Quake on Long Island, Northeast triggered by stress in shifting plates of Earth's crust

In the hours after an earthquake shook the northeastern United States Friday, scientists calculated its magnitude — 4.8 on the Richter scale — and triangulated its epicenter near the New Jersey town of Whitehouse Station.

But the earthquake’s causes, and the reasons behind why, when, and where it occurred, are more complicated.

“It’s too early to say specifically what might have triggered the earthquake, but we know there are many faults in the area, and we do have faults that can accommodate earthquakes,” said Folarin Kolawole, a Columbia University structural geologist.

Earthquakes happen when blocks of earth suddenly slip past each other. A fault is a fracture, weakness or zone of fractures between the blocks where the slip occurs. The energy released in that slippage travels in waves through the crust and causes the shaking that we feel on the Earth's surface.

Those faults exist in a geological environment that is constantly shifting and under force, though much of the shifting is so gradual as to be imperceptible in human time.

While the area near where the quake occurred is hundreds of miles from the edge of the North American tectonic plate — plates that divide the Earth's crust, where the friction and slippage that produce earthquakes is stronger — it is still marked by hundreds of faults that make up the Ramapo and Flemington fault zones.

Those faults compose what Patrick Fulton, a Cornell University Earth and Atmospheric Sciences assistant professor, described as “a failed rift system” associated with the breakup of the supercontinent Pangea 200 million years ago when North America began to drift away from Europe and Africa. The actual rift, where the breakup took place, occurred under what is now the Atlantic Ocean, but the area of failed rift has been an area of moderate seismic activity in geologically recent times, with a magnitude 3.0 quake in the 1980s and other quakes in the 1700s and 1800s.

The latest U.S. Geological Survey National Seismic Hazard Model predicts a 10% chance of a Modified Mercalli scale Level VI earthquake occurring in a given 50-year span in northern New Jersey. The MMI scale is not the same as the Richter scale; Level VI is intense enough to be felt by all in range and to cause slight property damage.

“The force that’s generating these earthquakes is a large-scale compression in the Northeast to Southwest direction, a squeezing,” said John Armbruster, senior staff associate at Columbia’s Lamont-Doherty Earth Observatory.

Forces at work could include the immense weight of the Earth’s crust, said USGS geophysicist William Barnhart. “That stress gets focused onto faults. There’s also some evidence of glacial rebound from the last ice age, tens of thousands of years ago,” when ice sheets miles thick covered much of what is now the United States and Canada. “The rebound of Canada creates stress,” he said.

Other possibilities, said Stony Brook University geophysicist William Holt, include “mantle convection,” a process by which hot material rises and cold material sinks in the earth’s mantle, affecting tectonics on the crust, and a phenomenon called “ridge push” that transfers force from the edge of a plate inward.

Kolawole offered another possibility: human-induced seismic activity like fracking or quarrying. “If you abandon those quarries, it would allow water to impound; water can flow into a fault zone and cause it to activate,” he said.

Armbruster said there was precedent for that happening in Reading, Pennsylvania, and Wappingers Falls, New York, but Barnhart said there was no evidence of that causing the New Jersey quake. “To the best of our knowledge, this is a completely natural earthquake,” he said.