Turning standalone seismometers into strainmeters using tidal strain and ambient noise: a feasibility study

Solid Earth is subjected to gravitational tidal deformations in the nanostrain order. We focus on detecting seismic shear wave velocity changes caused by this phenomenon. Their estimate, together with reliably-computable synthetic solid Earth tidal strains, could allow for assessments of in situ non-linear stress-strain relationships beneath seismic stations. We use ambient seismic noise and passive image interferometry to retrieve time series of seismic velocity changes. We inspect them in the frequency domain. The tide-related frequency components in the time series are observed at numerous permanent seismometer stations. However, their occurrence is related to station proximity to nearby oceans. This contradicts the conventional hypothesis that velocity changes are induced by solid Earth tidal strains. We propose two possible explanations: (1) The observations of tide-related velocity changes are false and they appear as an artifact of changing properties of ambient noise sources in nearby oceans; or (2) We observe true velocity changes that are caused by underestimated tide-induced loading of the oceanic water. The effect of oceanic loading, especially in coastal areas, might be underestimated, since it is much less predictable than that of the solid Earth tides.