Gravitational and elastic energies stored in crustal volumes activate normal versus strike-slip and thrust seismogenic faults

Shallow crustal faults are passive features mobilized by the dissipation of the potential energy and the shear stress accumulated in the brittle volume surrounding them. However, the stored energy in the volume differs from the tectonic setting, i.e., it is mainly gravitational in extensional tectonic settings, whereas it is elastic in strike-slip and contractional tectonic environments. In extensional settings, below about 1 km, the horizontal tensile stress is overwhelmed by the confining pressure of the lithostatic load, and it becomes positive, i.e. compressive. Therefore, there is no horizontal tension in extensional tectonic settings and the pro-gravity motion of the crustal volume is provided by the lithostatic load, which is the vertical maximum principal stress. The elastic energy is rather accumulated by the maximum horizontal principal stresses, i.e., iso-gravity in transcurrent settings and counter-gravity in contractional tectonic settings. The different relation with the gravitational force in the different tectonic settings generates several relevant differences in the three main tectonic environments. The extensional tectonic settings, both in continental and oceanic rift zones generate normal fault-related earthquakes, i.e., pro-gravity movements, or graviquakes. They differ from the other tectonic setting because are marked by (i) lower energy and lower differential stress to activate faults with respect to strike-slip and contractional tectonics; (ii) lower maximum earthquake magnitude; (iii) a larger number of low magnitude earthquakes in extensional settings because the crust moves downward as soon as it can move, whereas contractional settings require larger accumulation of energy to move counter-gravity; (iv) consequently, the b-value of the Gutenberg-Richter is higher than 1 and the aftershocks are more numerous and last longer in extensional settings; (v) the downward motion of the hangingwall determines more diffuse cataclastic deformation with respect to the other tectonic settings because the lithostatic load works everywhere, whereas in the other tectonic settings is concentrated where the elastic energy accumulates; (vi) in extensional settings the volume dimension is determined by thickness of the brittle layer, and its length is in average three times the seismogenic thickness; in strike-slip and contractional settings dominates the elastic energy (elastoquakes), and the mobilized volume may be ten to thirty times longer in a single seismic sequence, being its size proportional both to the brittle thickness and the relative speed of plates. These differences characterize the seismic cycle of graviquakes with respect to the elastoquakes. The bigger the volume, the wider the seismogenic fault in all tectonic settings. The interplay between the horizontal tectonic forces and the lithostatic load, which is ubiquitous, varies in the three main tectonic settings, generating different seismotectonic styles and an increase of magnitude as the effect of the vertical gravitational force becomes a minority relative to the elastic storage and coseismic rebound.