Rapid determination of the shakemaps for the L'Aquila main shock: a critical analysis

This paper describes the progressive generation of the shakemap of the L'Aquila, M(w) 6.3 April 6, 2009, main shock at the Centro Nazionale Terremoti of the INGV. Since 2006 and as part of the national projects funded by the Italian Civil Protection and by the EU SAFER project, the INGV has been determining shakemaps for M >= 3.0 using the USGS-ShakeMap software package and a fully automatic procedure, based on manually revised location and magnitude. Focus of this work is on the importance that the data and the extent of the finite fault have in the determination of faithful ground motion maps. For the L'Aquila main shock, we have found that the data alone are not sufficient to replicate the observed ground motion in parts of the strongly affected areas. In particular, since the station coverage toward the SE where the earthquake rupture propagated is scantier, prompt availability of a rupture fault model would have been important to better describe the level of strong ground motion throughout the affected area. The final maps, obtained using all the data available and a likely estimate of the causative fault, appear to provide a faithful description of the ground motion experienced throughout a large region in and around the epicentral area. A critical review of the various aspects relevant to the generation of the maps indicates that availability of strong motion data in the near source region is critical not only to the generation of the shakemaps but also to more routinely seismological analysis. It follows that data exchange among those institutions acquiring strong motion data is of fundamental importance for rapid characterization of the seismic source and of the area affected by the strong ground motion.