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Abstract The earthquake size distribution is described by an exponential function governed by the b-value parameter. It has already been proven that the b-value depends on the differential stress and tectonic settings. Here, we propose a new method to group earthquakes using the kinematics of the interseismic geodetic strain rates and horizontal stress directions. We select the Italian peninsula as a case study, and we find that the b-value is significantly larger in the extensional setting than in the compressional one, although these differences are much smaller than previously reported. We also show that spatial fragmentation of uniform tectonic regimes leads to inaccurate b-value estimation due to the undersampling of earthquake size distribution. Given these results, we conclude that stress directions and geodetic data complement other geological or geophysical information and reduce the arbitrariness in drawing zones for a seismotectonic model.