Two medium earthquakes rattled the Azores islands within 12 hours, Portuguese weather services and the U.S Geological Survey reported.

A magnitude-5.1 tremor Sunday night had its epicenter 24 kilometers (15 miles) southeast of the Formigos islet. It was preceded by five smaller tremors and followed by another low intensity one, Portuguese weather services said.

A second earthquake at 0832 GMT Monday was reported by the the U.S Geological Survey as having a magnitude of 5.1, while the Portuguese weather services recorded it at 4.8.

The Azores islands, located about 1,500 kilometers (930 miles) off Portugal and prone to frequent seismic activity, have experienced several mid-intensity earthquakes since last week, with no injuries or damages reported.