Revisión y ampliación del catálogo sísmico del noroeste de la Península Ibérica previo a 1755 y sus implicaciones en la actividad intraplaca

The basis of the seismic hazard assessments is a seismic catalogue as complete as possible. In intraplate regions, it is essential to compile a catalogue covering a wide temporal interval due to the long recurrence periods between large earthquakes. The northwest of the Iberian Peninsula (Galicia, Asturias, Cantabria, Castilla y Leon and north of Portugal) is an intraplate region, which was considered seismically stable in the past, because it is far away from the seismically active plate boundaries. Nevertheless, some seismic crisises such as in Sarria-Triacastela-Becerrea (Lugo) (1995 y 1997; maximum earthquake of magnitude 5.3) expose a moderate seismic activity in this region. In this paper a revision and enhancement of the seismic catalogue prior to 1755 (the worst studied period) is carried out, with the aim of characterizing this intraplate seismicity and to recognize if the seismicity after 1755 is anomalous in this region. For this purpose, we have based on the Spanish official seismic catalogue of the Instituto Geografico Nacional (IGN), the agency responsible for the seismic network and warning. The research was first carried out for the original sources of the earthquakes described in this catalogue, in order to review and assess possible errors in the epicentral location, date or geographical areas affected by the earthquake. In addition to the 13 IGN catalogued earthquakes, 10 from the Ces Fernandez (2015) catalogue and 4 new ones have been included, generating a new catalogue with 27 events. Following a review of the data sources, we modified 6 date parameters, 8 locations and 10 geographical areas where the earthquake was felt. Subsequently, the intensity of 18 earthquakes on two macro-seismic scales (EMS-98 and ESI-07) has been estimated. The resulting EMS-98 values range from IV to X and from VII to XI in ESI-07, similar or even higher than those ocurring after 1755, which have not surpassed intensity VI or VII, as in the crisis of Sarria-Triacastela-Becerrea (Lugo) of 1997. Moreover, the geographical distribution of the events is similar before and after 1755. Taking altogether, such informations indicate that the actual seismicity and the seismicity after 1755 is not an anomalous phenomenon. This paper highlights the necessity of having a seismic catalogue as complete as possible, which should be extended using a multidisciplinary integration of historical and instrumental seismicity and paleosismological studies. For this reason, future studies of active tectonics and paleosismicity should pay attention to seismic faults that may be active under the current tectonic regime.