instead, three rowhouses were damaged in Newark, and New York City residents reported shaking at level 4 on the Mercali scale, and about 150 buildings reported minor damage. Even New Hampshire reported a level 3 earthquake about 280 miles away, the release said.

“So it's very puzzling why a lot of people felt it in the Northeast, but a lot fewer people felt it south to Philadelphia and further south to Virginia – a lot less than people in the Northeast,” Kim said FOX Weather on Friday. “As well as very minor damage around the epicenter area.”

The tremor waves went in a different direction

Kim says a typical earthquake would send much of its energy directly to the surface, usually making the epicenter the most dangerous place.

But in this case, the energy was sent downward until it reached the boundary between the Earth's crust and mantle about 20 miles underground. There the energy wave bounced back up, emerging beneath the New York City area and causing the great jolt there.

After another down cycle, the New England wave reemerged slightly weaker and repeated the cycle until the energy faded.

Kim's team's analysis suggests the quake occurred on a previously unmapped fault that runs south to north. But unlike traditional faults, which run vertically, this one dips into the Earth at an angle of about 45 degrees to the east, sending energy in unusual directions.

“In this case, the energy traveled northeast and to depth on the 45-degree dip of the fault,” Kim told FOX Weather.

How are northeast quakes different from California earthquakes?

Unlike the quakes that shake California and the West Coast with relative frequency and are caused by moving tectonic plates, northeast quakes are remnants of 200-million-year-old fault zones, researchers said.

At that point, what is now Europe broke away from North America, the study says. Some areas continue to settle and reorganize even after years, occasionally causing earthquakes.

“If the April earthquake were just a little stronger or a little closer to New York City, the impact would be much greater.” Wong Young Kim

Kim's colleagues have calculated that an April-magnitude quake occurs about every 100 years. However, they estimate that the region could experience magnitude 6 quakes every 700 years and magnitude 7 quakes every 3,400 years.

“No one knows whether such quakes have or could have occurred in human time,” wrote Kevin Krajick, the author of the press release. “But if you did, it would be disastrous.”

After the quake, researchers placed a temporary network of dozens of seismometers near the epicenter to measure the dozens of subsequent aftershocks, hoping to better map the region's faults.

The data could prompt a need to reassess how future earthquakes would affect the populous Northeast.

“Some (quakes) that aren't even that big could potentially concentrate energy on population centers,” Kim said. “If the April earthquake were just a little stronger or a little closer to New York City, the effect would be much larger. We need to understand this phenomenon and its implications for predicting ground motion.”

FOX Weather contributed to this report.