Stratigraphic evidence of coseismic faulting and aseismic fault creep from exploratory trenches at Mt. Etna volcano (Sicily, Italy)

Recognition of coseismic and aseismic slip in trench exposures is a major goal in paleoseismology. To define stratigraphic criteria for discriminating between (I) fast earthquake-related slip and (2) slow quasi-continuous creep, we carried out several exploratory trenches along the eastern flank of Mt. Etna, where capable faults (active fault. producing displacement at or near the surface) show both modes of movements with high slip rates and short recurrence intervals. Our sites have experienced predominant coseismic (Fondo Macchia) and aseismic (Mandra del Re) fault slip during histori cal times. At the Fondo Macchia site we trenched a normal fault scarp where -20 em of vertical olTsetoccurred in 1971 and three other similar earthquakes repeated in the past 150yr. Several erosional surfaces close to the fault zone in the footwall indicate that (1) a distinct and recognizable fault scarp free face retreated repeatedly, shaped byerosion and fault activity, and (2) the observed vertical displacement is a result of repeated scarp-forming earthquakes. At the Mandra del Re site a left-lateral, strike-slip fault with a large vertical component dams the drainage of a small valley.Avertical fault slip rate of -2 cmlyr and consequent high deposition rates of ponded, mainly well-layered, fine-grained sediments allow to reconstruct with excellent stratigraphic resolution the fault growth in the past few centuries. More than 3 m of vertical displacement has accumulated in the fault zone, leaving no indication of scarp-related erosion in the footwall deposits or of colluvial wedges in the hanging wall. This unequivocal stratigraphic evidence of "aseismites" (i.e., sedimentary features and relations generated by continuous fault creep) shows that earthquake surface faulting and aseismic creep generate completely different sedimentary responses.