Nonlinear Shallow-Water Tides Observed with Excellent Signal-to-Noise Ratio by Two Geodetic Laser Strainmeters at the Canfranc (Spain) Underground Laboratory.

Abstract:

Comparisons between observed Earth strain tides and model predicted ocean tidal loading can give an independent mean to validate the ocean models and finally the whole modelling process. In particular, the accurate representation of non-linear tides, such as quater-diurnal and higher-frequency components, and of their loading effects is a challenging task to be accomplished in the next years.
The Canfranc underground laboratory (LSC), excavated in the rock under the Central Pyrenees (Spain), is mainly devoted to the study of phenomena which needs “cosmic silence”, as the detection of cosmic neutrinos or “dark matter” particles, but, as the site is located in one of the most active seismic areas in Western Europe, it also hosts a geodynamic facility capable to record the whole geodynamic spectrum, from near-field seismicity to tectonic deformation, including Earth tides.
Using two near-orthogonally-oriented 70-m-long high-resolution laser strainmeters installed at LSC, we have obtained continuous precision measurements of local strain in terms of dL/L, where L is the interferometer length and dL its change over time. The laser strainmeters are operational since early 2012.
Spectra show clear tidal peaks whose frequencies range from the diurnal band to at least 8 cicles per day (cpd). Tidal peaks at frequencies higher than 2 cpd are ascribable to ocean loading of non-linear constituents, mainly from the Bay of Biscay. Strain amplitudes of observed non-linear tides range from few 10^-12 (e. g. 3*M2+K2) to 4x10^-10 (M4).
We will show comparisons between observations and predicted strain tides, computed using ocean-tide loading programs included in the SPOTL package (Agnew, 2013) and two ocean tidal models, namely TPXO8-atlas (enriched with the European Shelf 2008 regional solution) and FES2012.
In particular, M4, MS4, and MN4 will be compared with predictions by both ocean tidal models, while MKS2 (semidiurnal band), N4, and M6 will be compared with predictions by FES2012 only.