Earthquake supercycles in Central Italy, inferred from 36Cl exposure dating

We use 36Cl surface exposure dating to determine the slip release pattern over the last ~ 15 kyr for the Velino–Magnola fault, a major active normal fault in Central Italy. We sampled the fault at five well-separated sites along its length, and modeled the 36Cl concentrations measured in the 376 samples. We find that the fault broke in at least 9 large earthquakes that occurred in two 5–6 ka-long supercycles. Each cycle included a 4–5 ka-long phase of relative quiescence, followed by a cluster of at least 3 large earthquakes or earthquake sequences that released most of the accumulated strain in ~ 1 ka. All 9 identified events broke the entire fault and produced maximum surface slips of 2–3 m. Though the Velino–Magnola fault seems presently in a stage of relative quiescence, it may re-enter a phase of paroxysmal seismic activity in a few hundred of years.

► Long record of past earthquake slips and ages: a key for seismic hazard assessment.
► 36Cl exposure dating of limestone normal fault scarps provides such a long record.
► 5 sites along a 45 km long normal fault in Italy reveal 15 ka of seismic history.
► Past earthquakes occurred in supercycles, with centennial and millennial clustering.
► Mw 6.5 to 7 expected, with possibly next cluster in a few hundred of years.