Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005geoji.161..167k&link_type=abstract
Geophysical Journal International, Volume 161, Issue 6, pp. 167-178.
Astronomy and Astrophysics
Astronomy
5
Barometric Pressure, Horizontal Seismometers, Fe Modelling, Strainmeters
Scientific paper
Barometric pressure induced noise is one of the major limiting factors in analysing long-period seismological records. Especially the horizontal components are strongly affected. The barometric pressure influence originates from the attraction by the air masses and deformations of the crust of the Earth resulting in accelerations, strain and tilts. These factors are enhanced by the local topography and the installation site of the instrument. Although this influence is known for several years, the physical transfer mechanisms are still not well understood. By means of finite element (FE) analysis deformations caused by different pressure loadings are studied for Moxa observatory. The main topographic elements of the observatory surroundings like hill flanks and a valley in an area of approximately 2.5 km2 are included in the model. The gallery in which a long-period STS-1 seismometer and two quartz tube strainmeters are installed is modelled with a high resolution of 5 m. The FE model is exposed to three principle loading scenarios: (i) uniform barometric pressure loading, (ii) wind-induced pressure on a hill flank and (iii) passages of pressure fronts. As a linear elastic rheology is used, the loading effects can be superimposed and scaled as necessary. In order to get an estimate about the influence of cavity effects compared with that of topography, the computations are redone for a model containing only the gallery but no topography. Additionally, the influence of dipping layers and a possible cleft system on the pressure effects are studied.
From the FE analyses follows that all three scenarios can lead to significant tilts of approximately 0.1 to 0.7 nrad hPa-1 at the sites of the instruments. This is the same order of magnitude observed in the seismometer data. Also the directions of the tilts are in agreement. The deformation effects obtained for the strainmeters are too small by more than 1 order of magnitude. A qualitative agreement between model results and observations can be found and the change in polarity occurring during the passage of a pressure front can be explained. Besides, a direction dependence of the effects can be shown for the different components of the instruments as well as a dependence on the movement of the air masses. Anisotropy or a cleft system can modify the pressure influence regarding the order of magnitude up to a factor of 10 and the polarity of the effects. A cleft system might be the explanation for the small effects observed in the N-S component of the STS-1 seismometer.
Fischer K. D.
Jahr Th.
Kroner Corinna
Kuhlmann Salma
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