Inferring the depth and magnitude of pre-instrumental earthquakes from intensity attenuation curves

Abstract. The Italian historical earthquake record is among the richest worldwide; as such it allows for the development of advanced techniques for retrieving quantitative information by calibration with recent earthquakes. Building on a pilot elaboration of northern Italian earthquakes, we developed a procedure for determining the hypocentral depth of all Italian earthquakes from macroseismic intensity data alone. In a second step the procedure calculates their magnitude, taking into account the inferred depth. Hypocentral depth exhibits substantial variability countrywide but has so far received little attention: pre-instrumental earthquakes were routinely “flattened” at the upper-crustal level (∼10 km), on the grounds that the calculation of hypocentral depth is heavily dependent on the largely unknown local propagation properties. We gathered a learning set of 42 earthquakes documented by reliable instrumental data and by numerous macroseismic intensity observations. We observe (1) that within 50 km from the epicenter the ground motion attenuation rate is primarily controlled by hypocentral depth and largely independent of magnitude, (2) that within this distance the fluctuations in crustal attenuation properties are negligible countrywide, and (3) that knowing both the depth and the expected epicentral intensity makes it possible to estimate a reliable magnitude.