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

Solid Earth is subjected to nanostrain tidal deformations caused by gravitational attraction of the Moon and Sun. This causes periodic deformations of imperceptible fractures in the shallow rock that likely result into subtle variations of seismic velocities. It is possible to theoretically model the gravitational tidal deformations while the seismic velocities can be estimated, e.g., using ambient noise recordings processed with passive image interferometry. Combining these two pieces of information could allow for in-situ assessment of bedrock properties beneath seismic stations. In this study, we tried to accomplish this task using 18 standalone seismic stations (i.e., no array) from a network of the Integrated Plate Boundary Observatory Chile, complemented by several others in Europe and North America. The velocity changes were mostly estimated for frequencies of 1-4 and 4-7 Hz, using hourly Green's functions acquired after temporal stacking. Analysed coda lapse time windows of the Green's functions were 1-6, 5-10 and 8-13 seconds. Tide-related velocity changes were observed (mostly the M2 component). However, our results show that observability of such tide-related velocity variations seems to be strongly related to the station proximity to oceanic coastlines. This raises reasonable doubt about the required solid Earth tides origin of the observed tidal signals.