Local effects on tidal strain measurements at Esashi, Japan

Abstract

SUMMARY Finite element modelling has been used to make a quantitative investigation of local factors affecting the measurement of tidal strain at the Esashi earth tide station. Factors investigated, with the approximate magnitude of the perturbation produced by each, are topography (12 per cent), regional geology (1 per cent), inhomogeneity of the elastic properties of the granodiorite from which the strainmeter tunnels were excavated (3 per cent), and the cavity effect (2 per cent). Correction for these factors reduces the average discrepancy between the observed strains (observational accuracy ~1 per cent) and those predicted from the solid earth and ocean loading tide from 23 to 8 per cent. After such correction the ratios of observed to modelled amplitudes of the O1 and M2 tides on each strainmeter are in fair agreement, but these ratios differ from strainmeter to strainmeter; they are about 7 per cent too large on the N-S and NE-SW strainmeters and 9 per cent too small on the E-W instrument. These studies illustrate the difficulties involved in relating strains measured in an observatory such as Esashi to those representative of the surrounding region and suggest that, with the level of detail and sophistication used, this relationship cannot be modelled to much better than 10 per cent. Systematic differences between results from the mid-point and free-end strainmeters imply that the effects of small-scale rock inhomogeneities have not been modelled correctly.