earthquake in gulf
Photo provided by U.S. Geological Survey

An unusual seismic event shook the Gulf of Mexico on Sunday morning when an earthquake occurred about 120 miles southeast of Grand Isle, according to the United States Geological Survey.

Dozens of people had reported feeling vibrations from the quake as of Sunday evening, their locations spanning from southern Louisiana into Mississippi, Florida and Texas, according to USGS online data

The earthquake was assigned a preliminary magnitude of 4.6 at its epicenter with the strongest vibrations reported in Slidell, Walker and Baker. But even in those areas, the impact was classified as light according to a USGS scale that ranks reports based on a series of questions answered by residents.

Paul Caruso, a geophysicist at USGS, said the earthquake was large considering its location, which doesn't typically see much seismic activity. But the impact was small because it happened in the ocean, reducing the potential for injuries and damage to buildings.

Caruso said scientists don't anticipate significant damage unless an earthquake registers about magnitude 5.5 or higher. The largest earthquake ever recorded was a magnitude 9.5 and occurred in Chile in 1960, according to USGS data.

Caruso also said anything under a magnitude 7 doesn't typically generate a tsunami, which was not a potential risk in this case.

He said all earthquakes are the result of faults in the Earth's crust, though the largest quakes usually happen at fault lines when two plates rub against each other. The location of Sunday morning's seismic movement in the Gulf — about 160 miles southeast of New Orleans — was not on or near a fault line. 

Studying the cause of the earthquake could be difficult considering it happened under the ocean, Caruso said. But scientists sometimes look at wave patterns to learn more about underwater seismic movement.

The last time an earthquake shook the upper Gulf was on Feb. 26, when a magnitude 4.3 occurred almost 500 miles south of Baton Rouge, USGS data show. Others have dotted that area in recent years, with two of the largest — magnitudes 5.3 and 5.9 — in 2006 and another in 1986 that happened in almost the same spot as Sunday's.