Observation of the Earth liquid core resonance by extensometers

The rotation axis of the fluid outer core of the Earth and the axis of the mantle do not coincide in space therefore restoring forces are set up at the core-mantle boundary which try to realign the two axes causing a resonance effect. In celestial reference system it is called the “Free Core Nutation” (FCN), which can be characterized by a period of 432 days while in the Earth reference system it is called the “Nearly Diurnal Free Wobble” (NDFW). Free core nutation of the Earth causes fluid core resonance (FCR) in the diurnal tide band, especially in P1 and K1 waves. Due to its resonance effect this phenomenon can be detected also by quartz tube extensometers suitable for Earth tides recording. The period of the FCN strongly depends on the ellipticity of the core-mantle boundary, and on the electro-magnetic coupling of the core-mantle boundary and anelasticity of the Earth. Therefore the investigation of the FCR effects on Earth tides is an effective tool to gain information about the geodynamics of the Earth’s deep interior. In this study data series measured in several extensometric stations were used to reveal the presence of the FCN resonance on tidal strains. In the Pannonian Basin there are five observatories where extensometric strain measurements were carried out in different lengths of time. Four stations in Hungary: Sopronbánfalva Geodynamical Observatory (2000-2014), Budapest Mátyáshegy Gravity and Geodynamic Observatory (2005-2012), Pécs uranium mine (1991-1999), Bakonya, near to Pécs (2004-2005) and in Slovakia: Vyhne Earth Tide Observatory (2001-2013). Identical instrumentation in different observatories provides the opportunity to compare measurements with various topography, geology and environmental parameters. The results are also compared to values inferred from extensometric measurements in other stations.
This research was funded by the Hungarian National Research Fund (OTKA) under project K 109060.