Free Core Resonance parameters from strain data: sensitivity analysis and results from the Gran Sasso (Italy) extensometers

Abstract

We analyse eight years of discontinuous strain records from two crossed 90-m long laser interferometers (extensometers) operating in the Gran Sasso underground observatory (Central Italy) to investigate the FCN parameters. At first, inverting realistic synthetic tidal parameters (amplitude and phase of eight diurnal tidal components) we show that the resolving power of strain tides is comparable to that of gravity tides if tidal parameters are inverted minimizing the L-2 misfit (as usually done). Both resolving powers improve if data are inverted minimizing the L-1 misfit, and this improvement is particularly notable for gravity tides. The analyses of strain records have been performed after correcting measured strain for local distortion of the regional strain field and ocean loading. Data are inverted minimizing the L 1 misfit. Parameter uncertainty assessment is performed both inverting synthetic tidal parameter sets, obtained from the real one by adding a random noise, and estimating the Bayesian marginal probabilities from an ensemble of parameter space samples. Even if we analyse data from a single station, our results for the period of the Free Core Nutation (about 429 sidereal days) are robust and comparable to those from gravity tides, obtained from the joint inversion of data from several stations. The quality factor is not well constrained because of the large uncertainty on the Psi 1 phase; however our results are consistent with recently published values (approximate to 20 000). The agreement between observations and predictions looks better than in any previous work that makes use of strain tides. Our inversions provide also an independent estimate of Re(h(1) - 3l(1)), which is the dominating term in the resonance strength expression, in agreement with IERS Conventions (2003)

Investigation of the time variability of diurnal tides and resonant FCN period

“…The FCN resonance parameters can also be inverted from strain data (e.g. Amoruso et al 2012). The usual approach in determining FCN parameters is linearized least-square method (LSQ) (Defraigne et al, 1994;Sun et al, 2004).…”

Estimation of free core resonance parameters based on long-term strain observations in the diurnal frequency band

“…Суточные гармоники приливных деформаций в азимутальном направлении η могут быть представлены выражением [Mukai et al, 2004;Amoruso et al, 2012]…”

“…Таблица 2. Оценки периода свободной нутации ядра Земли по деформационным данным T FCN (сидерические сутки) Публикация 1 427.5 ± 11.1 [Mukai et al, 2004] 2 429 ± 10 [ Amoruso et al, 2012] 3 428.5 ± 8.3 Настоящая работа…”

Nonlinear and minor ocean tides in the Bay of Biscay from the strain tides observed by two geodetic laser strainmeters at Canfranc (Spain)

“…The heart of the geodynamical facility are two laser strainmeters (interferometers), similar to those operating in Central Italy for about 20 years [ Crescentini et al ., ; Amoruso and Crescentini , ]. These latter produced valuable results on slow earthquakes [ Crescentini et al ., ; Amoruso and Crescentini , ], long‐period toroidal Earth free oscillations [ Park et al ., ], earthquake nucleation [ Amoruso and Crescentini , ], the free nutation of the Earth core [ Amoruso et al ., ], and the correlation between groundwater flow and deformation in a fractured carbonate aquifer [ Amoruso et al ., ]. For the moment, strain data from the LSC interferometers have been used to investigate the local link between groundwater flow and deformation [ Díaz et al ., ].…”