Statistical Analysis of Intraplate Seismic Clusters: The Case of the NW Iberian Peninsula

The NW Iberian Peninsula is an intraplate region with significant seismicity, but it has rarely been studied. Despite the increase in seismicity during recent years, which has even forced changes to the building codes, a comprehensive statistical analysis of this seismicity has not been carried out. Thus, this study aims to characterize seismicity clusters on an intraplate region, based on comprehensive qualitative and quantitative analysis. The b value of the Gutenberg–Richter law is estimated and the Epidemic Type Aftershock Sequence model is employed. Additionally, qualitative analysis is carried out using the Mogi classification, and the results of this work are compared with those of other statistical studies in the literature. We identify clustered seismic activity in the NW Iberian Peninsula, which is classified into 18 swarms and eight sequences. A high b value and low efficiency of aftershock generation are characteristic of the swarms. These parameters, along with the similarities to other regions, point to fluid migration as the swarm seismic source mechanism. Earthquakes of higher magnitude than swarms and a b value close to 1 define the sequences of this region. The statistical results in sequences (high k, c, and p values) and the similarity to other regions would indicate a mix of tectonic origin and aseismic processes (such as fluid migration or slow slip events) as the driving force of seismic sequences. The presence of crustal fluids associated with fault activity in the NW Iberian Peninsula also supports the hypothesis of fluid migration as the source of seismic events.