Abruzzi region (central Italy) producing vast damage in the L'Aquila town and surroundings. In this paper we present the location and geometry of the fault system as obtained by the analysis of main shock and aftershocks recorded by permanent and temporary networks. The distribution of aftershocks, 712 selected events with M L ! 2.3 and 20 with M L ! 4.0, defines a complex, 40 km long, NW trending extensional structure. The main shock fault segment extends for 15-18 km and dips at 45°to the SW, between 10 and 2 km depth. The extent of aftershocks coincides with the surface trace of the Paganica fault, a poorly known normal fault that, after the event, has been quoted to accommodate the extension of the area. We observe a migration of seismicity to the north on an echelon fault that can rupture in future large earthquakes.

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Regressions over a data set of broadband seismograms are performed to quantify the attenuation of the ground motion in the Apennines (Italy), in the 0.25-5.0 Hz frequency band. The data set used in this article consists of over 6000 horizontal-component seismograms from 446 events, with magnitude ranging from M w Ӎ 2 to M w ‫ס‬ 6.0. Waveforms were collected during recent field experiments along the Apennines. Data from two MedNet broadband stations, located in central and southern Apennines, were also used. Seismograms are bandpass-filtered around a set of sampling frequencies, and the logarithms of their peak values are written as AMP(f, r) ‫ס‬ EXC(f, r) ‫ם‬ SITE(f) ‫ם‬ D(r, r , f). ref ref EXC(f , r ref) is the excitation term for the ground motion at the hypocentral distance r ref. SITE(f) represents the distortion of the seismic spectra induced by the shallow geology at the recording site. D(r, r ref , f) includes the effects of the geometrical spreading, g(r), and of a frequency-dependent crustal attenuation Q. It is determined as a piecewise linear function, allowing to consider complex behavior of the regional attenuation. A first estimate of D(r, r ref , f) is obtained using a coda normalization technique (Aki, 1980; Frankel et al., 1990) and used as a starting model in the inversion of the peak values. Then, by trial and error, the empirical D(r, r ref , f) is fitted using a trilinear geometrical spreading, with crossover distances at 30 and 80 km, and the crustal parameter 0.10 f Q(f) ‫ס‬ 130 ; f ‫ס‬ 1.0 Hz ref f ref These results suggest a low-Q crust in the entire Apennines in the 0.25-5.0 Hz range, implying that the seismic hazard in the region may be dominated by the local seismicity. The final section is devoted to highlight the limitations of the formula proposed by Console et al. (1988) to estimate duration magnitudes M d in Italy.

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