The contribution of the NEMO-SN1 seafloor observatory to improve the seismic locations in the Ionian Sea (Italy)

The Western Ionian Sea is characterised by an active and diffuse seismicity, directly related to the convergence of the European and African Plates and by gravitational sinking and rollback of the oceanic lithosphere. In this area, the location of earthquakes is characterised by considerable uncertainties due to large azimuthal gaps, resulting in notable location errors. This problem was partially overcome with the use of data recorded by NEMO-SN1 seafloor observatory (October 2002 - February 2003; June 2012 - May 2013). We relocated 1130 crustal and sub-crustal earthquakes using land network and NEMO-SN1 data. As most events occurred on Mt. Etna, we focused on 358 earthquakes in the offshore area and near the coasts of Sicily and Calabria. The use of the combined land-marine networks has improved the earthquake locations in terms of azimuthal GAP, as well as in horizontal and vertical errors. The comparison between locations performed with and without NEMO-SN1 data shows that differences in latitude, longitude and depths are more evident in the Western Ionian Sea and in the coast of Sicily, where values of the differences over 5 km correspond to structural heterogeneities. The increased number of seismic stations deployed on land from 2003 to 2012 did not influence the location of events occurring offshore, where NEMO-SN1 continued to be the distinctive tool in the location process. Moreover, the new 73 focal mechanisms computed with P-wave polarities from NEMO-SN1 and land stations are in agreement with the regional structural model, showing a prevalent normal, normal/oblique, and strike-slip kinematics. The similarity of two new focal solutions with the mechanisms of the main shock and aftershock of the 1990 earthquake demonstrates that the seismic structures are still active and potentially dangerous. The P-wave travel time residual analysis confirms the activity along the main structural alignments. A single point of observation in the Ionian Sea can significantly improve the quality of locations, giving an opportunity to focus on the seismogenic structures responsible for the occurrence of medium-to-high magnitude earthquakes

Seismic location improvements from an OBS/H temporary network in Southern Tyrrhenian Sea

We present the first investigation performed on the seismicity of Southern Tyrrhenian Sea, off-shore Sicily with the contribution of data from broad-band ocean bottom seismometers and hydrophones (OBS/H). Offshore data were recorded during the TYrrhenian Deep sea Experiment (TYDE) from December 2000 to May 2001 in the Southern Tyrrhenian Sea. Hypocenter locations of a cluster of 53 seismic events occurred in March 2001 in north-eastern Sicily were estimated by the integration of land (permanent network) and offshore (temporary network) data and compared with locations estimated from land data only. The scatter of the cluster was evaluated by dispersion parameters. The off-shore data significantly reduced the scatter of the swarm hypocenters also restricting the depth range of the cluster. Moreover, space trends of the event distribution originally shown by the land data were only partially confirmed by the land-sea joint data. In order to assess the efficiency in terms of hypocenter mislocations in the subject area, of a land-sea integrated network with respect to a land-based network, we performed simulations by assuming a grid distribution of earthquakes and a recent local 3D velocity model, computing synthetic arrival times of body waves to the stations of both network configurations (integrated and land-based) perturbing the computed times and relocating earthquakes by inversion. The results of the synthetic tests demonstrated that the presence of sea bottom stations in the Tyrrhenian basin can reduce the mislocations of large magnitude and/or superficial earthquakes in the southernmost Calabria and Messina Strait and of low magnitude and/or deep earthquakes in north-eastern Sicily. The major accuracy of synthetic earthquake locations obtained including OBS/H data provides an additional support to the interpretation of the cluster occurred in March 2001 and to the opportunity of long-term installation of an off-shore network like TYDE in the study region

Geophysical multidisciplinary investigation of the structure of an unstable flank: the NE sector of Mt. Etna.

