Offshore Eastern Sicily Research Articles

Structure and morphology of an active conjugate relay zone, Messina Strait, southern Italy

AbstractMessina Strait is a narrow fault‐bounded marine basin that separates the Calabrian peninsula from Sicily in southern Italy. It sits in a seismically active region where normal fault scarps and raised Quaternary marine terraces record ongoing extension driven by southeastward rollback of the Calabrian subduction zone. A review of published studies and new data shows that normal faults in the Messina Strait region define a conjugate relay zone where displacement is transferred along strike from NW‐dipping normal faults in the northeast (southern Calabria) to the SE‐dipping Messina‐Taormina normal fault in the southwest (offshore eastern Sicily). The narrow marine strait is a graben undergoing active subsidence within the relay zone, where pronounced curvature of normal faults results from large strain gradients and clockwise rotations related to fault interactions. Based on regional fault geometries and published age constraints, we infer that normal faults in southern Calabria migrated northwest while normal faults in NE Sicily migrated southeast during the past ca. 2–2.5 Myr. This pattern has resulted in tectonic narrowing of the strait through time by inward migration of facing normal faults and rapid mantle‐driven uplift.

Faulting, sediment loading, and flow of underlying ductile units: A case study from the Western Ionian Basin Offshore Eastern Sicily

Tectono-stratigraphic analysis coupled with digital 3D surface modelling derived from high-resolution seismic profiles is performed along a narrow turbidite basin offshore E-Sicily in order to increase understanding on the processes that contributed to the shaping of the Western Ionian Basin. Seismic-reflector patterns of the identified Pliocene-Quaternary sequence point to syn-depositional deformation during the Pliocene associated with the simultaneous activity of regional faults and underlying ductile units. Long-wavelength sediment fanning results from the extensional activity of the Malta Escarpment faults. Conversely, internal reflector architecture and lateral terminations indicate localized subsidence associated with the growth of uprising structures in the easternmost part of the basin. Lateral shifting of basin depocenters is in line with withdrawal effects observed in basins floored by ductile units (salt or shale). 3D modelling of time-reference surfaces highlights sub-circular depressions associated with nearby structural culminations. This pattern is similar to salt-withdrawal minibasins commonly reported in evaporite-floored basins. Accordingly, salt migration/flow triggered by sediment loading, locally enhanced by fault activity, is proposed as the process controlling basin evolution during the Pliocene in the Western Ionian Domain. Nevertheless, the possibility of shale/mud tectonics as the ductile source of deformation cannot be discounted.

Reconstruction of the 1908 Messina gravity flow (central Mediterranean Sea) from geophysical and sedimentological data

Earthquakes, tsunamis and gravity flows are common processes offshore Eastern Sicily and pose a significant hazard to coastal communities and infrastructure. The 1908 Messina earthquake and tsunami resulted in >60,000 casualties. It caused a large turbidity current, which broke the Malta-Zante telegraph cable. Yet, this gravity flow remains poorly characterised in terms of its route and flow behaviour. A comprehensive analysis of multibeam echosounder data, sub-bottom profiles, and sediment cores has been carried out to improve our understanding about gravity flow activity within conduit systems of the western Ionian Basin to reconstruct the characteristics of the 1908 sediment flow (e.g., erosion, velocity, source region). Three main canyon-channel systems can be distinguished within the study area. The easternmost system (C3) appears to be the most active in terms of sediment transport. There are numerous erosional and depositional bedforms, including large-scale scours (>100 m-long), turbidite sediment waves and channel wall collapses that are not overprinted by younger events. The other two canyon-channel systems (C1, C2) do not show many bedforms indicative of repeated and recent gravity flow activity. Indeed, the transport of the majority of sediment discharged into the western system (C1) is limited to <25 km downslope from the continental slope, while the central system (C2) facilitates sediment deposition from gravity flows. C3 is, thus, suggested to have been the main passageway of the 1908 sediment flow. It also leads directly to two of three cable break locations. The most likely source areas for the gravity flow are north-eastern Sicily and southern Calabria. Bedforms indicate a flow thickness of >170 m along the upper channel portion of C3 and > 140 m along its lower portion close to the cable breaks. An average flow velocity of 5.6 to 6.3 ms−1 is reconstructed, given the timing of the breaks and length of the canyon-channel system. The flow may have locally decelerated and accelerated while bypassing morphologic highs and knickpoints. These new findings significantly improve our understanding of the 1908 gravity flow (e.g., passageways, depositional/erosional behaviour, thickness, velocity) and provide important insights into gravity flow events in general, especially those with a large run-out. This knowledge is needed to assess potential hazards associated with these events.

