Those rumbles at the Jersey shore? Sonic booms and the occasional earthquake

Along the Jersey Shore, they are called "sonic booms."

And the rumbles and shaky tremors that seem almost as frequent as nor'easters have been known to slide glassware right off shelves and rattle doors and windows. The booms - there have been three this year - often mystify coastal residents about their source.

They have been happening for years.

"It can be a pretty good jolt when you get one," said Jim Eberwine, a former National Weather Service meteorologist who is now the emergency management coordinator for Absecon, a mainland gateway community to Atlantic City.

Many of the explosion-like booms are verified to have occurred during training and testing by regional military installations, while fewer are blamed on seismic activity. And some have never been explained.

Eberwine, who is also a meteorological instructor at Atlantic Cape Community College, says that these days, when he hears one of the booms or gets the report of one, he immediately checks with various sources to try to determine a cause. One resource he uses is the online earthquake map provided by the Lamont-Doherty Cooperative Seismographic Network at Columbia University in New York. The map, which monitors activity 24/7, recorded a magnitude 2.7 earthquake about 200 miles off the coast of New Jersey on Sept. 30.

"I've been here since 1977, and there have probably been at least 10 major incidents involving sonic booms," said Eberwine, who previously lived in Southern California, where there was a greater threat of such tremors from earthquakes rather than military testing. "In between, there have been lots of reports of little tremors here and there."

The latest - the second such occurrence this year - happened this month when a military aircraft was practicing late-afternoon maneuvers off the coast and generated a sonic boom that officials said was felt as far north as Harvey Cedars on Long Beach Island and south to Wildwood, and inland as far as Hammonton.

Sonic booms occur when shock waves are created when an object travels through the air faster than the speed of sound. Waves of pressure are created in front of and behind the object.

Patrick Gordon, a public affairs spokesman for the Naval Air Station Patuxent River in Maryland, said an F-36 Joint Strike Fighter conducted exercises off the mid-Atlantic coast and specifically near South Jersey on Dec. 2. The aircraft was cleared by military officials to "go supersonic" as part of a training mission, but it is unclear whether the aircraft achieved supersonic speed, Gordon said.

The area, about three miles off the coastline between South Jersey and Virginia, is considered a "test track" for military aircraft, Gordon said.

It was the second time in 2016 that the air station at Patuxent was responsible for generating sonic booms in the region, with the previous occurrence reported in January, Gordon said.

At times, sonic noises have also been created by practice maneuvers by the 177th Air National Guard Fighter Wing, which operates out of the Atlantic City International Airport in Pomona. The fighter wing is located in a key position between New York and Washington, and patrols the East Coast, providing combat-ready aircraft and personnel for deployment worldwide.

Fewer sonic booms and tremors in this region are caused by earthquakes, according to Jonathan E. Nyquist, a professor of geophysics at Temple University.

Nyquist said seismic activity does not produce actual sonic booms - but sonic booms can produce earthquake-like tremors that people sometimes mistake for seismic activity.

And while substantial seismic activity is a rarity in the region, small earthquakes have been documented in diaries and newspapers as long as history has been recorded, Nyquist said.

The most substantial East Coast seismic event felt in this region may have been an August 2011 earthquake. The epicenter of the 5.8 magnitude quake was in Louisa County, Va., about 38 miles northwest of Richmond, but the quake was felt up and down the mid-Atlantic, and as far north as the Maritimes in Canada and as far south as Georgia.

Nyquist said one such earthquake hit the Philadelphia region just as Civil War troops were returning from Gettysburg in 1863. It felled the brick chimneys on homes and "stopped pendulums on clocks and pulled glassware off apothecary shop shelves," he said.

"It had people at the time talking about the earthquake and the damage it caused instead of the troops returning from Gettysburg," Nyquist said. "People are always fascinated by the occurrence of such geologic events. . . . They want to know the how and why."

Contact Jacqueline L. Urgo at 609-652-8382 or jurgo@phillynews.com @JacquelineUrgo