Spectral analysis of seismic signals and noise recorded at the NORESS high-frequency element

Abstract

The high-frequency seismic element (HFSE) installed within the NORESS array in southern Norway represents a major advance in the recording of high-frequency seismic energy with high resolution and large dynamic range across a bandwidth from less than 1 Hz to more than 50 Hz. Based on two years of recorded HFSE data, a seismic noise model has been formulated that conforms to a spectral slope of 24 dB octave⁻¹ below 2 Hz and 12 dB octave⁻¹ above 2 Hz. The noise level standard deviation is 3–5 dB across the entire frequency band of 0.5–50 Hz and, at high frequencies, generally reflects diurnal variations associated with cultural activities. Analysis of more than 100 low magnitude (M_L ∼ 3.0) seismic events at regional distances has shown that the Pn and Sn phases contain significant signal energy at frequencies up to more than 50 Hz at distances as large as 500 km and energy up to 10 Hz at a distance of 1500 km. This has important implications, e.g. for the monitoring of underground nuclear explosions using data from regional networks.