A “Geodetic Gap” in the Calabrian Arc: Evidence for a Locked Subduction Megathrust?

Abstract

Subduction of old Ionian seafloor beneath the Calabrian Arc (southern Italy) is the geological process with the greatest mass flux in the central Mediterranean, yet its seismogenic behavior is largely obscured. No unambiguous evidence of subduction-related earthquakes exists in historical times, and local GPS velocities indicate very low strain rates. Nevertheless, the region hosted some of the deadliest normal-faulting earthquakes of the entire Mediterranean basin. We show that the low strain rates recorded in southern Calabria can be reconciled with the regional vigorous seismic moment release by assuming high interseismic coupling but low seismic coupling of the subduction interface. The alternative scenario of steadily creeping subduction cannot be ruled out but requires the historical seismicity record to be dismissed as unrepresentative. We refer to the peculiar spatial pattern of short-term strain rates in southern Calabria as a “geodetic gap” resulting from destructive interference between upper-plate extension and temporary compression due to locking along the subduction interface. Seismic hazard modelers must understand that within such gaps, the long-term seismic hazard is greater than that suggested by the low geodetic strain rates.