Abruzzo earthquake of April 2009: seismic sequence,ground motion attenuation, simulation scenario and losses

The April 6, 2009 Abruzzo earthquake was the third strongest earthquake recorded in Italy since 1972, after the 1976 Friuli (Mw 6.4), and 1980 Irpinia (Mw 6.9) ones. This event, considering also the long aftershock sequence, produced the largest amount of experimental data ever obtained in Italy for a single earthquake, including broad-band and strong ground motion recordings, GPS and interferometry data, macroseismic surveys, microzonation mapping, damage surveys, etc. Globally, the earthquake affected a territory of about 2,400 km2 with a population of 140,000, causing high death toll (308 victims and 1,600 injured) and damage (about 23,000 unusable buildings), in particular, to the largest town in the area, L’Aquila, where the macroseismic intensity reached IX degree of the EMS-98 scale. The main shock was recorded close to the centre of L’Aquila by fifty-eight digital accelerometers with very high values of PGA (0.3-0.65 g). The strong motion recordings are clearly affected by source effects and show a SE directivity, with a systematic decrease of PGA and PGV at sites located in the backward direction of the rupture propagation. The predictive equations available in literature, underestimate the PGA values closest to the epicenter and overestimate those in the backward directivity direction. The overestimation of the far data is reduced when considering PGV and lower frequency response spectral values. The response spectra of the recordings closest to L’Aquila town, show very high values of acceleration in the interval 2-10 Hz, corresponding to the fundamental frequencies of most of the buildings in the area. The acceleration spectra, in the short period range, are higher than those considered by the new Italian building code NTC-08. The simulation scenario, available about 30 minutes after the main shock and giving preliminary estimates of the expected damage and losses, underestimates the effective losses subsequently obtained from the field. In this paper, the characteristics of the strong ground motions recorded during the Abruzzo seismic sequence and of their attenuation with distance are presented together with a comparison of the loss simulation scenario with the damage building surveys performed after the earthquake.