Probing the in situ Elastic Nonlinearity of Rocks with Earth Tides and Seismic Noise.

Heterogeneous materials such as rocks, concrete, and granular materials exhibit a strong elastic nonlinearity. The sensitivity of the elastic nonlinearity to the applied stress and pore pressure in principle allows the use of seismic waves for remote observations of stress or pore pressure changes. Yet the nonlinearity of rocks is difficult to quantify in situ as active deformation tests are not possible in the field. We investigate the elastic nonlinearity in a fully natural experiment using the ambient seismic noise of a single seismic station to sense changes of the seismic velocity in the subsurface reaching 0.026% in response to the minute deformation caused by various constituents of the tidal forces exerted by the Sun and Moon.