Registration and analysis of the Earth's free oscillations with the 30m vertical pendulum in the salt mine of Berchtesgaden

Since February 2007, DGFI operates a 30m vertical pendulum in the salt mine of Berchtesgaden. The location in Berchtesgaden was chosen because of its very good geological characteristics (good connection of the mountain and the Earth's crust). The not longer aerated part of the salt mine allows to protect the pendulum from air-pressure and temperature fluctuations and the hygroscopic rock keeps the air moisture at a nearly constant low level.

The measured movements of the Earth's crust could have various causes . On the one hand, oscillations of the Earth are excited by external forces and torques and on the other hand by abrupt events like e.g. an earthquake. Elastic oscillations of the Earth are excited by earthquakes with a magnitude bigger than 6.5 on the moment magnitude scale.

In general, the free oscillations are classified according to their moving direction in spheroidal and toroidal modes. Thereby, spheroidal modes describe radial and tangential movements relative to the Earth's crust (only measured with gravimeters and seismographs) whereas toroidal modes describe only tangential movements (only measured with pendulums and strain-meters).

: Upper part: Measured signal (north-south direction) during the Chile-Maule earthquake on February, 27th 2010. Lower part: Fourier spectra of the measured signal with allocated modes of free oscillations.

The upper part of the figure shows the measured movement of the pendulum during the Chile-Maule earthquake on February, 27th in 2010 which has a magnitude of 8.8 on the moment magnitude scale. The lower part shows the Fourier spectra of the measured signal (red). One could see very clearly the frequencies of the free oscillations. For comparison, the Fourier spectra of a time period without a big earthquake (blue) is also displayed.

The aim of this project is the permanent registration of the pendulum's movement and the analysis of the movement after big earthquakes. The analysis is done with the wavelet-analysis. Therefore, the time series have to be corrected for periodic and non-periodic effects. Periodic effects are, e.g., signals which are caused by the frequently done calibration of the registration unit. Non-periodic effects are, e.g., jumps and drifts. The aim of the wavelet-analysis is the accentuation of the connection between big earthquakes and the Earth's free oscillations and the investigation of the damping of the free oscillations.

Together with the "Black Forest Observatory" (BFO) in Schiltach, new methods are investigated to further improve the data registration (especially the signal-to-noise ratio).

Selected publications

Bloßfeld M., Schmidt M.: Eigenschwingungen der Erde in Berchtesgaden. Akademie Aktuell Heft 3/2010, Bayerische Akademie der Wissenschaften, 2010

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