Insights into tectonic zonation models from the clustering analysis of seismicity in South and South-eastern Spain

Abstract. The South and South-eastern part of Spain exhibits the highest seismicity rate in the country. However, although the recently developed Quaternary Active Fault database of Iberia (QAFI, García-Mayordomo et al. (2012)) collected the available information existing in the study area regarding fault data for their use in seismic hazard applications, this information is of limited use since data quality is very heterogeneous: few earthquakes are associated to specific fault segments and occurrence time periods (when indicated) are affected by high uncertainties (Gaspar-Escribano et al., 2015). This fact has motivated the definition of alternative tectonic zonation models, to be used for evaluating the seismic hazard. So far, the clustering properties have not been considered in this regard, though they can provide essential information about the features of seismic energy release, depending on the tectonic style of a region (Talebi et al., 2024). This is why in this work the properties of the seismicity in terms of clustering are evaluated by applying the Nearest-Neighbor (NN) algorithm on the South-eastern Spain region. The scale parameters needed for the NN algorithm are optimised through the study of the z-score and the temporal anomalies between events in the identified clusters for each run. The tree structure under the graph theory notation has been proved useful in the determination of the critical threshold that separates the background (independent) seismicity from the clustered (dependent) seismicity in the NN algorithm. Once the clusters have been identified, the properties of the clusters have been quantified in terms of a selection of complexity measures: outdegree, closeness, and average node depth. This procedure has been applied by considering two different completeness magnitudes: Mw3.0 (the mean completeness magnitude for the entire catalogue) and Mw2.1 (accounting for the most recent part of the catalogue). The results are similar in terms of proportion of foreshocks, mainshocks and aftershocks, and indicate a clear distinction between the western-most part (higher complexity) and eastern-most part (lower complexity). To check this result, three different zonation models have been examined and cross-compared; two of them passed the Kolgomorov-Smirnov test, meaning the distributions of the selected complexity measures are not the same for the different zones defined in the models. These zonations can be used in order to assess the seismic hazard, as they account for the influence of the tectonic setting on the patterns of earthquakes occurrence, including the features of background and clustered seismicity components.