GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 1.1 37 The model predicts that the dilated wedge formed during the interseismic will be contracted during the coseismic stage. An example of this inversion occurred during the 1986 Kalamata (south Peloponnesus) normal fault-related earthquake, which was associated with several compressional focal mechanisms. We discuss the mechanics of crustal normal fault-related earthquakes, and show that they represent dissipation of gravitational potential energy (graviquakes) and their magnitude increases with the involved volume (delimited by the seismogenic fault and an antithetic dilated wedge in its hangingwall), and the fault dip. The magnitude increases with the deepening of the brittle-ductile transition (BDT), which in turn enlarges the involved volume. The fault dip seems rather controlled by the static friction of the involved crustal layers. We apply the model to the extensional area of the Italian peninsula (Fig. 2), whose geodynamics is controlled by theAlpine and Apennines subduction zones. The latter has a well-developed backarc basin and a large part Fig. 2 – The maps show the maximummagnitude (M) expected by gravitational collapse along normal faults in the Italian area and surroundings undergoing extensional tectonics, assuming a slip (D) increasing with magnitude as reported in the lower diagram. The values are inferred assuming the collapse of the volumes computed in the previous figure (45 left and 60 right). It is hypothesized a constant static friction, and normal fault planes activated during an earthquake seem to have in average a mean maximum length three times the depth of the hypocentre. Normal fault earthquakes have their maximummagnitude close to the brittle-ductile transition (BDT), which is deeper where the surface heat flow is lower. The deeper the BDT, the larger the volume and the higher will result the graviquake magnitude. These maps of the extensional areas affecting the central Mediterranean, are only a preliminary attempt to relate brittle volumes and maximum potential magnitude, if active normal faults are present (after Petricca et al., 2015).