International Research Project - GEO03


TITLE: TILDE - Tidal InterplateLithosphericDeformationof Earth(TILDE)
FUNDING AGENCY: European Space Agency

START DATE: Gennaio 2021
END DATE: Dicembre 2023
The lithosphere shows a rotation relative to the underlying mantle estimated, with respect to the NNR model, of about 0.2°-1.2°/year (Crespi et al., 2007). Although its origin is still debated (Bostrom, 1971; Scoppola et al., 2006; Riguzzi et al., 2010), it is probable it could be due to the tidal drag induced by Moon and Sun. As proof of this hypothesis there is certainly the fact that the lithosphere follows a flow whose maximum speed has a peculiarly defined shape called “tectonic equator” (Doglioni, 1990) and it is inclined 28° with respect to the geographic equator. This angle can be explained by the Earth’s precession and the Maxwell time of the lithosphere. In fact the ratio between viscosity and rigidity is about 1012 s, the tectonic equator is the bisector of the angle generated by the oscillating Earth’s axis that lasts 20,000-26,000 years. The tectonic equator best represents also the net rotation or the westward drift of the lithosphere; moving toward the poles of the tectonic equator, plate velocities and seismicity at plate boundaries decrease (Riguzzi et al., 2010), as well as ocean spreading rates and size of the orogens rates. Moreover a tidal modulation in relative plate motion has recently been shown highlighting the presence of low frequency harmonics compatible with some astronomical motions such as the annual revolution, the apsidal lunar and solar precessions and the nodal lunar precession (Zaccagnino et al., 2020; e.g. Fig.5) in the residuals of velocity. The drag of Moon and Sun determines a tidal bulge, which can be decomposed into its vertical and horizontal components that depend on latitude, the bulge is misplaced relative to the gravitational Earth-Moon alignment, being about 0.3-2.4° eastward of it (Carcaterra & Doglioni, 2018) due to the delay in reaction for the anelastic component of the Earth as a response to the tidal pull. This phase displacement produces the westerly-directed horizontal drag acting on the lithosphere and the continuous slowing of the Earth rotation (Varga et al., 1998). In fact the tidal forces, besides the symmetric oscillatory motions, the nonlinear response of the LVZ breaks the symmetry of the tidal traveling wave on the Earth surface: the small asymmetry produces a net drift motion of any material point interacting with the gravitational wave force, in the direction following the Moon’s rotation. Therefore, detailed properties of the combined tidal oscillation and tidal drift depend on the degree of deformability of the lithosphere, due to its local temperature and geochemistry.These modulations act through two different mechanisms on the outer layers of the Solid Earth:

  1. The brittle and outermost part of the crust is elastically affected by the solid tidal waves, which induce a stress variation in rocks that, depending on the local geodynamics, promotes or, on the contrary, can disadvantage the achievement of the critical breaking point, so it plays a statistically significant role in fault activation (Dieterich, 1987; Cochran et al., 2004; Scholz et al., 2019; Kossobokov & Panza, 2020).
  2. The tidal force actively contributes to the plate motions, as mentioned above, over geological periods (Bostrom, 1971; Doglioni, 1990; Tanaka et al., 2015) due to a partial dissipation of the tidal torque at the LVZ level, i.e. a depth interval with slightly reduced seismic velocity located below a depth of 150 km.

Our work suggest the analysis of the historical series collected by single and pairs of GNSS stations to derive the variations in the distance between them looking for deviations from seasonal and Solid Earth Tides models. Just the possible link between residuals theoretical forecast and experimental observation and the occurrence of seismic phenomena or aseismic slip is at the center of the second part of our project. We introduce a confidence interval for the relative speed between plates based on the deviations recorded in the historical GPS data series available and we derive, by using both statistical and machine learning tools, the experimental probability distribution that seismic events with a given magnitude may occur as a result of a certain deviation. The analysis of the obtained results can be applied for studies aimed at acquiring a better understanding of the energy accumulation processes in the crust and release during earthquakes. We work at both high frequency (seconds, minutes, hours, days) and low frequency (months, years) in search of qualitative and quantitative anomalies. In a similar way we will analyze the data collected through the optical fibers installed in Italy: the use of the interferometric technique will allow us to observe displacements with an order of magnitude of a 10-6 m and therefore to be able to reveal movements invisible to GPS. Thus we will look for possible correlations between tides and the preparation of seismic events of moderate magnitude. The study of soil deformations will also be performed through InSAR data, in order to achieve complete mappings of a given territory. We will try to identify both high and low frequency tidal signals. Particular attention will be devoted to the volcanic areas of Italy, adding a surveillance of the parameters like volcanic tremor, bradyseism, micro-seismicity, explosions and gas emissions. The aim is to identify an expected link between the intensity and some features of these phenomena with the influence of Solid Earth Tides.

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