Long-term monitoring of cetacean bioacoustics using cabled observatories in deep-sea off East Sicily

The EMSO Research Infrastructure operates multidisciplinary seafloor-cabled observatories in a deep-sea area offshore Eastern Sicily (2100 m of depth). In a data-lacking zone, Passive Acoustic Monitoring activities revealed new information on cetacean bioacoustics over multiple ecological scales. Expert operators investigated the presence of cetacean vocalizations within the large acoustic datasets acquired. Then, algorithms were developed to provide information on the behavior and ecology of the recorded species. In 2005-2006, the acoustic activity of toothed whales was investigated through the OvDE antenna (100 Hz to 48 kHz). The assessment of the size distribution of sperm whales was acoustically possible and the tracking of the animals showed the direction of movement and the diving profile. The biosonar activity of dolphins resulted mostly confined in the nighttime, linked to seasonal variation in daylight time and prey-field variability known for these deep-pelagic waters. Furthermore, in 2012-2013, we monitored the annual acoustic presence of fin whales thanks to the NEMO-SN1 station (1 Hz to 1 kHz). The results showed that the species was present throughout all seasons, with peaks in call detection rate during spring and summer months, and that the fin whale calls were mostly detected in low background noise conditions.

Annual Acoustic Presence of Fin Whale (Balaenoptera physalus) Offshore Eastern Sicily, Central Mediterranean Sea.

In recent years, an increasing number of surveys have definitively confirmed the seasonal presence of fin whales (Balaenoptera physalus) in highly productive regions of the Mediterranean Sea. Despite this, very little is yet known about the routes that the species seasonally follows within the Mediterranean basin and, particularly, in the Ionian area. The present study assesses for the first time fin whale acoustic presence offshore Eastern Sicily (Ionian Sea), throughout the processing of about 10 months of continuous acoustic monitoring. The recording of fin whale vocalizations was made possible by the cabled deep-sea multidisciplinary observatory, “NEMO-SN1”, deployed 25 km off the Catania harbor at a depth of about 2,100 meters. NEMO-SN1 is an operational node of the European Multidisciplinary Seafloor and water-column Observatory (EMSO) Research Infrastructure. The observatory was equipped with a low-frequency hydrophone (bandwidth: 0.05 Hz–1 kHz, sampling rate: 2 kHz) which continuously acquired data from July 2012 to May 2013. About 7,200 hours of acoustic data were analyzed by means of spectrogram display. Calls with the typical structure and patterns associated to the Mediterranean fin whale population were identified and monitored in the area for the first time. Furthermore, a background noise analysis within the fin whale communication frequency band (17.9–22.5 Hz) was conducted to investigate possible detection-masking effects. The study confirms the hypothesis that fin whales are present in the Ionian Sea throughout all seasons, with peaks in call detection rate during spring and summer months. The analysis also demonstrates that calls were more frequently detected in low background noise conditions. Further analysis will be performed to understand whether observed levels of noise limit the acoustic detection of the fin whales vocalizations, or whether the animals vocalize less in the presence of high background noise.

Dataset from Annual Acoustic Presence of Fin Whale (Balaenoptera physalus) Offshore Eastern Sicily, Central Mediterranean Sea

Dataset from Annual Acoustic Presence of Fin Whale (Balaenoptera physalus) Offshore Eastern Sicily, Central Mediterranean Sea

Reply to comment on the article “Propagation of a lithospheric tear fault (STEP) through the western boundary of the Calabrian accretionary wedge offshore eastern Sicily (Southern Italy)” by Gallais et al., 2013 Tectonophysics

In his Comment, A. Argnani argues that our interpretation of the STEP fault (named STEP-1 fault, in the Comment) is poorly constrained by the seismic data used in Gallais et al. (2013), and he particularly disagrees with our identification north of the Alfeo Seamount. A. Argnani is convinced that the tear fault is expressed by a belt of surface deformation, close to the Malta Escarpment. However, none of the seismic images published by A. Argnani show a crustal scale structure that could be associated with the presence at depth of a lithospheric tear. In that paper we documented the presence of a sub-vertical crustal scale fault, located 50 km east of the Malta Escarpment, with a northward increasing vertical offset (Gallais et al., 2013). The track of this fault towards the north could be extended following the position the fault “F6”, mapped 20 km east of the Malta Escarpment by Nicolich et al. (2000). In fact these authors reported an offset of the crust associated with activity the fault “F6”, suggesting the lithospheric tear at depth at the edge of the Ionian slab is distinct from the Malta Escarpment.

