The relative locations of multiplets in the vicinity of the Western Almería (Southern Spain) earthquake series of 1993-1994

Abstract

We have analysed 721 earthquakes (1.5 ≤ mb ≤ 5.0) of the 1993-1994 Western Almeria (southern Spain) series and the following seismicity in the area until 1998. Among the data there are several multiplets, events characterized by very similar seismograms at the short-period stations of the local network. We detected similar seismograms using cross-correlation analysis of the P and S arrivals and classified similar events into families, or clusters. We found 39 multiplet clusters of 3-33 events. Within each cluster, relocations relative to a master event have been calculated by using the interpolated cross-correlation maxima for the precise relative timing of P and S phases at each station. Relative arrival times have been compared for all the possible selections of the master event, and adjusted by forming the mean value after removing the outliers. The distribution of the stations does not permit a satisfactory resolution of focal depths, but relative epicentres have been determined with an accuracy of a few tens of metres. Typically they draw well-defined lineaments and show two dominant strike directions: N120°-130°E and N60°-70°E. These directions are coincident with known fault systems in the area and with the source parameters of three of the largest events (Mw = 4.8, 3.6 and 4.9), which were estimated from waveform modelling of near-field acceleration records at a single station. Consistent with previous studies, distances within multiplets (typically several tens of metres) are for the most part clearly smaller than the fracture radii of these events. This indicates repeated slip on the same fault segment. It was possible to obtain precise relative locations between several nearby clusters, thereby imaging a very heterogeneous seismotectonic fine structure of the source area, i.e. the positions of adjacent active fault segments and the fragmentation of the crust into small (approximately 1 km) tectonic blocks.