Robustness analysis of HVSR-derived soil fundamental frequency estimates in an urban seismic network

Microtremor data acquired using permanent and temporary seismic deployments within the city of Barcelona were used to systematically analyze the robustness of the soil fundamental frequency and horizontal to vertical spectral ratio (HVSR) amplitude estimates, leveraging the long recording period, in the order of several months, available for most of the stations. The analysis considers different time-window lengths, times of day, and seasons of the year. Overall, the results are consistent; however, we identified specific time intervals in which the results, particularly the amplitude of the HVSR peak, deviate significantly from the mean. We have observed that during these intervals, the seismic energy in the microseismic peak is lower than average, due to reduced sea wave activity. Therefore, we interpret that during the periods of calm seas, the HVSR results may be biased because of insufficient seismic energy. The new set of HVSR estimates has been integrated with existing data to obtain an updated map of the soil fundamental frequency in Barcelona, contributing to improved microzonation and seismic risk assessment in the city. Furthermore, we show that the soil fundamental frequency map serve as a proxy of the thickness of the Cenozoic sedimentary cover, yielding results consistent with previous observations based on variations in the amplitude of ambient seismic noise in the 2–20 Hz band during the period of low anthropogenic activity caused by the lockdown measures decreed during the COVID-19 pandemic. • Horizontal to Vertical Spectral Ratio (HVSR) estimation in urban environments. • Robustness of fundamental frequency estimates. • Influence of oceanic noise on HVSR analysis. • Updated map of soil fundamental frequency in Barcelona, Spain.