Characteristics of strong ground motions from the L’Aquila (Mw = 6.3) earthquake and its strongest aftershocks

Strong motion data during the L’Aquila seismic sequence, were mainly recorded by
the Italian accelerometric network (Rete Accelerometrica Nazionale, RAN), operated
by the Department of Civil Protection (DPC). Several records were obtained also by a
temporary network installed the day after the mainshock by the Istituto Nazionale di
Geofisica e Vulcanologia (INGV). The L’Aquila earthquake is the third strongest
seismic event producing strong-motion records in Italy, after the Irpinia (1980, Mw 6.9)
and Friuli (1976, Mw 6.4) earthquakes. This event, together with its largest aftershocks
(Mw > 4.0) provided a unique strong-motion data set in Italy, especially due to the
amount and intensity of near-fault records. The data set included in the Italian strong
motion database, ITACA, consists of about 300 digital accelerograms (270 of which
belonging to RAN), with a very good signal-to-noise ratio, recorded by about 70
stations, installed on different site conditions at distances ranging from 0 to 300 km.
The national and international relevance of this data set is enhanced by its contribution
to filling gaps in the magnitude-distance distribution of worldwide strong motion
records, especially for normal-fault earthquakes. Near-fault records were obtained by
(i) an array of 6 stations installed by DPC in 2001 in the Aterno Valley to study seismic
site effects; (ii) station AQK close to downtown L’Aquila; (iii) station AQU, belonging
to the broad-band Mednet network, located in the L’Aquila historic castle; (iv) the
stations of the INGV temporary network, installed in the epicentral region one day
after the mainshock. These stations are located less than 5 km from the mainshock
epicenter and are inside the surface projection of the fault rupture. This work presents
an overview of the main features of seismic ground shaking during the L’Aquila
sequence, referring to records of the mainshock and of the two strongest aftershocks.
The dependence of the strong-motion parameters on distance, azimuth and site
conditions as well as the characteristics of near-fault strong-motion records are
discussed