Seismic noise in northcentral North Carolina

Abstract

Ambient seismic noise in the frequency range 0.5 to 7 Hz is investigated for a portion of northcentral North Carolina. Three-component, digital data were collected as part of the state's bid for the Superconducting Super Collider. Stations were established along the proposed collider ring position and were intentionally located near sources of transient vibration. The absence of transient vibrations over all or a significant portion of the recording interval at six stations allowed seismic noise to be investigated. Noise spectra display similarities in shape; all spectra decay with approximately constant slope (roughly f −3) below 1 Hz and all flatten in the frequency range 2 to 3 Hz. Vertical-component spectra decay with roughly constant slope ( f −2.1) for f > 4 Hz. At most stations, spectral levels for horizontal components are greater than vertical-component levels for f > 4 Hz. Vertical-component spectra are similar in shape and magnitude to vertical spectra computed for RSTN station RSCP. The most likely source for the ambient seismic noise is higher-mode surface waves produced by cultural activity. Ambient noise cannot be attributed to system noise or to the direct effects of traffic. Higher-mode surface waves are suggested by low coherence values computed for orthogonal components and extremely complex particle motion diagrams. Large horizontal particle motions and high spectral levels for horizontal components suggest that a significant portion of the noise, particularly for f > 4 Hz, is being produced by Love waves. Rayleigh waves contribute to the noise in the band 2 to 3 Hz.