Spatial characterization of Agron, southern Spain, 1988–1989 seismic series

During November 1988 to April 1989, a seismic swarm of about 400 earthquakes of magnitude between 0.4 and 4.0 occurred near the village of Agron, Andalusia, southern Spain. The pattern of the energy release versus time clearly shows that the dataset is representative of a seismic swarm, where it is not observable a typical mainshock–aftershock sequence. Basing on the Gutenberg–Richter relationship the catalogue is considered complete down to a magnitude of 1. We revise the dataset by selecting those digital data which were recorded by at least four stations. The revised dataset is relocated using a probabilistic approach based on a Bayesian formalism. The epicenters are barely aligned along two main structures trending NNE–SSW, NW–SE and WNW–ESE, respectively. The focii depth is confined in the 5–20 km interval. Application of a clustering technique based on the principal component analysis (PCA) of the local rupture ellipsoids depicts a spatial clustering of the seismicity according to five main planes, of which only one coincides with the horizontal alignment of epicenters. Cross-correlation among waveforms allows to individuate five different groups of similar events (multiplets). High-resolution estimate of time differences among events belonging to the same multiplet and least-squares adjustment of arrival times allow for precise relative locations, from which we derive the corresponding rupture planes. Focal mechanisms are calculated for three of the five multiplets: for these cases, the planes obtained from the relative locations are coincident with one of the fault plane solutions. The results are consistent with those previously inferred from PCA, and indicate a high variability in the spatial setting of fault planes over a short distance range. These observations are consistent with a fragmentation of the upper crust into small (2–4 km) tectonic blocks, and indicate an heterogenous stress regime, with different stresses acting simultaneously in adjacent areas.