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

We have analysed 721 earthquakes (1.5 less than or equal to m(b) less than or equal to 5.0) of the 1993-1994 Western Almeria (southern Spain) series a nd the following seismicity in the area until 1998. Among the data there ar e several multiplets, events characterized by very similar seismograms at t he short-period stations of the local network. We detected similar seismogr ams using crosscorrelation analysis of the P and S arrivals and classified similar events into families, or clusters. We found 39 multiplet clusters o f 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 ar rival times have been compared for all the possible selections of the maste r event, and adjusted by forming the mean value after removing the outliers . The distribution of the stations does not permit a satisfactory resolutio n of focal depths, but relative epicentres have been determined with an acc uracy of a few tens of metres. Typically they draw well-defined lineaments and show two dominant strike directions: N120 degrees -130 degreesE and N60 degrees -70 degreesE. These directions are coincident with known fault sys tems in the area and with the source parameters of three of the largest eve nts (M-w = 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 se veral tens of metres) are for the most part clearly smaller than the fractu re radii of these events. This indicates repeated slip on the same fault se gment. 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 s egments and the fragmentation of the crust into small (approximately 1 km) tectonic blocks.