Summary The uncertainty that may affect seismic strain estimates in the Mediterranean is investigated using a distribution-free numerical approach based on the bootstrap resampling technique, applied to more than 2000 seismic source mechanisms of shallow earthquakes that occurred from the Azores to Iran in the period 1905–1999. This analysis shows that the short time interval covered by the available data set may be the main source of uncertainty on long-term strain estimates, since it could imply a biased representation of contributions from large earthquakes. Our results also indicate that, as a possible consequence of this bias, the condition of strain field uniformity is poorly verified in most of the zones considered. The confidence limits obtained for scalar strains and directions of principal strain axes indicate that both the amount and the style of seismic deformation are poorly defined for roughly half of the zones considered, mostly located in the western and central Mediterranean area. These results suggest that one should be cautious in using seismic strain estimates when not accompanied by satisfactorily uncertainty evaluation
