Calibrating the Bakun Wentworth's Method for the Macroseismic Estimation of Earthquake Parameters in Italy

We present the results of a calibration analysis performed on Italian seismicity datasets of the approach proposed by Bakun and Wentworth (1997) to bound earthquakes epicentral area and magnitude from macroseismic data only. The calibration is required as the method derives an intensity magnitude (equal in the mean to moment magnitude Mw) from macroseismic observations by using a regionally suitable attenuation relationship of intensity as a function of Mw and source distance. To this purpose, a training set of earthquakes occurred in Italy since 1980 was selected, for which a large number of intensity observations and reliable instrumental determinations of Mw and epicentral location are available. Following the Bakun-Wentworth’s method, the distribution of intensity data with epicentral distance was analyzed for each one of the events considered and three different functional relations (linear, logarithmic and cubic dependence on distance) were calibrated. We also considered two better constrained (defined on a much larger dataset) intensity attenuation relationships recently proposed for Italy, originally derived as a function of epicentral intensity I0 and, thus, “converted” through an empirical I0-Mw relation. Performance of all the above relationships was then checked both on the training set and on an independent set of recent Italian earthquakes.