Global pattern of earthquakes and seismic energy distributions: insights for the mechanisms of plate tectonics

In this paper, we analyse the distributions of number of events (N) and seismic energy (E) on the Earth’s surface and along its radius as obtained from the global declustered catalog of large independent events (M ≥ 7.0), dissipating about 95% of the Earth’s elastic budget. The latitude distribution of the seismic event density is almost symmetric with respect to the equator and the seismic energy flux distribution is bimodal; both have their medians near the equator so that they are equally distributed in the two hemispheres. This symmetry with respect to the equator suggests that the Earth’s rotational dynamics contributes to modulate the long-term tectonic processes. The distributions of number and energy of earthquakes versus depth are not uniform as well: 76% of the total earthquakes dissipates about 60% of the total energy in the first ~50km; only 6% of events dissipates about 20% of the total amount of energy in a narrow depth interval, at the lower boundary of the upper mantle (550-680 km). Therefore, only the remaining 20% of energy is released along most of the depth extent of subduction zones (50-550 km). Since the energetic release along slabs is a minor fraction of the total seismic budget, the role of the slab pull appears as ancillary, if any, in driving plate tectonics. Moreover the concentration of seismic release in the not yet subducted lithosphere suggests that the force moving the plates acts on the uppermost lithosphere and contemporaneously all over the Earth’s outer shell, again supporting a rotational/tidal modulation.