LAUSR

Tidal signals are analyzed in the Plate Boundary Observatory borehole strainmeters in western North America. While the extracted diurnal strain tidal constituents (except daily-cycle S1) respond to the tidal forces in similar proportion, the S1 strain tide exhibits an abnormally strong amplitude, reaching tens to hundreds times larger than theoretical prediction. The S1 strain tide exhibits phase lags and little amplitude correlations with the corresponding S1 barometric pressure tide, while no phase lags are observed at the other frequencies between these two observations. Several mechanisms are examined and are excluded for explaining the strong S1 strain tides, including integral effect of barometric pressure loading, viscoelastic response to barometric pressure, elastic loading of ocean water and hydrological water variation, and thermal leakage from the surface to the strainmeter instruments. The strong S1 strain tides can be attributed to the results of two physical mechanisms: 1) elastic loading of surface barometric pressure and 2) thermoelastic deformation induced by temporally and spatially varying surface temperature. The barometric pressure produces a component of elastic strain in phase with the pressure, while the surface temperature induces thermoelastic strain exhibiting comparable amplitude with the pressure-induced component but with various phase lags. As a result, the superposition of these two S1 strain tides exhibits a phase difference with respect to the corresponding S1 tides of both barometric pressure and temperature, and explains the large S1 strain tides observed in the strainmeter data. Our results indicate the importance of thermoelastic deformation being a near-surface physical process.