The Problem of Regionalization in the Study of Magnitude-intensity Correspondences in the Mediterranean Area: A Distribution-free Approach

Abstract

—A distribution-free statistical approach is proposed for tackling the problem of regionalization devoted to the study of magnitude-intensity relationships in the Mediterranean area. The training-set constitutes a compilation of more than 2000 earthquakes which occurred in the Mediterranean region since the end of the nineteenth century, when instrumental data became available, with epicentral or maximum intensity ≥ VI and MS magnitude values. Firstly an empirical magnitude-intensity correspondence has been computed for each intensity class by using the entire data set in the assumption of homogeneity at the regional scale. Residuals of such relation have been analyzed by a distribution-free statistical approach in order to evaluate the opportunity of a regionalization able to locally improve the performances of magnitude-intensity relationships. The analysis indicates that data concerning larger earthquakes (intensity ≥ VII) do not suggest the opportunity of zonation, and that unbiased estimates of macroseismic magnitude can also be obtained in the assumption that magnitude-intensity correspondences are uniform over the entire Mediterranean area. Therefore, better constrained relations determined for the entire Mediterranean region should be preferred to ill-defined local ones. As concerns smaller events (intensity VI), the procedure suggests that medium/small-scale lateral variations (on a wavelength lower than 102 km) should be taken into account if an efficient estimate of magnitudes from maximum observed intensity is pursued, but that data presently available are not sufficient to suggest any reliable zonation of the area under study.