WASHINGTON - A strong earthquake measuring 6.1 shook northern Argentina on Monday, US researchers reported.

The epicenter of the tremor, which occurred at 4:46 am (3:46 pm local time), was located 111 kilometers (69 miles) east-southeast of the city of Santiago del Estero, according to the US Geological Survey.

Thye depth of the epicenter was 550 kilometers (342 miles).

There were no immediate reports of casualties or damage.

The reading was based on the open-ended Moment Magnitude scale used by US seismologists, which measures the area of the fault that ruptured and the total energy released.