Tidal effects in the Earth’s crust

Disturbance of geophysical media as a result of gravitational interaction within the Earth – Moon – Sun system is one of the main factors governing most of geophysical processes’ regimes. Tidal impact of the Moon and the Sun on the continental Earth’s crust defines not only mechanics of its deformation and conversion of its fabric, but also regimes of several geophysical fields and intensity of interactions between the geospheres on the boundary between the atmosphere and the Earth’s crust.

Decompression of substance during passage of the tidal wave causes transformation of the Earth’s crust structure, peculiar movements of blocks and variation in regimes of underground fluids (underground water, Radon). The effects are especially distinct in local zones characterized by contrasting features against the background of the neighboring rocks (e.g. zones influenced by tectonic faults).

This report presents measurements of high frequency seismic noise, intensity of Radon emanation, number of electric pulses in the Earth’s crust and underground water table changes in an open borehole all caused by tidal force. The observations were performed and collected between 2004 and 2015 in various regions of Russia.

The obtained data evidence a significant influence of tidal deformation on amplitude variation of seismic noise. The solar/lunar tide, though its influence in a wide frequency range is not always perceptible, causes regular variations in mean-squared amplitude of noise with periods close to 12 and 24 hours in certain frequency bands (i.e. an amplitude modulation of seismic background takes place). At this, the stronger tidal effects are observed within the zones influenced by faults, while the frequency band modulated by tide turns out different for each observed spot of the Earth’s crust, which can be used as a characteristic of geodynamic properties of the spot.    

Our observations reveal a significant role of tidal factor in emanation of underground gases. Changes in permeability of underground gas migration channels as a result of decompression during passage of tidal wave causes characteristic periodicities and cyclic occurrences, for example, in Radon emanation. At this, there has been registered a relevant correlation between the intensity of Radom emanation and tidal force amplitude, whereas the former value can be characterized by volumetric activity of the subsurface Radon. It was also found that spectra of underground water level variations contain accentuated spectral maxima corresponding to the periods of main tidal waves.

The tidal deformation of the Earth’s crust manifested as alternating phases of expansion and contraction strongly influences the intensity of relaxation processes characterized at its simplest by the number of relaxation events. The analysis of these observations shows the number of microseismic pulses of relaxation type and electric pulses in the soil correlate to variation in tidal force.

The analysis of tiltmeter data shows that motion of a crustal block is characterized by precession around a certain axis. The precession exhibits periodicity in variation of tilt angle that corresponds to periodicity in variation of vertical component of the tidal force. This enables considering the solid tide as one of the main causes of variation in the block’s tilt. It should be also stressed the spectra of tilt variation reveal the presence of close maxima predefined by the block response to close in their period tidal forces of М₂, S₂, K₁ and О_1.