Sources of short-term fluctuations in the seismic noise level at NORESS

Abstract

Short-term fluctuations in the seismic noise field at the NORESS array have been investigated by analyzing the two-year output of an automatic short-term to long-term average (STA/LTA) power detector. A remarkable feature is the occasional occurrence of a vast increase in the number of noise detections, sometimes lasting for several days. From slowness analysis, the majority of these noise bursts of a few seconds duration are found to originate in the azimuth range 95–105° from NORESS. Apparent velocities less than 3.0 km s −1 suggest that they are propagating Rayleigh waves. The temporal distribution of the noise detections originating in the azimuth range 95–105° shows a strong correlation with the water flow in the nearby River Glomma, although there is in some cases a time lag of 3–5 days between peak water flow and peak detection rates. A detailed analysis of a typical six-hour period has revealed the presence of a continuous source generating Rayleigh waves with azimuth 99.5 ± 0.8°, apparent velocity 2.83 ± 0.03 km s −1 and dominant frequencies between 2.5 and 2.9 Hz. There is a strong possibility that these Rayleigh waves are caused by the operation of a nearby hydroelectric power plant, although the source and generation mechanisms of these disturbances have not been explicitly determined.