2009 April 06 Central Italy Earthquake

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April 07, 2009
April 08, 2009
Strong Motion Seismology Group

A Mw 6.2 earthquake occurred in the central Italy at 01:32:39 UTC on April 06, 2009. According to the USGS preliminary report, the earthquake epicenter is located close to L'Aquila in the central Appenine mountains. The earthquake was generated by the normal faulting along the active Celano-L'Aquila fault system, which extends NW-SE being parallel to the axis of the Appennine mountains. The initial location of the hypocenter according to the USGS and INGV, Italy is lat = 42.334 N, lon = 13.334 E, and depth = 10 km.

To investigate the source process of this earthquake, we have performed the teleseismic waveform inversion of Kikuchi and Kanamori (1991) using displacement records observed at FDSN stations from IRIS DMC. The 34 vertical components of the P-wave data at epicentral distances of 30 - 100 deg were used in the inversion (Fig. 1). According to the aftershock distribution (Fig. 2) and the previous studies of the active fault system in Celano-L'Aquila (e.g., Salvi et al., 2003), we assumed that the fault plane is dipping to the southwest with a dimension of 25 km in length by 15 km in width. We also assumed strike = 147 deg and dip = 44 deg, based on the residuals of the point source analysis and the aftershock distribution. We fixed the epicenter to the location determined by USGS and searched for a focal depth that maximizes the waveform fit. The optimal depth is 6 km as shown in Fig. 3.

In the obtained slip distribution over the fault plane, we can find a single large asperity. The peak slip of about 0.5 m appears on the shallow part of the fault plane. The slip distribution also suggests that the rupture propagated towards the southeast. The results of the inversion are summarized in Fig. 3.

Fig. 1. Stations used for the teleseismic waveform inversion.

Fig. 2. Distribution of aftershocks (black dots) and cross sections along the lines A-A' (red dots) and B-B'(blue dots).

Fig. 3. Source time function, point source solution, slip distribution, and comparison of the obsereved and synthetic displacement waveforms.

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