Investigating rupture directivity effects on macroseismic intensity data in Italy

Abstract. Macroseismic intensity data represent a fundamental measure of ground shaking, particularly for events lacking instrumental recordings. Empirical Intensity Prediction Equations (IPEs) are fundamental to relate earthquake source parameters (i.e., location and magntiude) to site intensities, and have today a broad range of applications. While residual analysis has become a standard procedure in non-ergodic ground-motion studies to isolate source, site and rupture directivity contributions, similar approaches have not yet been systematically applied to macroseismic intensity data.  This study investigates the potential signature of rupture directivity in macroseismic intensity data of recent Italian earthquakes by adapting the residual decomposition framework commonly adopted in non-ergodic ground-motion studies. The analysis is based on 213 earthquakes in the period 1907-2020 and more than 18,000 intensity data. Residuals are computed using two recent IPEs and decomposed into between-event, locality-to-locality, and remaining aleatory variability terms. The spatial distribution of the aleatory residuals is subsequently investigated for the 31 earthquakes which show uniform azimuthal coverage to identify coherent directional patterns potentially associated with rupture directivity. The results show that the aleatory component represents the dominant source of residual variability and that 8 out of 31 investigated earthquakes exhibit coherent directional patterns consistent with fault strikes and rupture propagation directions. These results indicate that intensity residuals preserve measurable source-related directional information and highlight the potential of residual decomposition as a promising framework for investigating rupture directivity using macroseismic intensity data.