Frequency variation in site response over long and short time scales, as observed from strong motion data of the L’Aquila (2009) seismic sequence

摘要

Previous works based mainly on strong-motion recordings of largeJapanese earthquakes showed that site amplification and soil fundamental frequencycould vary over long and short time scales. These phenomena were attributedto non-linear soil behaviour due to inelastic, softening non-linearity: thestarting fundamental frequency and amplification were both decreasing and notrecovering for a time varying from few hours to several months. The recent April6th 2009 earthquake (MW 6.3), occurred in the L'Aquila district (central Italy),gave us the possibility to test hypotheses on time variation of amplification functionand soil fundamental frequency, thanks to the recordings provided by a preexistingstrong-motion array and by a large number of temporary stations. We performedspectral ratio studies for the permanent stations of the Aterno Valley arraywhere a reference station was available. The temporary stations and permanentones were studied using time-frequency analyses through the S-Transform approach(Stockwell et al., 1996). Finally, analyses on noise recordings were performed,in order to study the soil behaviour in linear conditions. The results providedpuzzling evidences. Concerning the long time scale, little variation wasobserved at the permanent stations of the Aterno Valley array. As for the shorttime-scale variation, the evidence was often contrasting, with some station showinga time-varying behavior, while others did not change their frequency with respectto the one evaluated from noise HVSR. Even when a time-varying fundamentalfrequency was observed, it was difficult to attribute it to a classical,softening non-linear behaviour. Even for the strongest recorded shocks, with PGAreaching 0.7 g, variations in frequency and amplitude seems not relevant frombuilding design standpoint. The only exception seems to be the site named AQV,where the analyses evidence a fundamental frequency of the soil, shifting from 3Hz to about 1.5 Hz during the mainshock.