Spatial analysis of the La Paca, SE Spain, 2005 seismic series through the relative location of multiplets and principal component analysis

On 29 January 2005, a MW 4.8, Imax VI-VII, earthquake occurred near the village of La Paca, SE Spain. The aftershock sequence shows distinct heterogeneity, manifested in a non-uniform temporal decay of activity and different source mechanisms for the main shock and the largest aftershocks. We analysed 262 earthquakes of the seismic series in order to characterize the active seismic structures. Moving-window cross-correlation of the P- and S-waveforms recorded at local broad-band stations and the application of a selection algorithm have allowed to identify events with similar waveforms (multiplets) and to classify them into 16 clusters of 3-25 earthquakes each. In each multiplet cluster, events have been relocated with respect to a master event by the linear inversion of cross-correlation time lags for P- and S-waves. Clusters show epicentral alignments with a predominant direction N145°E and two secondary directions: N6°E and N40°E. Spatio-temporal patterns of the seismicity have been analysed using principal component analysis (PCA), upon the relocated epicentre distribution and we have obtained three trends, N42°E, N14°E and N145°E, present during all the sequence. These directions inferred from relative location and PCA are coincident with nodal planes of moment tensor solutions for the larger events within the series. They can be interpreted as the simultaneous activity of conjugate strike-slip faults under a N160°-170°E orientation of the principal compressive stress σ1.