Monochromatic components of the seismic noise have been detected in the NORSAR area. The most prominent are near 2·08 Hz and 2·78 Hz. The geographical distribution and the variation with time is studied, and also the particle motion and wave propagation. A source for the 2·78 Hz peak has been found at a hydroelectric power plant, but the sources for the other spectral peaks are still unknown.

Seismic velocities in the siting area of the Norwegian Seismic Array (Norsar) have been monitored over a time period of 1 week by using a hydroelectric power plant as a continuous wave generator. Propagational phase angle differences have been measured over travel distances ranging from 4.7 to 13.7 km, and group velocities of the order of 3.5 km/s are derived. This is close to the expected (phase) velocity for S waves, and the particle motions derived at a distance of 4.7 km also correspond well with those for S waves. The obtained precisions are 10 −3 for a time period of about 2 hours and 10 −4 when 1 week of data is used. The phase difference data exhibit a clear semidiurnal variation which probably corresponds to variations in the seismic velocities with peak to trough amplitudes around 10 −3 . There is some possibility that the observed variations could be caused by tidal effects. Observed spatial variations show that the influence of the local geology is significant.