Strainmeters at Moxa observatory, Germany

Abstract

Since 1997, two quartz tube strainmeters at the Geodynamic Observatory Moxa, located 30 km south of Jena, are used to observe long-period horizontal deformation signals. Both strainmeters are 26 m long with orientations NS and EW and are installed in a gallery. To this system a third component was added in 1999, which connects the ends of the quartz tubes diagonally. This component is realised as a laser strainmeter, running through a 38 m long horizontal borehole. The first data analyses show high signal-to-noise ratios for the tidal frequencies and also the free oscillations caused by the Sumatra earthquake in December 2004 are clearly detectable. It can be shown that the quartz strainmeter extending in EW direction generally contains significant more noise induced by barometric pressure than the NS-component. The laser strainmeter record shows strong influences of changing barometric pressure, due to the fact that the beam does not run in a vacuum. This influence is reduced in the higher frequencies by sealing the ends of the horizontal borehole with high quality glass. In addition, the observations are clearly temperature dependent and the influence of rainfall could be verified by two irrigation experiments.