Hundreds of millions of years ago - eons before Long Island even existed - this part of the world routinely seized with earthquakes, volcanoes spewed lava, and the violent collision of tectonic plates thrust up mountains taller than the Himalayas.

"To be able to live here at that time, there would have been countless earthquakes," said William Holt, a geophysicist at Stony Brook University. "A truly big one once a century."

Today? Not so much.

Long Island is one of the most stable geological areas in the world, experts say, squarely inside the massive North American tectonic plate, far from earthquake-producing friction where two plates meet, such as in California and Japan.

While temblors occur daily on Long Island, such local seismic activity is usually imperceptible.

"Within a plate, the geological tectonic formation is subdued to the point that it's often not measurable," said Leonardo Seeber of Columbia University's Lambert-Doherty Earth Observatory in upstate Palisades.

Earthquakes as intense as Tuesday's 3.9 temblor occur hourly somewhere in the world but are rare locally, Holt said. The last comparable quake, a 4.0 magnitude centered in Westchester County, occurred in 1985. The only earthquakes that were more damaging occurred in the 18th and 19th centuries, before the development of the Richter scale.

"The likelihood of a major earthquake here is very, very small," said Vic DiVenere, a geologist at C.W. Post Campus of Long Island University in Brookville. "They could happen, but the time span between significant earthquakes is measured in thousands of years."

Local earthquakes are echoes of the ancient past, occurring in faults - weak points in rock where pressure is concentrated - created more than 200 million years ago as the continents broke apart and drifted, geologists said.

Few of those faults are underneath Long Island, which was formed by glacial deposits about 60,000 years ago.

Tuesday's quake doesn't appear to have been caused by activity on any of the region's known fault lines, such as the Ramapo Fault in New Jersey or the 125th Street Fault in Manhattan. Instead, it appears to have been centered about 80 miles south of the Hamptons, below several miles of sediment near a steep drop from the continental shelf to deep ocean floor.

Its location may hinder identification of the fault, said Won-Young Kim, director of the Lamont-Doherty Cooperative Seismographic Network.

"This is a very difficult case," he said. "We cannot just go out and take a look. It's four miles below the ocean bottom."

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