Classification of seismic strain estimates in the Mediterranean region from a 'bootstrap' approach

The uncertainty that may affect seismic strain estimates in the Mediterrane an is investigated using a distribution-free numerical approach based on th e 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 cove red 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 con tributions from large earthquakes. Our results also indicate that, as a pos sible consequence of this bias, the condition of strain field uniformity is poorly verified in most of the zones considered. The confidence limits obt ained for scalar strains and directions of principal strain axes indicate t hat both the amount and the style of seismic deformation are poorly defined for roughly half of the zones considered, mostly located in the western an d central Mediterranean area. These results suggest that one should be caut ious in using seismic strain estimates when not accompanied by satisfactori ly uncertainty evaluation.