Understanding seismogenic processes in the Southern Calabrian Arc:a geodynamic perspective

Abstract

For any scientist working in seismotectonics, the Calabrian Arc represents the most challenging area of Italy. Lying on top of a subduction zone, it is characterised by a complex geological structure largely inherited from the early stages of the collision between the Africa and Eurasia plates. The current and extremely vigorous seismogenic processes, although generated by a mechanism driven by the subduction, are no longer a direct consequence of plate convergence. About one fourth of the largest Italian earthquakes concentrates in a narrow strip of land (roughly 200x70 km) corresponding to the administrative region of Calabria. The present-day seismicity, both shallow and deep, provides little help in detecting the most insidious seismogenic structures, nor does the available record of GPS-detected strains. In addition to its fierce seismicity, the Calabrian Arc also experiences uplift at rates that are the largest in Italy, thus suggesting that active tectonic processes are faster here than elsewhere in the country. Calabrian earthquakes are strong yet inherently elusive, and even the largest of those that have occurred over the past two centuries do not appear to have caused unambiguous surface faulting. The identified active structures are not sufficient to explain in full the historical seismicity record, suggesting that some of the main seismogenic sources still lie unidentified, for instance in the offshore. As a result, the seismogenic processes of Calabria have been the object of a lively debate at least over the past three decades. In this work we propose to use the current geodynamic framework of the Calabrian Arc as a guidance to resolve the ambiguities that concern the identification of the presumed known seismogenic sources, and to identify those as yet totally unknown. Our proposed scheme is consistent with the location of the largest earthquakes, the recent evolution of the regions affected by seismogenic faulting, and the predictions of current evolutionary models of the crust overlying a W-dipping subduction zone.

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

“…Published GPS measurements and deformation models of the forearc D'Agostino et al, 2011;Kreemer et al, 2014) exhibit rather low strain rates (~10-20 nanostrain/yr). Yet different clusters of earthquakes above m = 6 are reported for onshore Calabria (Rovida et al, 2016;Tiberti et al, 2017), that is, for the portion of the forearc lying approximately above the subduction megathrust, in open contrast with the low observed geodetic strain rates. Yet different clusters of earthquakes above m = 6 are reported for onshore Calabria (Rovida et al, 2016;Tiberti et al, 2017), that is, for the portion of the forearc lying approximately above the subduction megathrust, in open contrast with the low observed geodetic strain rates.…”

“…The combination of slow trenchward motion with the lack of thrust faulting mechanisms in the instrumental record (Neri et al, 2009;Presti et al, 2013;Totaro et al, 2016) led some researchers to hypothesize that the Calabrian subduction is no longer active (Monaco et al, 1996;Perouse et al, 2012). Yet different clusters of earthquakes above m = 6 are reported for onshore Calabria (Rovida et al, 2016;Tiberti et al, 2017), that is, for the portion of the forearc lying approximately above the subduction megathrust, in open contrast with the low observed geodetic strain rates. Since clustering is a dominant character of seismic release of Calabria, is the seismicity of the past few centuries truly representative of its long-term pattern?…”

Hybrid GMPEs for Region-Specific PSHA in Southern Italy

“…Sketch map that illustrates the main tectonic features of the study area. Faults from Monaco and Tortorici [15], Minelli and Faccenna [16], Polonia et al [17], Tiberti et al [18]. Subduction interface from Maesano et al [19].…”

“…1908 and 1905 earthquakes, and part of the 1783 earthquake sequence ( [15,[23][24][25], and references therein). Strike-slip faults are responsible for the along-strike segmentation of the Calabrian Arc, which divided it into portions that are characterized by a diverse range of geomorphological features and by different uplift rates ( [18,26], and references therein). To the southwest, a NW-SE series of faults reflects the presence of a major deep-seated shear zone that bounds the active portion of the Ionian subduction (e.g., [19,27,28]).…”

“…No large earthquake is known for these sources; nevertheless, the size and geometry of the accretionary wedge thrust faults are fully consistent with an expected maximum magnitude of ≥7.0. The Calabrian Arc is characterized by a series of blind, low-angle, normal faults that alternate with strike-slip structures ( [18], and references therein). The main fault system of Calabria runs onshore along the Tyrrhenian coast for its entire length and consists of large normal faults that are believed to be responsible for the largest earthquakes in Calabria (and the whole of Italy), such as the M > 7.…”