This is a low to moderate seismicity zone. The historical catalogue includes some earthquakes located in the Atlantic Ocean and Cantabrian Sea, such as those of 1783 and 1899, senses with lesser intensity of VII, and earthquakes close to the coast senses with intensity VII such as those of 1909 in Viana do Castelo (Portugal), 1910 in El Ferrol and 1920 in Pontevedra.
In the instrumental time we can point out the earthquake of Zamora of 1961 of M5,3 and sense with intensity VI and the three seismic series of Sarria-Becerreá (Lugo) in 1979, 1995 and 1997 with earthquakes of up to M4,6, M4,7 and M5,2 respectively, being VI the maximum intensity in each one of these series. In the region of Sarria-Becerreá where most of the earthquakes in the seismic catalogue are concentrated in this area, which may be related to the faults of Vilachá-Baralla and Becerreá.
The basement of Central and Western Europe is formed by the Varisco Orogen which in the Iberian Peninsula is called the Iberian Massif. The area represented here corresponds to Galicia and part of Asturias, in the NW sector of the Iberian Massif. It is part of a large varicose orogen that extends throughout Europe, from the Guadalquivir River to the Bohemian Massif. In this area, the orogen presents a curvature (Ibero Armorican or Asturian arch) whose nucleus is located in the eastern part of Asturias. The south of Galicia is the terrain that registers more earthquakes in the Iberian Massif.
Of the six areas into which the Iberian Massif is divided, three are in this area: the Cantabrian, the Asturoccidental-Leonese and the Central Iberian.
The Cantabrian zone is a large lateral strip of the varicose orogen. It includes the Cantabrian mountain range (the western sector of the Pyrenean mountain range) which is characterised by superficial tectonics with the presence of dipping and take-off mantles. Stratigraphically, all the Paleozoic systems are represented without metamorphism or tectonic foliation.
The Asturoccidental-Leonese zone presents a powerful series of cambro-Ordovician siliciclastic rocks with verging folds to the east and regional metamorphism of low to medium degree. The western boundary of this zone is the Vivero fault.
The Central Iberian zone is an already more central strip of orogen. It has two domains: "Ollo de sapo" (glandular gneisses) along with syntectonic granitoids and areas with high grade regional metamorphism, and the "Complejo Esquisto-grauváquico" Domain composed of materials from ages prior to the lower Ordovician with low to medium grade regional metamorphism. In this area there are also abundant tardi and post-torogenic granitoids.
On the previous zone, there has been distinguished another one of minor extension and autochthonous character, reduced to Galicia and north of Portugal denominated Galicia-Tras-os-Montes that this structurally riding on the autochthonous relative Centroibérico. The north-south extension of these dismembered outcrops is about 300 km. According to a general pattern of movement, advanced from the west to the east, and within it two domains are distinguished. The Lower Domain (Schistose Domain) has a para-autochthonous character and is composed of Paleozoic metasediments (schists, phyllites, quartzite, lidite and vulcanite) with ages from the Lower Ordovician to the Lower Devonian. The Upper Domain is totally allochthonous and initially rode over the Schistose Domain, later experiencing a greater movement through the Central Iberian Zone that reached the north of Portugal. It is composed by a superposition of diverse units (ofiolitas, máficos and ultramáficos complexes) corresponding to oceanic lithospheres, of ages from the Neoproterozoic to the superior Cambrian. In the Spanish part there are two main complexes: Cabo Ortegal and Órdenes. There is a third with an elongated shape that goes from Malpica, to the north, to Tuy in the south.