International audienceCosmic-ray exposure dating of preserved, seismically exhumed limestone normal fault scarps has been used to identify the last few major earthquakes on seismogenic faults and recover their ages and displacements through the modelling of the content of in situ [36Cl] cosmonuclide of the scarp rocks. However, previous studies neglected some parameters that contribute to 36Cl accumulation and the uncertainties on the inferred earthquake parameters were not discussed. To better constrain earthquake parameters and to explore the limits of this palaeoseismological method, we developed a Matlab® modelling code (provided in Supplementary information) that includes all the factors that may affect [36Cl] observed in seismically exhumed limestone fault scarp rocks. Through a series of synthetic profiles, we examine the effects of each factor on the resulting [36Cl], and quantify the uncertainties related to the variability of those factors. Those most affecting the concentrations are rock composition, site location, shielding resulting from the geometry of the fault scarp and associated colluvium, and scarp denudation. In addition, 36Cl production mechanisms and rates are still being refined, but the importance of these epistemic uncertainties is difficult to assess. We then examine how pre-exposure and exposure histories of fault-zone materials are expressed in [36Cl] profiles. We show that the 36Cl approach allows unambiguous discrimination of sporadic slip versus continuous creep on these faults. It allows identification of the large slip events that have contributed to the scarp exhumation, and provides their displacement with an uncertainty of +/- ~25 cm and their age with an uncertainty of +/-0.5-1.0 kyr. By contrast, the modelling cannot discriminate whether a slip event is a single event or is composed of multiple events made of temporally clustered smaller size events. As a result, the number of earthquakes identified is always a minimum, while the estimated displacements are maximum bounds and the ages the approximate times when a large earthquake or a cluster of smaller earthquakes have occurred. We applied our approach to a data set available on the Magnola normal fault, Central Italy, including new samples from the buried part of the scarp. Reprocessing of the data helps to refine the seismic history of the fault and quantify the uncertainties in the number of earthquakes, their ages and displacements. We find that the Magnola fault has ruptured during at least five large earthquakes or earthquake clusters in the last 7 ka, and may presently be in a phase of intense activity

[1] We recover the Holocene earthquake history of seven seismogenic normal faults in the Fucino system, central Italy. We collected 800 samples from the well-preserved limestone scarps of the faults and modeled their 36 Cl concentrations to derive their seismic exhumation history. We found that > 30 large earthquakes broke the faults in synchrony over the last 12 ka. The seven faults released strain at the same periods of time, 12-9 ka, 5-3 ka, and 1.5-1 ka. On all faults, the strain accumulation and release occurred in 3-6 ka supercycles, each included a 3-5 ka phase of slow (≤ 0.5-2 mm/yr) strain accumulation in relative quiescence, followed by a cluster of three to four large earthquakes or earthquake sequences that released most of the strain in < 1-2 ka. The large earthquakes repeated every 0.5 ± 0.3 ka during the paroxysmal phases and every 4.3 ± 0.9 ka between those phases. Earthquakes on the northern faults produced twice larger surface slips (~2 m) and had larger magnitudes (Mw 6.2-6.7) than those on the southern faults. On most faults, the relative strain level was found to control the amount of slip and the time of occurrence of the next large earthquake. Faults entered a phase of clustered activity once they had reached a specific relative strain threshold. The Tre Monti fault is identified as the most prone to break over the next century. Our data document earthquake synchrony and clustering at a broader space and time scale than has been reported to date.

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We provide a database of the coseismic geological surface effects following the Mw 6.5 Norcia earthquake that hit central Italy on 30 October 2016. This was one of the strongest seismic events to occur in Europe in the past thirty years, causing complex surface ruptures over an area of >400 km2. The database originated from the collaboration of several European teams (Open EMERGEO Working Group; about 130 researchers) coordinated by the Istituto Nazionale di Geofisica e Vulcanologia. The observations were collected by performing detailed field surveys in the epicentral region in order to describe the geometry and kinematics of surface faulting, and subsequently of landslides and other secondary coseismic effects. The resulting database consists of homogeneous georeferenced records identifying 7323 observation points, each of which contains 18 numeric and string fields of relevant information. This database will impact future earthquake studies focused on modelling of the seismic processes in active extensional settings, updating probabilistic estimates of slip distribution, and assessing the hazard of surface faulting.