Monitoring Time Variations in Seismic Noise Amplitude at Permanent Seismic Networks

The quality control process usually followed at broad-band seismic networks includes the calculation of the power spectral density and their probability density functions. These results do not make possible a quick estimation of temporal variations that can result from non-continuous sources of noise, meteorologic phenomena, etc. We propose the use of the SeismoRMS package, originally developed to analyze the seismic amplitude variations associated with the COVID19 lockdown, to monitor the time evolution of seismic noise sources in a permanent network, using as a case example the dataset collected during 2023 by the CA network in NE Iberia. Frequencies above 1 Hz show remarkable differences between the stations, despite sharing similar installation settings. Most of the sites show day/night and working day/weekend variations, suggesting a relevant contribution of anthropic sources, but the amplitude of such variations differs strongly among the sites. Our study allows us to identify specific sources of noise affecting some sites during short and regular time periods, an aspect that needs to be taken into account when evaluating the overall quality of each site. We conclude that a systematic analysis of the amplitude variations at different frequency bands can be a tool of interest for the management of a broad-band seismic network.