Ambient Noise Levels in Greece as Recorded at the Hellenic Unified Seismic Network

We investigate the characteristics of ambient noise across Greece as recorded at the Hellenic Unified Seismic Network (HUSN). Power spectral densities (PSDs) and their corresponding probability density functions (PDFs) have been estimated for 110 broadband seismic stations using the continuous waveform data for a four‐year period from 2007 to 2010. Using PDFs we monitor and show network performance in terms of overall station quality and the level of noise at each site. At high frequencies (>7 Hz), the main source of noise is cultural with strong diurnal variations. Stations with constantly increased noise levels across this band indicate poor vault construction or poor site selection and provide an indication to the network operators for possible structural improvements or even station relocations. The microseismic noise levels show a clear seasonal variation at all stations. Specifically, for the double‐frequency (DF; period range 4–8 s) band, the average noise level differs between stations on the mainland and those located on the islands, reaching a value as high as 10 dB. Furthermore, the DF noise peak is observed at all of the HUSN stations, and it is correlated well with local sea wave height measurements at buoys deployed in the Aegean and the Ionian seas. This indicates that the HUSN seismic network also monitors local sea–weather conditions within a range of a few hundred kilometers. The longer period single‐frequency (SF) band is affected by sea–weather conditions at much longer distances in the North Atlantic. Finally, we calculate the HUSN mode noise model (HMLNM) that represents the highest probability ambient noise level in Greece. This model is a realistic noise threshold for future seismic station installations. Online Material: Table of seismic sensor description, and tables, figures, and movie describing microseismic noise measurements.