The East Coast earthquake came from a fault that’s been asleep for hundreds of millions of years but suddenly reactivated

The earthquake that just shook the East Coast was caused by one of dozens of ancient fault lines that have lain dormant for hundreds of millions of years, top scientists announced on Friday.

Millions of New Yorkers and other East Coasters may have been shocked to feel the swaying of the 4.8-magnitude quake Friday morning. The area is not really known for its seismic activity.

But the earthquake didn’t come from nowhere. Scientists knew of a potential hazard deep below the Earth’s surface — ancient faults sleeping quietly.

“This is an area of older, generally inactive faults, but they can become reactivated at any time,” Jessica Jobe, a geologist at the US Geological Survey, said in a briefing on Friday.

Ancient faults from the formation of the Appalachian mountains

The earthquake’s epicenter seems to have been in New Jersey, preliminary USGS data suggests.

Cones cordon off fallen debris from the historic Taylor’s Mill in Lebanon, New Jersey, after the earthquake Friday morning.

Matt Rourke/AP Photo

That means it came from a region of faults called the Appalachian fold-and-thrust belt, which is much more ancient and quiet than fault lines on the West Coast.

“Many of these faults, we don’t know where they are until you have an earthquake,” Lingsen Meng, associate professor of geophysics at UCLA, told Business Insider.

These faults come from the collision of two continents 500 to 300 million years ago. As the continental plates crashed together, closing an existing ocean and opening up the Atlantic Ocean, the plates crumpled against each other to give rise to the Appalachian mountains.

The Appalachian Trail near Hot Springs, North Carolina.

George Rose/Getty Images

That extreme event also opened fault lines in the Earth’s crust throughout the region.

“There were many dozens of faults that were active millions of years ago, and those faults are still present in the Earth’s crust,” Jobe said. “As we have tectonic plates move, and stresses kind of moving around in the Earth’s crust, every now and then one of those faults will become infrequently or intermittently active, just in a single earthquake, or a series of small earthquakes.”

As far as scientists know, those faults haven’t been active since they formed “a couple 100 million years,” ago Jobe said.

It remains to be seen whether this is just one quake or a series. Aftershocks are possible in the coming weeks, USGS warned. Most likely any aftershocks will be smaller quakes, but there is a small chance that an ensuing quake will be of similar or larger magnitude.

Because there are dozens of faults in the Appalachian fold-and-thrust belt, USGS isn’t yet able to attribute Friday’s quake to any specific fault line, though early data suggests the culprit was about 3 miles deep.

The next question is what caused the fault to reactivate on Friday.

This quake may be a remnant of the Ice-Age glaciers’ retreat

On the West Coast, earthquakes often come from movement along the boundaries of tectonic plates. The East Coast, however, is located in the middle of a plate and doesn’t experience much of that movement.

A map shows the boundaries between today’s tectonic plates, in the middle of the Atlantic Ocean.

NOAA

Instead, East Coast quakes come from stress that builds up on those ancient fault lines.

“When that stress is released, they give rise to earthquakes. And that appears to have been what’s happened this morning in New Jersey,” Ben Fernando, a post-doctoral fellow studying seismology at Johns Hopkins, told BI.

A likely source of that stress, Meng said, is the Earth’s crust rebounding from the weight of the glaciers — yes, the glaciers that covered North America during the last Ice Age about 20,000 years ago.

“That was a huge amount of weight that deformed the underlying rock in the crust,” Fernando said.

The glaciers heavily weighed down and compressed the northern part of the East Coast. Though that weight has been lifted for more than 10,000 years, the Earth’s crust is still rising back to its former shape.

“It’s constantly rebounding,” Meng said. “During that process, it will put stress on whatever the weak structures [are] inside of the crust.”

That has slowly put mild stress on the fault lines, he said, but “given enough time, you can still build enough stress to cause the earthquakes.”

Scientists should be able to discern whether glacial rebound contributed to Friday’s tremor as they continue to analyze data.

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