Comment on the article “Propagation of a lithospheric tear fault (STEP) through the western boundary of the Calabrian accretionary wedge offshore eastern Sicily (Southern Italy)” by Gallais et al., 2013 Tectonophysics

Abstract The oceanic lithosphere of the Ionian Sea is subducted under the Calabrian Arc, and it has been proposed that the Ionian lithosphere has been torn apart (STEP fault sensu Govers and Wortel, 2005) on its western side, close to the Malta escarpment, a major morphological feature inherited from the Mesozoic palaeogeography. In a recent paper Gallais et al. (Tectonophysics, 2013) have traced a right-lateral strike-slip fault that extends from south of the Alfeo Seamount all the way to the Tyrrhenian Sea. They also argue that the position of the STEP fault is distinct from, and located ca. 50 km east of, the Malta escarpment. Gallais et al. document with some additional details a fault segment that was already reported in the literature and that is located south of the Alfeo Seamount. However, Gallais et al. have little constraints to trace the STEP fault further to the north; in this sector a belt of surface deformation that can be related to the STEP fault has been mapped from a denser grid of seismic profiles, and is located west-ward, close to the Malta escarpment (Argnani and Bonazzi, 2005). This undermines the conclusion of Gallais et al. that the lithospheric tear is distinct from the Malta escarpment. It is therefore likely that surface deformation associated with the lithospheric tear fault is more complex and is not represented by a single strike-slip fault.

Propagation of a lithospheric tear fault (STEP) through the western boundary of the Calabrian accretionary wedge offshore eastern Sicily (Southern Italy)

The rollback of a segmented slab of oceanic lithosphere is typically accompanied by vertical lithospheric tear fault(s) along the lateral slab edge(s) and by strike slip movement in the upper plate, defined as a STEP fault (Subduction Tear Edge Propagator). The Neogene evolution of the Central Mediterranean is dominated by the interaction between the slow Africa–Eurasia convergence and the SE-ward rollback of the Ionian slab, that leads to the back-arc opening of the Tyrrhenian Sea. Here, we present post-stack time migrated and pre-stack depth migrated Archimede (1997) multichannel seismic lines, that were acquired offshore eastern Sicily, at the foot of the Malta escarpment . First, we identify the recent deformation along the lateral ramp of the Calabrian accretionary wedge. Towards the east, the Calabrian wedge is formed by the accretion of the post-evaporitic sediments, above a decollement at the base of the Messinian evaporites . At the latitude of Syracuse, 50 km east of the Malta escarpment, a major N150°E trending crustal scale and vertical fault slices through the entire accretionary wedge. This fault cuts by several kilometers, through the pre-evaporitic Messinian sediments and into the basement. The vertical offset along this vertical fault decreases from north to south, and the fault is no longer observed on the seismic lines, 50 km SE of the Alfeo seamount . A previously published Moho depth isocontour map, offshore Sicily and the recent GPS data, combined with the presence of strike slip movements NE onshore Sicily, allow us to identify this 200 km long crustal-scale fault as the surface expression of a STEP fault. The presence of syntectonic Pleistocene sediments on top this crustal-scale fault suggests a recent lithospheric vertical movement of the STEP fault, in response to the rollback of the Ionian slab and to the SE-ward advance of the Calabria-Peloritan block. ► Depth migrated seismic reflection profiles from the Calabria/Sicily active margin. ► A N150°E striking, lithospheric scale fault, 50 km east of the Malta escarpment ► Interpreted to be a “Subduction Tear Edge Propagator” (STEP) fault ► Fault offset decreases to the S until termination (at the propagating tip) ► Deformed sediments suggest recent fault activity (related to Calabrian subduction).

Seismic facies and seabed morphology in a tectonically controlled continental shelf: The Augusta Bay (offshore eastern Sicily, Ionian Sea)

