Watching the Wind: Seismic Data Contamination at Long Periods due to Atmospheric Pressure-Field-Induced Tilting

Following the installation of a new broadband seismic station near the Pacific coast of the Olympic Peninsula, Washington, the Pacific Northwest Seismic Network recorded elevated background seismic noise at periods exceeding 30 s. We observed power spectral density variations of about 15-20 dB in the horizontal-component seismograms between times of high and low noise, and differences of up to about 25 dB between the horizontal and vertical components during noisy intervals. The amplitude of the signal at long periods exhibited clear correlations with cyclic daytime fluctuations of air temperature and wind speed. Our observations provided measurable evidence that the variations of the local atmospheric pressure field associated with local wind systems represents an effective source of ground displacements and rotations. The rotational component of the pressure-field-induced motion is recorded by horizontal-component seismometers that, below their low-frequency corner, are sensitive to surface tilts. The response of seismometers to ground rotations is characterized by a transfer function, which we use to estimate the amplitude of tilt. We measured a pressure-field-induced surface tilt of the order of several tenths of mu rad. Based on simple modeling, tilt contamination is expected to attenuate rapidly with depth and to become negligible just a few hundreds of meters below the surface. Removal of this unwanted effect from seismograms remains a challenging task as the details of tilting depend on the nonstationary character of the pressure field, the compliance of the near surface to pressure variations, and the design and construction of the seismic station.