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.

Deformation and Fault Propagation at the Lateral Termination of a Subduction Zone: The Alfeo Fault System in the Calabrian Arc, Southern Italy

“…The migration of the Calabrian Arc subduction system have significantly slowed down over time (Wortel and Spakman, 2000;Goes et al, 2004;Rosenbaum and Lister, 2004;Faccenna, 2005;Mattei et al, 2007) and it is still weakly active (Palano et al, 2017). This characteristic of the subduction process provides a critical constraint for modeling the present day activity and potential earthquake production of the slab interface (Carafa et al, 2018;Nijholt et al, 2018). Both the propagation and displacement rates of the AFS also significantly slowed down over time, implying that the subduction process and AFS development are interdependent.…”

“…The identification and characterization of lateral terminations of the slab was mainly based on tomography data and on the geologic and geodetic evidence of active tectonics, along with a rather abrupt lateral termination of intraslab seismicity (Guarnieri, 2006;Del Ben et al, 2008;Orecchio et al, 2014Orecchio et al, , 2015Barreca et al, 2016;Chiarabba and Palano, 2017;Cultrera et al, 2017;Milia et al, 2017;Palano et al, 2017;Carafa et al, 2018).…”

A sanity check for earthquake recurrence models used in PSHA of slow deforming regions: the case of SW Iberia

“…In more active areas, like subduction zones, a more complex interplay between seismicity and strain is possible. Carafa et al (2018) investigating the Calabrian Arc proposed that the low strain rates observed by GPS together with large seismic moment release may be explained by a model with high interseismic coupling and low seismic coupling of the subduction interface.…”

Effect of Shallow Slip Amplification Uncertainty on Probabilistic Tsunami Hazard Analysis in Subduction Zones: Use of Long-Term Balanced Stochastic Slip Models

“…Available estimates of the convergence rates in these subduction zones are very variable, and we recall just some of them here. The Calabrian Arc convergence rate is estimated at 5 mm/year by Devoti et al (2008) and recently at 1.5-1.6 mm/year if creeping, or 2:7 À 3:0 if temporarily locked by Carafa et al (2018). The Hellenic Arc convergence rate is estimated at 23 mm/year in the Ionian Islands (Hollenstein et al 2008), 35 mm/year in the western Figure 1 a Rigidity profiles as a function of the depth computed from the end-member case of Bilek and Lay (1999) (blue curve) and interpolating the PREM (Dziewonski and Anderson 1981) values (green line).…”

“…The values of the coupling coefficients in these three subduction zones are also highly debated. In the Calabrian Arc two competing interpretations-ranging from partially-locked to unlocked or inactive-were recently proposed (Carafa et al 2018;Nijholt et al 2018). In the Hellenic Arc interpretations range from full locking (Ganas and Parsons 2009) to low coupling (Shaw and Jackson 2010;Vernant et al 2014), but also the presence of important along-strike coupling variations was proposed (Laigle et al 2004).…”