FAST DETERMINATION OF MOMENT RUPTURE HISTORY: APPLICATION TO THE APRIL 6TH L’AQUILA EARTHQUAKE

Abstract

On April 6th 2009, a magnitude Mw=6.1 earthquake strucks the Abruzzi region in central Italy. Despite its moderate size, the earthquake caused more than 300 fatalities and partially destroyed the city of L’Aquila and many villages in its surroundings. The main shock was preceded by an earthquake swarm which started at the end of 2008. The largest earthquakes of the swarm included a Mw=4.0, occurred on 2009/03/30 at 13:38 (UTC), and Mw=3.9 and Mw=3.5 events that occurred on 2009/04/05 at 20:48 and 22:39 (UTC) respectively. By the end of November 2009, more than 16,000 aftershocks with ML>0.5 have been recorded by the INGV seismic network (Figure 1). Current advances in data transmission and communication yield high quality broadband velocity and strong motion waveforms in near real-time. These data allow for the rapid characterization of earthquake sources in terms of fault geometry, focal depth and seismic moment. For the L’Aquila main shock, the velocimeter data of the Italian National Sei- smic Network (INSN, code IV), MedNet (code MN, station PDG), the North-East Italy Broadband Network (code NI, stations ACOM and PALA) and the SudTirol Province (code SI, station KOSI) were available in real-time. In the following days, the strong motion data of the RAN network (“Rete Accelerometrica Nazionale”) and the displacement data recor- ded by the INGV GPS Network (Anzidei et al. 2009) also become available. In this study we present the results of the rapid source parameters determination procedure developed at the Istituto Nazionale di Geofisica e Vulcanologia (INGV) (Scognamiglio et al., 2009) as applied to the L’Aquila seismic sequence. Our approach consists of two stages – the near real-time determination of the seismic moment tensor, that is already routinely performed for all ML ≥ 3.5 earthquakes, and the rapid imaging of the rupture history on a finite fault for earthquakes with ML ≥ 6.0. We present the moment tensor so- lutions computed for all the earthquakes of the L’Aquila sequence with ML ≥ 3.5, and examine the effect of the velocity structure on the main shock moment magnitude. Then we provide a detailed description of the kinematic source model of the main event by inverting both strong motion and GPS data