New seismological data from the Calabrian arc reveal arc-orthogonal extension across the subduction zone

Tiziana Sgroi,Graziella Barberi,Alina Polonia,Luca Gasperini,Andrea Billi

doi:10.1038/s41598-020-79719-8

Abstract

The Calabrian Arc subduction-rollback system along the convergent Africa/Eurasia plate boundary is among the most active geological structures in the Mediterranean Sea. However, its seismogenic behaviour is largely unknown, mostly due to the lack of seismological observations. We studied low-to-moderate magnitude earthquakes recorded by the seismic network onshore, integrated by data from a seafloor observatory (NEMO-SN1), to compute a lithospheric velocity model for the western Ionian Sea, and relocate seismic events along major tectonic structures. Spatial changes in the depth distribution of earthquakes highlight a major lithospheric boundary constituted by the Ionian Fault, which separates two sectors where thickness of the seismogenic layer varies over 40 km. This regional tectonic boundary represents the eastern limit of a domain characterized by thinner lithosphere, arc-orthogonal extension, and transtensional tectonic deformation. Occurrence of a few thrust-type earthquakes in the accretionary wedge may suggest a locked subduction interface in a complex tectonic setting, which involves the interplay between arc-orthogonal extension and plate convergence. We finally note that distribution of earthquakes and associated extensional deformation in the Messina Straits region could be explained by right-lateral displacement along the Ionian Fault. This observation could shed new light on proposed mechanisms for the 1908 Messina earthquake.

Highlights

The Calabrian Arc subduction-rollback system along the convergent Africa/Eurasia plate boundary is among the most active geological structures in the Mediterranean Sea
Our new dataset confirms the paucity of compressional earthquakes, except for those observed at the northern tip of the Ionian Fault (Figs. 5 and 6), and for some events with horizontal axis offshore southern Calabria, in the vicinity of out-of-sequence thrust faults located in the inner accretionary wedge (Fig. 4a)
The Calabrian Arc (CA) accretionary prism in the wester Ionian Sea is crossed by NW–SE striking deeply rooted faults, whose geodynamic significance is still enigmatic since seismological observations are scant

Summary

Introduction

The Calabrian Arc subduction-rollback system along the convergent Africa/Eurasia plate boundary is among the most active geological structures in the Mediterranean Sea. Spatial changes in the depth distribution of earthquakes highlight a major lithospheric boundary constituted by the Ionian Fault, which separates two sectors where thickness of the seismogenic layer varies over 40 km This regional tectonic boundary represents the eastern limit of a domain characterized by thinner lithosphere, arc-orthogonal extension, and transtensional tectonic deformation. Two major oppositely dipping fault systems (Fig. 1), the NW–SE-striking transtensional Ionian Fault (IF) and the Alfeo-Etna Fault (AEF), have been described based on analysis of multiscale geophysical data[3,11] Since these faults systems are regional deep-seated structures, they are likely candidates as seismogenic sources for the large magnitude earthquakes occurred in the western Ionian Sea during historical times[4,12,13] as suggested by the analyses of the seismically triggered turbidite r ecords[14,15]. The eastern Sicily/Calabria margins have been struck repeatedly by high magnitude earthquakes during historical times, including the largest events ever recorded in Italy[21] (e.g., 1169, Mw 6.6; 1693, Mw 7.4; 1908, Mw 7.2; 1990, Mw 5.7; Fig. 1), but location and geometry of tectonic sources are still uncertain, for recent events such as the 1908 Messina e arthquake[22]

Findings

Discussion

Conclusion