Morphological and seismic–stratigraphic features of the Augusta Bay (offshore eastern Sicily) have been characterized by means of Sub-bottom Chirp profiles, Multibeam data and Side Scan Sonar images, covering an area of about 110 km 2 . The marine data set was combined to mainland stratigraphy, derived from coastal boreholes and geological maps, to achieve a land–sea correspondence of main morpho-structural and stratigraphical features. The study area is located in the western sector of the Ionian Sea which represents a key area to reconstruct the geological parameters that control the processes of convergence between Africa and Europe in the Mediterranean. In particular, the area of interest draws the attention for problems concerning high seismicity and environmental pollution from oil and chemical industries. The Augusta Bay offshore displays a marked variability of morphologies and seismic–stratigraphic characters, related to the influence of both large-scale geological processes and local variables. It consists of an irregular surfaced narrow shelf, generally tracing out the shape of the relative coastline seaward bounded by a steep continental slope. This latter is deeply engraved by gullies and shows evidences of recent withdrawal, favored by the occurrence of structural lineaments. The Augusta offshore reflects the corresponding on-land geological pattern, being characterized by a central basinal area filled with Pleistocene–Holocene successions, bounded by two relieved sectors corresponding to the seaward prosecution of the Mt. Tauro and Magnisi–St. Panagia horsts, where the Miocene substratum outcrops. The seismic–stratigraphic architecture of the basinal area not only is the result of sea-level variations related to the Late Pleistocene sea-level fall and subsequent transgressive phase, responsible for the formation of a main angular unconformity, but also depends on pre-existing substratum arrangement, which leads to a wide lateral facies variability, on short distances. Morphological and seismic–stratigraphic evidences outline the presence of structural lineaments whose orientation is consistent with that of the on-land sectors. Slight movements are testified by the dislocation of recent sedimentary covers, pointing to active tectonic processes during Holocene. Such vertical movements are also responsible for a southward differential subsidence of the basinal area, whereas the proximal portion of the shelf seems to be strictly connected to the uplifting emerged sectors. This evidence is consistent with the recognition of recently active tectonic structures where the distal offshore represents the down-faulted sector. Our results, providing a detailed knowledge of the submerged sector of a highly populated area, affected by high seismicity and environmental pollution risk may significantly contribute to an appropriate management of the area in the next future. ► We give a description of the Augusta offshore through integration of geophysical data. ► Offshore stratigraphical and structural patterns reflect those of the emerged area. ► Facies distribution is strongly controlled by preexisting morpho-structural features. ► Current tectonic activity is responsible for subsidence of the distal shelf. ► Tectonic is also responsible for the slope withdrawal.

Vertical crustal motions from differential tide gauge observations and satellite altimetry in southern Italy

Our goal is to determine vertical crustal movement rates from tide gauge and satellite altimetry measurements. Tide gauges measure sea level, but as they are fixed to the crust, they sense both sea surface height variations and vertical crustal movements. The differential sea level rates of sufficiently nearby stations are a good means to determine differential crustal movement rates, when sea level height variations can be assumed to be homogeneous. Satellite altimetric measurements determine sea surface height variations directly and can be used to separate the crustal signal from the sea surface height variations in tide gauge measurements. The correction of the tide gauge sea level rates for the sea surface height contribution requires collocation of the satellite pass and the tide gauge station. We show that even if this is not the case, the satellite altimetric observations enable correction of differential tide gauge rates for the effects of sea surface rate inhomogeneities. We apply the methodology to an area of broad scientific interest, due to its high seismic risk and its location as standpoint for a proposed major bridge connecting Sicily to the Italian mainland. We find that the Southern Calabria and the eastern Sicily tide gauges have a deficit in sea level increase of 1–2 mm/yr with respect to the north western Sicilian tide gauge. The satellite altimetric observations show that this differential movement must be caused by a tectonic component, because the sea surface rates are higher offshore eastern Sicily compared to offshore western Sicily. The satellite altimetric rates show that the sea surface rates are inhomogeneous in the Mediterranean and have larger amplitudes as we move away from the coast than immediately offshore. Our technique can be applied to any part of the world where tide gauge observations are available, because satellite altimetric observations are global. © 2010 Elsevier Ltd. All rights reserved.

GEMS: Underwater spectrometer for long-term radioactivity measurements

GEMS (Gamma Energy Marine Spectrometer) is a prototype of an autonomous radioactivity sensor for underwater measurements, developed in the framework for a development of a submarine telescope for neutrino detection (KM3NeT Design Study Project). The spectrometer is highly sensitive to gamma rays produced by 40K decays but it can detect other natural (e.g., 238U, 232Th) and anthropogenic radio-nuclides (e.g., 137Cs). GEMS was firstly tested and calibrated in the laboratory using known sources and it was successfully deployed for a long-term (6 months) monitoring at a depth of 3200 m in the Ionian Sea (Capo Passero, offshore Eastern Sicily). The instrument recorded data for the whole deployment period within the expected specifications. This monitoring provided, for the first time, a continuous time-series of radioactivity in deep-sea.

