Attenuating intensities

Abstract

The study presents a methodology in which fractile distances for a given macroseismic intensity are used as a point measure on which to do attenuation characterization. The use of statistically derived distances expected not to be exceeded at a given probability avoids ambiguous definition of isoseismal radii, and consequent mis- interpretation of attenuation parameters with a physical content. The utility and definiteness of such a fractile distance is evident when treating many earthquakes, of different magnitudes and for which the macroseismic data-sets are of different sizes. The methodology is applied to 55 Italian earthquakes of the last four centuries, with epicentral intensity ranging from VII to XI MCS. Propagation properties in volcanic districts are peculiar, with a rapid decay of observed intensity from the epicentre. The attenuation properties of the other crustal en- vironments are not clcarly correlated with the geodynamic domain; the earthquakes show a mean behaviour that appears to depend on relatively homogeneous conditions of propagation. Source depth, tectonic style and finiteness of the source are not addressed because only macroseismic surveying had been applied to most of the earthquakes, and they cannot be linked to surficial fault ruptures. The study is intended to provide mean attenuations that can be used in probabilistic seismic hazard studies; the variabiljty of individual earthquake attenuations for a region introduces the need to use different parameters in future deterministic earthquake scenarios.