Background noise characteristics at the IberArray broadband seismic network

Abstract

The characteristics of the background seismic noise recorded at the IberArray broadband seismic network have been analyzed using power spectral den-sity estimates and their corresponding probability density functions (PDFs). The net-work is composed of 55 stations that have been deployed in southern Iberia and northern Morocco beginning in the summer of 2007. PDFs provide a useful tool for monitoring the network performance, allowing identification of stations with anomalously high noise levels. They can also be used to investigate the major sources of noise at different frequency bands and the seismic background noise variations related to time of day, season, weather, location, and type of installation. At high frequencies (>1 Hz), the main contribution seems to arise from the cultural noise, and therefore significant variations are observed between noise levels at dif-ferent stations based on proximity to populated areas. At microseismic frequencies (0.05-1 Hz), the noise level is more uniform among stations, although sites along the Gulf of Cádiz show a slightly increased level. At long periods, the vertical com-ponent noise level usually lies 15 dB above the new low-noise model and the horizontal components are much noisier, sometimes exceeding the new high-noise model for periods longer than 20 sec. Infragravity waves can be identified along the network for periods close to 100 sec, even for stations located inland. The dis-tribution of the median noise levels across the IberArray network shows a clear correlation with major geological features, such as the Guadalquivir and Gharb sedimentary basins, where noise is high, and the Iberian Massif where the noise levels are lowest. Among the main temporal variations observed we can highlight the diurnal differences in the noise level, particularly significant for high frequencies, and the noise level variations correlated to the significant wave heights in the surrounding water bodies.