NEW YORK (Reuters) – Residents from New Jersey to Connecticut reported feeling earthquake-like shaking on Thursday afternoon, but U.S. seismologists said the vibrations were likely the result of a series of sonic booms.

The U.S. Geological Survey, which monitors earthquake activity, said no quake had struck. The agency reported at least nine sonic booms had been recorded over 90 minutes starting at 1:24 p.m. near Hammonton, New Jersey, about 35 miles southeast of Philadelphia.

On Twitter, users said they felt several tremors, particularly in southern New Jersey.

It was not immediately clear what had caused the sonic booms, which are generated by airplanes traveling in the air faster than the speed of sound.

Some news reports suggested that military aircraft from McGuire Air Force Base, approximately 35 miles north of Hammonton, were the likely source.

But the McGuire base said on Twitter that its training ranges were clear on Thursday and that none of its aircraft are capable of creating sonic booms. In a subsequent post, the base said it was working with local authorities to determine a cause.

Seismologists at the Lamont-Doherty Earth Observatory at Columbia University in New York also confirmed that no earthquake had occurred in the region and that they had measured vibrations and low-frequency sound waves consistent with about eight sonic booms from approximately 1:20 p.m. to 2:40 p.m.

Won-Young Kim, a research professor at the observatory, said residents on the ground likely would not have heard the booms but would have experienced 15 to 20 seconds of shaking during each one.

The tremors produced numerous emergency calls to local police departments, some of which took to Twitter to ask residents not to flood their emergency lines with any more reports.

No damage was immediately reported.

Some Twitter users offered a lighthearted response. One person using the handle @VixenRogue quipped: “Aliens are invading New Jersey. What’s the best way to let them know the other 49 states are just fine with this?”

The reports came two days after residents in Charleston, South Carolina, said they felt tremors. The shaking was likely caused by sonic booms from F-18 fighter jets on a training run from a nearby military base, according to media reports.

Sonic booms are often mistaken for seismic activity, according to the USGS website.

(Reporting by Joseph Ax; Additional reporting by Curtis Skinner in San Francisco; Editing by Andrew Hay and Sandra Maler)