Search

Ocean bottom seismometers (OBS) commonly record short duration events (SDEs), that could be described by all of these characteristics: (i) duration < 1 s, (ii) one single‐wave train with no identified P‐ nor S‐wave arrivals and (iii) a dominant frequency usually between 4 Hz and 30 Hz. In many areas, SDEs have been associated with gas or fluid‐related processes near cold seeps or hydrothermal vents, although fish bumps, instrumental or current‐generated noise have been proposed as possible sources. In order to address some remaining issues, this study presents results from in situ and laboratory experiments combined with observations from 2 contrasting areas, the Sea of Marmara (Turkey) and the Chilean subduction zone. The in situ experiment was conducted at the EMSO‐Molène submarine observatory (near Brest, France) and consisted in continuously monitoring two OBSs with a camera. The images revealed that no fish regularly bumped into the instruments. Laboratory experiments aimed at reproducing SDEs’ waveforms by injecting air or water in a tank filled by sand and sea‐water and monitored with an OBS. Injecting air in the sediments produced waveforms very similar to the observed SDEs, while injecting air in the water column did not, constraining the source of SDEs in the seafloor sediments. Finally, the systematic analysis of two real data sets revealed that it is possible to discriminate gas‐related SDEs from biological or sea‐state related noise from simple source parameters, such as the temporal mode of occurrence, the back azimuth and the dominant frequency. Key Points Short duration high amplitude events routinely recorded on marine seismometers are not created by fish bumping into the geophone. In laboratory experiments, the waveforms of these events are well reproduced by fluid migration in the sediments. We propose 3 simple source parameters to discriminate gas‐related short duration events in ocean bottom seismometer data.

Keypoints This contribution is a reply on a comment submitted by A. Argnani. The alternate interpretation of the wide-angle seismic model is discussed. The Alfeo Fault system is proposed to be the current location of STEP fault. Abstract Andrea Argnani in his comment on Dellong et al., 2020 (Geometry of the deep Calabrian subduction (Central Mediterranean Sea) from wide‐angle seismic data and 3D gravity modeling), proposes an alternate interpretation of the wide-angle seismic velocity models presented by Dellong et al., 2018 and Dellong et al., 2020 and proposes a correction of the literature citations in these paper. In this reply, we discuss in detail all points raised by Andrea Argnani.

The Calabrian subduction zone is one of the narrowest arcs on Earth and a key area to understand the geodynamic evolution of the Mediterranean and other marginal seas. Here in the Ionian Sea, the African plate subducts beneath Eurasia. Imaging the boundary between the downgoing slab and the upper plate along the Calabrian subduction zone is important for assessing the potential of the subduction zone to generate mega‐thrust earthquakes and was the main objective of this study. Here we present and analyze the results from a 380 km long, wide‐angle seismic profile spanning the complete subduction zone, from the deep Ionian Basin and the accretionary wedge to NE Sicily, with additional constraints offered by 3‐D Gravity modeling and the analysis of earthquake hypocenters. The velocity model for the wide‐angle seismic profile images thin oceanic crust throughout the basin. The Calabrian backstop extends underneath the accretionary wedge to about 100 km SE of the coast. The seismic model was extended in depth using earthquake hypocenters. The combined results indicate that the slab dip increases abruptly from 2‐3° to 60‐70° over a distance of ≤50 km underneath the Calabrian backstop. This abrupt steepening is likely related to the roll‐back geodynamic evolution of the narrow Calabrian slab which shows great similarity to the shallow and deep geometry of the Gibraltar slab. Plain language abstract We investigate the deep crustal structure of southern Italy and the Central Mediterranean where some of the oldest oceanic crust on Earth is actively descending (subducting) into the earth's interior (Speranza et al., 2012). This process causes much of the moderate seismicity observed in this region and may be responsible for strong historical earthquakes as well (Gutscher et al., 2006). Deep seismic data recorded during a marine geophysical expedition performed in 2014, allow us to reconstruct the 3‐D geometry of this subduction zone. Our data reveal a 1‐4 km thick evaporitic (salt bearing) layer in the 13 km thick accretionary wedge. The thin underlying crust has characteristics of oceanic crust. The adjacent onshore domains (E Sicily and SW Calabria) are composed of 25‐30 km thick crust with velocities typical of continental crust. Together with earthquake travel‐time tomography (providing images of the subducting slab down to 300 km) and gravity modeling we can for the first time image the abrupt steepening of the subducting slab, the “slab hinge”, where slab dip increases from ≤5° to >60° over a downdip distance of 50 km. This slab dip is steep compared to other subduction zones, for example in Northern Honshu Japan or Sumatra, where the slab dip remains roughly 10° down to 40 km depth and therefore may have consequences on the seismicity of the region.

The nature of the Ionian Sea crust has been the subject of scientific debate for more than 30 years, mainly because seismic imaging of the deep crust and upper mantle of the Ionian Abyssal Plain (IAP) has not been conclusive to date. The IAP is sandwiched between the Calabrian and Hellenic subduction zones in the central Mediterranean. A NNE–SSW-oriented 131 km long seismic refraction and wide-angle reflection profile, consisting of eight ocean bottom seismometers and hydrophones, was acquired in 2014. The profile was designed to univocally confirm the proposed oceanic nature of the IAP crust as a remnant of the Tethys and to confute its interpretation as a strongly thinned part of the African continental crust. A P-wave velocity model developed from travel-time forward modelling is refined by gravimetric data and synthetic modelling of the seismic data. A roughly 6–7 km thick crust with velocities ranging from 5.1 to 7.2 km s−1, top to bottom, can be traced throughout the IAP. In the vicinity of the Medina seamounts at the southern IAP boundary, the crust thickens to about 9 km and seismic velocities decrease to 6.8 km s−1 at the crust–mantle boundary. The seismic velocity distribution and depth of the crust–mantle boundary in the IAP document its oceanic nature and support the interpretation of the IAP as a remnant of the Tethys lithosphere with the Malta Escarpment as a transform margin and a Tethys opening in the NNW–SSE direction.

International audience; In the Ionian Sea (central Mediterranean) the slow convergence between Africa and Eurasia results in the formation of a narrow subduction zone. The nature of the crust of the subducting plate remains debated and could represent the last remnants of the Neo-Tethys ocean. The origin of the Ionian basin is also under discussion, especially concerning the rifting mechanisms as the Malta Escarpment could represent a remnant of this opening. This subduction retreats toward the southeast (motion occurring since the last 35 Ma) but is confined to the narrow Ionian basin. A major lateral slab tear fault is required to accommodate the slab roll-back. This fault is thought to propagate along the eastern Sicily margin but its precise location remains controversial. This study focuses on the deep crustal structure of the eastern Sicily margin and the Malta Escarpment. We present two two-dimensional P wave velocity models obtained from forward modeling of wide-angle seismic data acquired onboard the R/V Meteor during the DIONYSUS cruise in 2014. The results image an oceanic crust within the Ionian basin as well as the deep structure of the Malta Escarpment, which presents characteristics of a transform margin. A deep and asymmetrical sedimentary basin is imaged south of the Messina strait and seems to have opened between the Calabrian and Peloritan continental terranes. The interpretation of the velocity models suggests that the tear fault is located east of the Malta Escarpment, along the Alfeo fault system.