Earth tide strain measurements in the Poorman Mine near Boulder, Colorado

Earth tide strains at a particular site are affected by the internal structure of the solid earth, ocean loads, local inhomogene- :.ties in elastic constants due to geologic struc- ture, topography near the observation site, and the c avity in which the measuring instrument is situated. All of these influences have been es- timated quantitatively for the Poorman site for a diurnal (01) and a semidiurnal (Mp) tide. The predicted strains agree with those observed to within 5% for M2 and 10% for 01 whereas without the topographic and geologic corrections there is an amplitude discrepancy of about 25%. The phase discrepancy is reduced from -15' to -6 for 01. The residuals can be reduced further by using terms quadratic in the a pplied potential, which may imply a breakdown of linear tide theory, a deficiency in classical tidal calculations, or a local effect that modulates the tidal transfer