Graviquakes in Italy

•Normal fault earthquakes (graviquakes) deliver gravitational energy.•The hangingwall collapse recovers dilatancy formed in the interseismic period.•The dilatancy is accumulated above the brittle–ductile transition (BDT).•The larger the involved volume (deeper BDT), the higher the earthquake magnitude.•Low extension rates have longer earthquake recurrence time, but not lower magnitude.

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, whose geodynamics is controlled by the Alpine and Apennines subduction zones. The latter has a well-developed backarc basin and a large part of the accretionary prism is affected by on-going extensional tectonics, which is responsible for most of peninsular Italy seismicity. Analyzing the seismic record of the Apennines, the length of seismogenic normal faults tends to be at most about 3 times the hypocenter depth. We compile a map of the brittle–ductile transition depth and, assuming a fixed 45° or 60° fault dip and a dilated wedge developed during the interseismic period almost perpendicular to the fault plane, we compute the maximum volume of the hangingwall collapsing at the coseismic stage, and estimate the maximum expected magnitude. Lower magnitude values are obtained in areas with thinner brittle layer and higher heat flow. Moreover, lower magnitude relative to those theoretically expected may occur in areas of higher strain rate where faults may creep faster due to lower frictional values.

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