Regression analysis of MCS intensity and ground motion parameters in Italy and its application in ShakeMap

In Italy, the Mercalli-Cancani-Sieberg (MCS) is the intensity scale in use to describe the level of earthquake ground shaking, and its subsequent effects on communities and on the built environment. This scale differs to some extent from the Mercalli Modified scale in use in other countries and adopted as standard within the USGS-ShakeMap procedure to predict intensities from observed instrumental data. We have assembled a new PGM/MCS-intensity data set from the Italian database of macroseismic information, DBMI04, and the Italian accelerometric database, ITACA. We have determined new regression relations between intensities and PGM parameters (acceleration and velocity). Since both PGM parameters and intensities suffer of consistent uncertainties we have used the orthogonal distance regression technique. The new relations are I-MCS = 1.68 +/- 0.22 + 2.58 +/- 0.14 log PGA, sigma = 0.35 and I-MCS = 5.11 +/- 0.07 + 2.35 +/- 0.09 log PGV, sigma = 0.26. Tests designed to assess the robustness of the estimated coefficients have shown that single-line parametrizations for the regression are sufficient to model the data within the model uncertainties. The relations have been inserted in the Italian implementation of the USGS-ShakeMap to determine intensity maps from instrumental data and to determine PGM maps from the sole intensity values. Comparisons carried out for earthquakes where both kinds of data are available have shown the general effectiveness of the relations.