Separation of source and site effects in ground motions recorded in the village of Onna during aftershocks of the 2009 April 6, Mw 6.1 L’Aquila earthquake

Abstract

The village of Onna, a 10 km apart from L’Aquila, central Italy, was dramatically struck by the 2009 April 6, Mw 6.1 earthquake with 80 per cent of buildings collapsed or severely damaged. The high vulnerability of predominantly ancient buildings and the propensity of site geology to amplify ground motion on the Holocene sediments of the Aterno river valley were unanimously thought as responsible for the huge destruction. To quantify site effects in the damaged zone of Onna and study source scaling over a wide magnitude range (2.0 ≤ ML ≤ 5.4), we have used recordings of 20 stations installed in Onna and other villages around L’Aquila. We analyse more than 1000 seismograms of 202 aftershocks occurring up to source-to-receiver distances of 50 km and infer site and source parameters by means of an inversion procedure. The source spectra inferred from the data inversion confirm the large variability in the high-frequency radiation already found by other authors for L’Aquila earthquakes, with Brune stress drops around 10 MPa at the highest magnitudes of the investigated range and spreading mostly between 0.1 and 1 MPa at smaller magnitudes. Moreover, the inversion of our data yields larger amplitudes of empirical transfer functions in the village of Onna confirming the role of the local geology on damage. The site functions of Onna show a common resonance mode around 2.7 Hz, with amplitudes attaining a factor of 4–5. Moreover, we find that the transfer function amplitude does not decrease below 2 in a large high-frequency band above the site resonant frequency, up to more than 10 Hz. This indicates a further broad-band contribution to the ground motion amplification in Onna. In a simulation of the main shock scenario applying the estimated source scaling to aftershock records, Onna results in the highest accelerations among the villages around L’Aquila. In distinct contrast, transfer functions close to unity in the entire frequency band are found for stations installed on harder rock formations (Mesozoic limestone and Pleistocene siltstone). Their scenario accelerations result in the smallest values, consistently with the lowest macroseismic intensities.