The earthquake, with its epicentre 16 kilometres northeast of Melgaço, was recorded at 04:31 am on 17 August in the mainland's Seismic Network stations, the IPMA said in a statement.
This shock did not cause personal or material damage and was felt with maximum intensity III/IV (modified Mercalli scale) in the municipalities of Arcos de Valdevez and Melgaço, in the district of Viana do Castelo.
It was also felt with lesser intensity in the Paredes de Coura municipality, also in the Viana do Castelo district.
Earthquakes are classified according to their magnitude as micro (less than 2.0), very small (2.0-2.9), small (3.0-3.9), slight (4.0-4.9), moderate (5.0-5.9), strong (6.0-6.9), large (7.0-7.9), important (8.0-8.9), exceptional (9, 0-9.9) and extreme (greater than 10).
The maximum intensity III (Weak) of the Mercalli scale means that the earthquake was "felt indoors". In these situations, “hanging objects shake and the vibration is similar to that caused by the passage of heavy vehicles”.
In intensity IV (Moderate), "hanging objects sway, parked cars sway, windows, doors and crockery shake, while glass and crockery rattle or tinkle", according to the IPMA.