GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 1.4 213 FREE CORE RESONANCE PARAMETERS FROM DIURNAL STRAIN TIDES RECORDED BY THE CANFRANC (SPAIN) AND GRAN SASSO (ITALY) UNDERGROUND GEODETIC INTERFEROMETERS A. Amoruso, L. Crescentini Department of Physics, University of Salerno, Italy In the last decades we have designed, installed and managed two underground systems, each consisting of two independent wide-band high-sensitivity geodetic laser strainmeters (interferometers). The former system has operated from summer 1999 to March 2013 under the Gran Sasso massif (central Italy) in the vicinity of the Laboratorio Nazionale del Gran Sasso (LNGS), taking advantage of the tunnels that surround the main halls of the laboratory (Amoruso and Crescentini, 2009). The latter system has operated since end 2011 under the Central Pyrenees Chain (Spain) in the vicinity of the Canfranc underground laboratory (LSC), taking advantage of a by-pass between a motorway tunnel and a decommissioned train tunnel and a narrow side hall parallel to and built at the same time as the train tunnel (Amoruso et al. , 2018). Each interferometer (70 m to 90 m in length) measures length changes over time and consequently extension with a nominal resolution of a few 10 -13 ; the dynamic range spans more than 7 orders of magnitude and the nominal bandwidth ranges hundreds of Hz to continuum. Unfortunately, strain data acquisition suffered several interruptions at both sites, because of technical problems, mainly due to laser tube replacements. Because of the extraordinary high signal-to-noise ratio in the tidal bands, we have analysed available data to study the Free Core Nutation (FCN) effect on strain tides. FCN is a rotational eigenmode arising from the dynamic influence of the rotating fluid outer core and the rotating, visco-elastic and elliptical mantle, whose rotation axes are slightly misaligned. The harmonics of the luni-solar gravitational forcing induce forced nutation terms; the frequencies of some of them (mainly the annual and semi-annual retrograde nutations) are close to the FCN frequency and this modifies their amplitude and phase. In the Earth-fixed reference frame, the frequency of FCN is inside the diurnal tidal band; thus, some diurnal tidal waves with frequencies close to 1 cycle per day (mainly P 1 , K 1 , Ψ 1 and Φ 1 ) are modified, hence the acronymous FCR (Free Core Resonance). The resonant modifications of these diurnal tidal waves and forced nutations can be employed to investigate the Earth’s FCN and calculate the related resonance parameters. In particular, in a space-fixed reference frame the FCN period is inversely proportional to the dynamical flattening of the Core Mantle Boundary. We have analysed about 7 years of LNGS data (ranging 2000 to 2011) and about 4.6 years of LSC data (ranging January 2012 to November 2018) in the diurnal band. FCR parameters have been estimated by comparing measured and computed diurnal strain tides. The comparison implies: (i) analysing strain data in the diurnal and semidiurnal tidal bands to obtain a tidal parameter set (amplitude and phase of Q 1 , O 1 , P 1 , K 1 , Ψ 1 , Φ 1 , N 2 , M 2 for each interferometer); (ii) computing the theoretical strain tide tensor, which is obtained by adding ocean loading to solid tides and depends on the FCR parameters; (iii) combining the components of the theoretical strain tide tensor to account for the local distortion of the strain field caused by tunnel installation, surface topography, and rock inhomogeneities (strain cross-coupling); (iv) estimating the FCR parameters by nonlinear optimization. In practice, items (ii) to (iv) are accomplished simultaneously; semidiurnal tides are used to make strain cross-coupling estimate more robust. Confidence intervals for the FCR parameter values and possible correlations are estimated by analysing 100000 synthetic sets of tidal parameters which we obtain from the measured one (item (i)) by adding a normal random noise whose standard deviation for each tidal parameter is given by its uncertainty as obtained from the tidal analysis. Results are fully consistent with those obtained by others by using nutation measurements from 3770 selected VLBI sessions and better than those obtained by using gravity records