An earthquake on the East Coast was caused by a fault that has been dormant for hundreds of millions of years but was suddenly reactivated

• Friday’s 4.8-magnitude East Coast earthquake came from dormant ancient faults.

• The faults were created when two continents collided about 500 million years ago, creating the Appalachians and the Atlantic Ocean.

• The stress left over from the retreat of the Ice Age glaciers may have reactivated a dormant fault.

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

It may have surprised the millions of New Yorkers and other East Coasters who felt the swing of the 4.8-magnitude quake on Friday morning. The area is not really known for its seismic activity.

But the earthquake did not come from nowhere. Scientists knew of a potential danger deep beneath the Earth’s surface – ancient faults sleeping quietly.

“This is an area where there are older faults, generally inactive, but they can be 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 epicenter of the earthquake appears to have been in New Jersey, preliminary USGS data suggests.

That means it came from a fault region known as the Appalachian fold and thrust belt, which is much older and quieter than the West Coast fault lines.

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

These faults result from the collision of two continents 500 to 300 million years ago. As the continental plates crashed together, closing off pre-existing oceans and opening up the Atlantic Ocean, the plates crashed against each other to form the Appalachian mountains.

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

“There were many, many faults that were active millions of years ago, and those faults are still in the Earth’s crust,” Jobe said. “As tectonic plates move, and stresses move around in the Earth’s crust, every now and then one of those faults will be active rarely or occasionally, just in one earthquake, or in a series of small earthquakes.”

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As far as scientists know, those faults have not been active since their formation “a few 100 million years,” Jobe said.

It remains to be seen if this is just a quiver or a series. Aftershocks are possible in the coming weeks, USGS warns. Any aftershocks will most likely be smaller tremors, but there is a small chance that the aftershock will be of equal or greater magnitude.

Because there are many faults in the Appalachian fold and thrust belt, the USGS is still unable to attribute Friday’s quake to any specific fault, although early data suggests the culprit was about 3 miles deep.

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

This tremor may be a remnant of Ice Age glacier retreat

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

Instead, the East Coast quakes come from stress that adds to those ancient fault lines.

“When that stress is released, they cause earthquakes. And that seems to be what happened this morning in New Jersey,” Ben Fernando, a postdoctoral fellow studying seismology at Johns Hopkins, told BI .

The likely source of that stress, Meng said, is the Earth’s crust rebounding from the weight of 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 bedrock in the crust,” said Fernando.

The glaciers greatly weighed and compressed the northern part of the East Coast. Although that weight has been lifted for over 10,000 years, the Earth’s crust is still going back to its former shape.

“He’s always rebounding,” Meng said. “During that process, it will stress whatever weak structures [are] inside the crust.”

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

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

Read the original article on Business Insider