Earth's tides, plate motions, graviquakes and elastoquakes

Tectonic plates move “westerly" along a flow described by the so-called “tectonic equator”. This great circle is inclined about 30° relative to the geographic equator and mimics the inclination of the Moon’s revolution. We infer that this geometric and kinematic feature is an indication of the active role of the Earth’s tides in driving plate tectonics. The drag exerted by the tidal lag and the permanent oscillation of the solid Earth tide should coexist and contribute with the thermal cooling that allow and control mantle convection. In fact upper mantle circulation is polarized “eastward”, whereas subduction zones contribute to mantle convection with three times larger volumes of recycled lithosphere along W-directed slabs. The pump provided by the horizontal component of the solid Earth tide is inferred as the effective mechanism determining plate motion and the net rotation of the lithosphere decoupled at the low velocity layer with respect to the underlying mantle. Therefore, the horizontal component of the Earth’s tide determines day after day the accumulation of stress gradients at plate boundaries. The vertical component of the Earth’s tide may rather trigger the dissipation of energy throughout earthquakes once the threshold of failure is reached in varying the lithostatic load with the oscillation of g. However, this mechanism is opposite as a function of the tectonic style. In fact during the low tide, g is higher, increasing the lithostatic load and most frequently triggering the activation of normal faults. Alternatively, the high tide has lower g value and the lithostatic load acting on faults decreases, possibly favoring the activation of thrusts. Since normal faults mostly dissipate gravitational potential energy, they are classified as graviquakes, whereas thrust faults release elastic energy and are therefore defined as elastoquakes. This differentiation is required in order to start recognizing the different phenomenologies associated to the earthquakes as a function of the tectonic setting.