Tidal analyses of long-base tiltmeters at Rustrel (France), Sainte-Croix (France) and BFO (Germany)

Horizontal instruments, like tiltmeters, are known to be noisier than vertical seismometers or gravimeters because of a higher sensitivity to ground tilts. A major source of noise on horizontal components is induced by the atmospheric pressure changes. Horizontal instruments are also much sensitive to local tilts associated with cavity, topographic and geological effects. Four hydrostatic long-base tiltmeters have been continuously recording at Sainte-Croix (two about orthogonal tiltmeters), in the mountainous Vosges area, at Rustrel, in the south of France and at the Black Forest Observatory in Germany. We analyze the tiltmeter records at these sites in three aspects: we first compare their noise levels at various frequencies. We decompose the observed signal into a basis of tidal predictions (in tilts and strains) in the search for possible local strain-tilt coupling effects. Finally, we compare the tiltmeter responses to local atmospheric pressure changes. From these analyses, we try to identify the main sources of horizontal noise at the three sites considered here.