Clues for a Relation between Rotational Effects Induced by the 2009 Mw 6.3 L'Aquila (Central Italy) Earthquake and Site and Source Effects

The 2009 M-w 6.3 L'Aquila earthquake produced an impressive number of rotational effects on vertically organized objects such as chimneys, pillars, capitals, and gravestones. We present a dataset of such effects that consists of 105 observations at 37 different sites and represents a compendium of earthquake-induced instances of rotational effects that is unprecedented in recent times. We find that the absolute majority of the reported effects were observed in the epicentral zone and that most of the observations are located where the Mercalli-Cancani-Sieberg intensity is between 7 and 8-9. The evident asymmetry in the distribution of the rotational effects resembles the southeastward directivity of the macroseismic effects and highlights a significant convergence between rotations and damage. Finally, we perform some qualitative analyses to recognize and evaluate which geological and seismological parameters can be significant contributors to local rotations. We find that surface geology and amplification of the seismic motion at each reported location strongly influence the occurrence and the nature of the earthquake-induced rotational effects. Conversely, the contribution of the pattern of slip distribution on the fault plane plays only a secondary role in enhancing the rotational motion at each site.