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Abstract

Tidal forces, by providing a well-known, though narrowband, input to the Earth and ocean, are very useful for detecting temporal changes in the response of these systems. Strain tides provide the most sensitive tidal measurement of local changes in the elastic behavior of the solid Earth for two reasons. First, unlike tilt and gravity, strain tides have no component from the direct attraction. Second, measuring the derivative of displacement makes strain measurements more sensitive to nearby changes than displacement measurements would be; changes in elastic constants are more likely at shallow depths. We report on temporal variations in strain tides observed on long-base laser strainmeters at five locations in Southern California. Since the overall data span is almost fifty years at some locations, and mostly more than a decade, these data provide an unmatched dataset for examining the stability of strain tides. The calibrations depend mostly on instrument length and the wavelength of light, both invariant with time, along with two electronic systems of high stability. A first look at a portion of the data from a few sites shows high stability for the semidiurnal tides; the diurnal tides show seasonal patterns driven by unavoidable thermal effects on the instrument systems, which produce a spurious S1 tide. Results from a fuller analysis will be compared with postseismic inelastic effects from large earthquakes in 1992, 1999, and 2010, along with water-well tides at one site, which have shown temporal variations.