GNGTS 2015 - Atti del 34° Convegno Nazionale

34 GNGTS 2015 S essione 1.1 We suggest that the fluid flow rates differ during the different periods of the seismic cycle (interseismic, pre-seismic, coseismic and post-seismic) and in particular as a function of the tectonic style. Regional examples indicate that tectonic-related fluids anomalies depend on the stage of the tectonic cycle and the tectonic style. Although it is difficult to model an increasing permeability with depth and several BDT transitions plus independent acquicludes may occur in the crust, we devised the simplest numerical model of a fault constantly shearing in the ductile deeper crust while being locked in the brittle shallow layer, with variable homogeneous permeabilities. The results indicate different behaviors in the three main tectonic settings. In tensional tectonics, a stretched band antithetic to the normal fault forms above the BDT during the interseismic period. Fractures close and fluids are expelled during the coseismic stage. The mechanism reverses in compressional tectonics. During the interseismic stage, an over- compressed band forms above the BDT. The band dilates while rebounding in the coseismic stage and attracts fluids locally. At the tip lines along strike-slip faults, two couples of subvertical bands show different behavior, one in dilation/compression and one in compression/dilation. This deformation pattern inverts during the coseismic stage. Sometimes a pre-seismic stage in which fluids start moving may be observed and could potentially become a precursor (Doglioni et al. , 2014). We propose that the brittle-ductile transition (BDT) controls the seismic cycle. In particular, the movements detected by space geodesy record the steady state deformation in the ductile lower crust, whereas the stick-slip behavior of the brittle upper crust is constrained by its larger friction. GPS data allow analyzing the strain rate along active plate boundaries. In all tectonic settings, we propose that earthquakes primarily occur along active fault segments characterized by relative minima of strain rate, segments which are locked or slowly creeping. We discuss regional examples where large earthquakes happened in areas of relative low strain rate. Regardless the tectonic style, the interseismic stress and strain pattern inverts during the coseismic stage. Where a dilated band formed during the interseismic stage, this will be shortened at the coseismic stage, and vice-versa what was previously shortened, it will be dilated. The interseismic energy accumulation and the coseismic expenditure rather depend on the tectonic setting (extensional, contractional, or strike-slip). The gravitational potential energy dominates along normal faults, whereas the elastic energy prevails for thrust earthquakes and performs work against the gravity force. The energy budget in strike-slip tectonic setting is also primarily due elastic energy. Therefore, precursors may be different as a function of the tectonic setting. In this model, with a given displacement, the magnitude of an earthquake results from the coseismic slip of the deformed volume above the BDT rather than only on the fault length, and it also depends on the fault kinematics (Doglioni et al. , 2015a). Earthquakes dissipate energy stored by pressure gradients at plate boundaries and still represent a major issue both for public safety and for their mechanisms. We assume that the lateral variations of the viscous-plastic basal mantle drag are determining the tectonics at plate boundaries and the deformation is transferred from the lithosphere base at the Earth’s surface. Due to the brittle nature of the upper crust, the shallow deformation occurs episodically, i.e., releasing in few seconds the energy accumulated in hundreds of years. We focus our study on the earthquakes generated by normal faults, which are widespread in several geodynamic environments, such as continental rifts, backarc basins, mid-ocean ridges, orogens and strike- slip settings. Earthquakes modify Earth’s gravitational energy. It was pointed out that the coseismic gravitational energy variation might be few orders of magnitude larger than the radiated seismic wave energy, which is usually referred as ‘seismic energy’. Unlike thrust or reverse faults, gravity favours normal faulting since the maximum stress axis is parallel to the lithostatic load. In fact, contrary to thrust faults, normal fault rupture tends to propagate upward. In extensional environments, the differential stress necessary to generate rock failure is on average 5–6 times smaller than that required in contractional tectonic settings. Accordingly, normal fault-related earthquakes never reached the magnitudes (e.g., >Mw 8.5) recorded