A strong earthquake struck Puerto Rico early Tuesday morning, the latest in a series of quakes to hit the U.S. territory in recent days.

The 6.4-magnitude quake was located near the southern coastal city of Ponce, according to the U.S. Geological Survey, revising its initial reading of 6.6-magnitude. The Pacific Tsunami Warning Center issued an alert immediately after the quake was reported, but it was later cancelled.

The Puerto Rico Electric Power Authority says power was been shut off across the the entire island after the auto protection systems in all of its power plants were activated.  Governor Wanda Vazquez says all non-essential government employees have been given the day off.  

The island has been shaken by numerous quakes of varying degrees since the night of December 28.  A 5.8-magnitude quake on Monday leveled several homes in the southern coastal town of Guanica and destroyed a coastal rock formation known as Punta Ventana in nearby Guayanilla.  The formation, shaped like a round stone window, was a popular tourist attraction.