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The maximum possible earthquake magnitude is one of the fundamental parameters to assess possible threats and effects on populations and infrastructures. Empirical scaling laws between fault/slip dimensions and earthquake magnitude can be used to assess this fundamental parameter. Upon the assumption of the reactivability of any fault, these seismic scaling laws are used at the national scale in Italy, considering all known faults regardless of their age, stress field orientation, or strain rate. Italy is a suitable case for comparing the fault-size-derived seismic magnitudes with the existing accurate catalogues of historical-instrumental earthquakes. To do so, (1) a comprehensive catalogue of all known faults is compiled by merging the most complete databases available, (2) the potential expected maximum magnitude of earthquakes (PEMM) is simply derived from fault dimensions, and (3) the resulting PEMMs are compared (i.e., the mathematical difference) with catalogued earthquake magnitudes. Results show that the largest PEMMs as well as the largest differences between PEMMs and catalogued magnitudes are observed for poorly constrained faults (inferred from subsurface data). Where, in contrast, the knowledge of faults is geologically well constrained, the calculated PEMM is often consistent with the catalogued seismicity, with the 2σ value of the distribution of differences being 1.47 and reducing to 0.53 when considering only the M ≥ 6.5 earthquakes. This overall consistency gives credibility to the used empirical scaling laws; however, some large differences between the two datasets suggest the validation of this experiment elsewhere. The largest differences are probably connected with the partial activation of presumed long continuous faults. The main advantages of this method are its rapidity, once the faults are mapped, and its independence from temporal and (paleo)seismological information. The novelty is not the method, but its use at the national scale also for faults considered inactive. This method cannot, however, be a substitute for time-dependent (paleo)seismological methods for seismic hazard assessments. Rather, it can rapidly provide a perspective time-independent seismic potential of faults. [ABSTRACT FROM AUTHOR]

The Mw 6.1 (6 April 2009) L'Aquila (Italy) earthquake occurred in one of the most seismically active areas of central Italy and was preceded by a three-month-long foreshock period. Thanks to recordings by a regional permanent network, we derive for the first time P- and S-wave velocity tomographic models of a major fault prone to an imminent main shock. Close to the Mw 6.1 hypocenter, we observe high Vp (>6.8 km/s) and high Vp/Vs (>1.9) consistent with thick dolomitic volumes filled with fluids sealed by impermeable anhydritic layers. Significant changes in velocities defined by time-lapse imaging during the foreshock period suggest rapid fluid migration through the locked fault zone. The complex positive feedback between fluid pressure buildup and hydrofracturing of the dolomitic reservoir, testified by foreshock production, eventually provoked the catastrophic coseismic breaching of the fault seal. Our results show that foreshock time-lapse tomography provides clues on the preparatory phase of a large normal-faulting earthquake. [ABSTRACT FROM AUTHOR]