Frequency variation in site response as observed from strong motion data of the L’Aquila (2009) seismic sequence

Abstract

Previous works based mainly on strong-motion recordings of large Japanese earthquakes showed that site amplification and soil fundamental frequency could vary over long and short time scales. These phenomena were attributed to non-linear soil behaviour: the starting fundamental frequency and amplification were both instantaneously decreasing and then recovering for a time varying from few seconds to several months. The recent April 6, 2009 earthquake (M W 6.3), occurred in the L'Aquila district (central Italy), gave us the possibility to test hypotheses on time variation of amplification function and soil fundamental frequency, thanks to the recordings provided by a pre-existing strong-motion array and by a large number of temporary stations. We investigated the intra-and inter-event soil frequency variations through different spectral analyses, including time-frequency spectral ratios and S-Transform (Stockwell et al. in IEEE Trans Signal Process 44:998-1001, 1996. Finally, analyses on noise recordings were performed, in order to study the soil behaviour in linear conditions. The results provided puzzling evidences. Concerning the long time scale, little variation was observed at the permanent stations of the Aterno Valley array. As for the short time-scale variation, the evidence was often contrasting, with some station showing a time-varying behavior, while others did not change their frequency with respect to the one evaluated from noise measurements. Even when a time-varying fundamental frequency was observed, it was difficult to attribute it to a classical, softening non-linear behaviour. Even for the strongest recorded shocks, with peak ground acceleration reaching 0.7 g, variations in frequency and amplitude seems not relevant from building design standpoint. The only exception seems to be the site named AQV, where the analyses evidence a fundamental frequency of the soil shifting from 3 Hz to about 1.5 Hz during the mainshock.

The Role of Site Effects at the Boundary Between Seismology and Engineering: Lessons from Recent Earthquakes

“…Mucciarelli et al (2011b) found a few percent decrease in fundamental frequency and amplification between the largest (M > 4) aftershocks and lesser aftershocks and noise. On the contrary, Puglia et al (2011) did not find any evidence on non-linearity in the response of the coarser, interdigited soils of the Upper Aterno valley. In Christchurch it was possible to observe hardening non-linearity in action.…”

Spectral models for ground motion prediction in the L'Aquila region (central Italy): evidence for stress-drop dependence on magnitude and depth

“…Site amplification effects are relevant for the L'Aquila data set (e.g. Bergamaschi et al 2011;Puglia et al 2011;Di Giulio et al 2014), since the epicentral area is located inside a river valley filled with quaternary deposits of various depths. Geological conditions are so variable that over a distance of few tens of metres the shear wave velocities of the shallower soil layers may differ substantially, as shown by the soil profiles available for some strong-motion stations from the Italian accelerometric archive (ITACA, http://itaca.mi.ingv.it; Luzi et al 2008;Pacor et al 2011).…”

Seismic site characterization of the Kastelli (Kissamos) Basin in northwest Crete (Greece): assessments using ambient noise recordings

“…The HVSR method has been used for site effects assessment in various geomorphological cases and microzonation studies (e.g Chavez-Garcia et al 1995;Teves-Costa et al 1996;Bard 1998;Mucciarelli et al 2003a;Cara et al 2003;Bindi et al 2009;Beroya et al 2009;Gallipoli et al 2011;Strollo et al 2011;Puglia et al 2011;Mucciarelli 2011). HVSR has been also applied in several Basins indicating the contribution of the local geomorphology on seismic ground motion (e.g Teves-Costa et al 1996;Gueguen et al 2000;Lebrun et al 2001;Gosar 2007;Bonnefoy-Claudet et al 2008;Özalaybey et al 2011 ;).…”