Tectonic constraints as the origin of heterogeneity in seismic style: new analyses and simulations

Faults are the boundaries where the slip between contiguous crustal volumes happens. The energy is mainly accumulated in the brittle lithosphere because of the action of tectonic and gravitational forces and it is released according to optimal dissipation criteria. The unbalance of stress components in the Earth’s crust, based on the relative motion of crustal volumes, produces a wide pattern of heterogeneities in local tectonics and seismic dynamics. Thrust faulting events, usually featured by dip angles ranging between 5°-30°, mostly take place along convergent plate boundaries and are caused by the elastic strain accumulation, which is released by multifaceted fault slip dynamics, ranging from almost periodic silent events to mega-quakes. Strike-slip-faulting earthquakes localize along steeply dipping faults (70°-90°) or transcurrent plate boundaries and transfer zones. Finally, normal faults develop along rift zones in extensional regimes having intermediate dip (45°-65°), with a dominant gravitational contribution to their energy budget. Structural, morphological, and geophysical differences have been highlighted among the three main tectonic settings. Normal faults cause fracturing mainly concentrated in the hanging walls and spaced clusters of parallel faults across rifting areas. Fractured volumes host complex fluid circulation during the seismic cycle. Intricate geometries are also typical of transcurrent regions, often accompanied by releasing and restraining bends or step-overs and other geological structures shedding light on complex spatial stress patterns. Conversely, thrust-faulting earthquakes usually involve geometrically simpler seismogenic sources. We propose a simple mechanism for faulting inspired by the physical properties of granular matter and optimization criteria to explain differences in seismic activity depending on the tectonic setting: while the dynamics of compressive and transcurrent zones is driven by elastic strain, seismic activity associated with normal faults is mainly fuelled by gravity. Computational simulations support our hypotheses.