New observations of local seismicity by the SN-1 seafloor observatory in the Ionian Sea, off-shore Eastern Sicily (Italy)

SUMMARY Seismicity in Eastern Sicily as recorded by the Submarine Network-1 seafloor observatory (SN1) in the period from 2002 October to 2003 May is examined with the aim of identifying the as yet poorly known seismogenic zones placed in the Ionian basin, where some of the strongest earthquakes have occurred. A comparison between the seismic recordings of land networks and the seafloor station has allowed us to focus on low-magnitude seismicity only recorded by SN-1. We have analysed 239 high-quality events from among a total of 485 seismic signals not included in the land-based network bulletins. The waveform features and the possible source zones for those events are investigated by means of polarization and particle motion techniques. Most of the 239 events (213) are characterized by high values of rectilinearity typical of P- and S-arrival particle motions, while the remaining 26 events have different polarization features, with an emergent first phase and prevalently planar polarization. We have interpreted the latter signals as being associated to submarine landslides. From particle motion analysis, we have determined the azimuthal distribution of the events and the incidence angles of P waves in respect to the Observatory with the aim of determining their distribution in relation to the active but scarcely known structural setting of the off-shore area. Moreover, the integrated locations of some earthquakes occurring in the study area and recorded by SN-1 and land stations was performed to determine the apparent P-wave velocity necessary to calculate source-station distances. As an additional result of the integration, we have obtained more accurate locations of earthquakes occurring in the coastal and off-shore areas of Eastern Sicily, associated with reduced horizontal and vertical errors and significantly lower values of azimuthal gaps. Lastly, a location distribution of the 213 analysed events was obtained by setting two conditions: (1) a maximum epicentral distance to a fixed depth coinciding with the depth of the seafloor station and (2) a minimum epicentral distance associated to the maximum depth of events. Accordingly, two patterns of seismicity were determined for the maximum and the minimum expected spread of the hypocentres. The main features of both patterns are a diffuse seismicity in the Western Ionian basin with a major epicentre density SE of SN-1 and a depth of most of the events within 60 km. Local magnitude determination was also performed, taking into account an attenuation law proposed for Southeastern Sicily. Despite the uncertainties in the location distribution using single-station recordings, the results show diffuse seismicity all around SN-1 and, in particular, in the off-shore area.

Malta Escarpment fault zone offshore eastern Sicily: Pliocene‐Quaternary tectonic evolution based on new multichannel seismic data

The Malta Escarpment represents the dominant morphological feature offshore eastern Sicily, linking the deep Ionian basin to the east with the Hyblean carbonate platform to the west. Interpretation of purposely acquired multichannel seismic data allows division of the Malta Escarpment into two portions characterized by different tectonic structures. Along the segment south of Siracusa the Malta Escarpment is not affected by recent faulting and appears as a steep surface that flattens out toward the Ionian basin. A recent deformation, characterized by a broad area of uplift, occurs 20–30 km east from the slope, along a NNW‐SSE trend. The segment of the Malta Escarpment extending north of Siracusa, on the other hand, is characterized by the presence of NNW‐SSE, east dipping recent extensional faults and related sedimentary basins. The observed structural features support the occurrence of a lithospheric tear between the Ionian oceanic basin and the Hyblean plateau.

Main shock and aftershocks of the December 13, 1990, Eastern Sicily earthquake

n this paper we describe the location and the fault plane solution of the December 13, 1990, Eastern Sicily earthquake (ML = 5.4), and of its aftershock sequence. Because the main shock location is not well constrained due to the geometry of the permanent National Seismic Network in this area, we used a "master event" algorithm to locate it in relation to a well located aftershock. The revised location is slightly offshore Eastern Sicily, 4.8 km north of the largest aftershock (ML = 4.6) that occurred on December 16, 1990. The main shock has a strike-slip mechanism, indicating SE-NW compression with either left lateral motion on a NS plane, or right lateral on an EW plane. Two days after the main event we deployed a local network of eight digital stations, that provided accurate locations of the aftershocks, and the estimate of source parameters for the strongest earthquake. We observed an unusual quiescence after the ML = 5.4 event, that lasted until December 16, when a ML = 4.6 earthquake occurred. The fault plane solution of this aftershock shows normal faulting on E-W trending planes. Between December 16 and January 6, 1991, a sequence of at least 300 aftershock" was recorded by the local network. The well located earthquakes define a small source region of approximately 5 x 2 x 5 km3, with hypocentral depths ranging between 15 and 20 km. The paucity of large aftershocks, the time gap between the main shock occurrence and the beginning of the aftershock sequence (3.5 days), their different focal mechanisms (strike-slip vs. normal), and the different stress drop between main shock and after- shock suggest that the ML = 5.4 earthquake is an isolated event. The sequence of aftershocks began with the ML = 4.6 event, which is probably linked to the main shock with a complex mechanism of stress redistribution after the main faulting episode.