According to the complex stress conditions in the peninsular centre, brittle strength  was calculated in thrust, strike-slip and normal fault regimes, and thus [alpha] was respectively 3.0, 1.2 and 0.75, assuming a coefficient of friction 0.75 (Ranalli, 1995).
Despite the complex stresses in Iberia, this result also agrees with the tectonic active regimes that mainly characterised its interior (Jimenez-Munt and Negredo, 2003; De Vicente et al., 2008; Olaiz et al.--2009); consequently, strike-slip regime ([alpha] in ) is the upper limit to estimate the brittle strength in this region.
When the material body encompasses a total number of i elementary units, the probability density function [g.sub.i]([sigma]) of the minimum value of brittle strength is given by (Cramer, 1946; Gumbel, 1960)
Determination of the median value of brittle strength, [sigma] , is straightforward (Pearson, 1902).
The value of [[beta].sub.RVE] indicates the acceptable scatter limit (= [s.sub.i] / s) for declaring the sample volume representative for brittle strength, and thereby defining it as homogeneous.