Peak Frequency Changes From HV Spectral Ratios in Central Italy: Effects of Strong Motions and Seasonality Over 12 Years of Observations

We analyzed 12 years of continuous seismic data acquired by two seismic stations in Central Apennines (Italy) with the aim to investigate the temporal variations of H/V spectral ratios. The two stations are located in the epicentral areas of the main strong motion earthquakes occurred in Central Italy in the last 12 years (L’Aquila 06 April 2009, Mw 6.1; Amatrice 24 August 2016, Mw 6.0; Visso 26 October 2016, Mw 5.9, and Norcia 30 October 2016, Mw 6.5). H/V ratios are computed both on continuous and earthquakes data recorded by the two stations. H/V ratios are subjected to abrupt changes after strong earthquakes. The results highlight that after strong shaking, there are remarkable drops of the peak frequencies (7%–10% of reduction). After this sudden decrease, the peak frequency slowly recovers to the initial value in a time ranging from a few months to a few years. We also observe a moderate frequency variation (of order of 2%–3%) linked to seasonal variations: the peak frequencies increase in the spring-summer months and decrease during the winter. The frequency changes after strong earthquakes have been interpreted in terms of velocity variations of the shallow layers. This effect must be taken into account for microzonation, geophysical, and engineering studies that are often carried out in the epicentral areas after strong earthquakes. H/V analysis over time proved to be a useful procedure for highlighting temporal changes relating to the properties of shallow layers of the propagation medium and could provide relevant information for the site characterization of permanent seismic stations.