Rupture Directivity of the Damaging Normal Faulting Earthquakes of the Last Twenty Years in Central Apennines.

Rupture directivity can lead to considerable amplification of ground motion in the propagation direction, especially when coupled with structural effects such as basin or low velocity waveguides. It is important to clarify whether earthquake directivity on a given fault system can be predicted statistically or is essentially random. We have faced this issue for normal faulting earthquakes in the Apennines, central Italy. We have determined along-strike rupture directivity of the strongest (Mw≤ 6.5) earthquakes of the 2016-2017 central Italy seismic sequence that filled a seismic gap between the 1997-1998 Umbria-Marche earthquakes to the north and the 2009 L'Aquila and Campotosto earthquakes to the south. The analysis technique is based on assessment of high-frequency S-wave amplitude variations versus source azimuth in an EGF-deconvolution approach. The results of the 2016-2017 earthquakes are compared with those of the two previous seismic sequences. All earthquakes show that along-strike rupture directivity is a persistent feature of normal faulting earthquakes in the Apennines. Different sectors of the Apennines show an alternating trend of preferential directions of rupture propagation along strike, with a significant stability in space and time and independently of magnitude. These results, if confirmed by further data, can lead to a more refined hazard assessment of the investigated region.