Strong Ground Motion in the Epicentral Region of the Mw 6.3, Apr 6 2009, L’Aquila Earthquake, Italy

In the night of Apr 6 2009, a Mw 6.3 earthquake struck the Abruzzi region and the whole Central Italy, causing about 300 deaths and vast destructions in the town of L'Aquila, one of the largest urban centres in Central Italy, and its surroundings. As most destructive earthquakes in the Italian Central and Southern Apennines mountain chain, this was caused by a normal fault rupture, the epicenter of which was estimated at less than 5 km from the center of L'Aquila. Several 3-components digital strong motion instruments were installed around L'Aquila at few km distance from the epicenter: 3 of them recorded the earthquake along a transept crossing the Aterno river valley, while two of them are located in the town centre. The near-fault conditions, the complex geological setting – L'Aquila lies on a fluvial terrace, consisting of calcareous breccias and conglomerates, lying on the top of lacustrine silty sediments – and the availability of several very good quality near-fault records, make this earthquake an important benchmark that provided an impressive and instructive picture of strong ground motion in the epicentral region of a normal fault earthquake. This paper illustrates some of the most interesting features of the L'Aquila earthquake near-fault dataset, including (i) peak values and spectral ordinates, together with their relationship with respect to some of the most up-to-date ground motion prediction equations, (ii) long period components, (iii) vertical vs horizontal ground motion. Finally, since the L'Aquila shallow subsoil is characterized by frequent natural buried cavities of karst origin, we briefly investigate their potential role on the seismic response.