Mount Etna is characterized by a complex regional tectonics with a N-S compression related to the Africa – Europe convergence that interacts with a WNW-ESE extension associated to the Malta Escarpment. A general eastward motion is present in the eastern flank. Although the existence of these phenomena is overt, the geometry of the sliding sector is still debated. The non-uniqueness of the geophysical inverse models and the different limitations in resolution and sensitivity of each technique spurred us to undertake, in the frame of the MEDiterranean Supersites Volcanoes (MED-SUV) project, a joint interpretation of independent data in order to better constrain the results. Seismic data come from the network run by the Istituto Nazionale di Geofisica e Vulcanologia (INGV) – Osservatorio Etneo, Sezione di Catania. The relocated seismicity defines two main seismogenic volumes in the NE sector of the volcano: the first cluster is related to the known Pernicana Fault system, while the second one is located southwards, beneath the northern wall of the Valle del Bove. The resistivity models come from a MT survey carried out on the eastern flank of the volcano and consisting of thirty broad-band soundings along N-S and NW-SE oriented profiles. The resistivity modeling of MT profiles reveal three major layers in a resistive-conductive-resistive sequence. A low resistivity volume is clearly identified on the NE flank of the volcano, between The Pernicana fault and the northern wall of the Valle del Bove. Ground deformation studies (GPS and InSAR) revealed the segmentation of the unstable flank and define the NE sector as the most mobile one; this sector is perfectly bounded by the two seismic clusters and corresponds to the low resistivity volume. The sliding surface modeled by ground deformation data inversions well matches in depth with a resistivity transition and with two seismogenic layers

THE SHALLOW MAGMA CHAMBER OF STROMBOLI

AbstractIn this work, we integrate artificial and natural seismic sources data to obtain high‐resolution images of the shallow inner structure of Stromboli Volcano. Overall, we used a total of 21,953 P readings from an active seismic experiment and an additional 2731 P and 992 S readings deriving from 269 local events. The well‐defined Vp, Vs, and Vp/Vs tomograms have highlighted the following: (i) the region where magma cumulates at shallow depths (2–4 km below sea level (bsl)), forming an elongated NE‐SW high‐velocity body (Vp ≥ 6.0 km/s and Vs ≥ 3.5 km/s), with a very fast velocity core (6.5 ≤ Vp < 7.0 km/s) of ~2 km3; (ii) the presence of some near‐vertical pipe‐like structures, characterized by relatively high P velocities values, mainly linked to past activity (e.g., Strombolicchio); and (iii) a near‐vertical pipe‐like volume with high Vp/Vs (1.78 ÷ 1.85), located beneath to the craters (down to ~1.0 km bsl), overlying a deeper region (1.0 to 3.0 km bsl) with low Vp/Vs (1.64 ÷ 1.69), interpreted as the actual and preferential pathway of magma toward the surface. Our results demonstrate the importance of combining passive and active seismic data to improve, in a tomographic inversion, the resolution of the volcanic structures and to discover where magma may be stored

The EARTHCRUISERS project (EARTH CRUst Imagery for investigating SEismicity, volcanism and marine natural Resources in the Sicilian offshore)

The EARTHCRUISERS project was developed for the MIUR’s call “Progetti Premiali 2015” by the “Istituto Nazionale di Oceanografia e di Geofisica Sperimentale” (Trieste, Italy) in collaboration with the “Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo” (Catania, Italy) and “Stazione Zoologica Anton Dohrn” (Naples, Italy). The main goals of the project are: (i) to identify and characterize the main crustal tectonic structures offshore Sicily and the Aeolian Islands, (ii) to better understand the geodynamic processes controlling seismicity and volcanism affecting this region, and (iii) to furnish a useful tool to estimate seismic, tsunamigenic and volcanic hazard in the highly populated coastal sectors. Furthermore, in order to contribute at the Blue Growth objectives, the project aims to analyze some relevant issues related to mineral prospecting offshore, such as the characterization of the hydrothermal systems in the Tyrrhenian Sea and the impact of the exploitation of oil and gas fields on the marine environment in the Sicily Channel. To achieve these objectives the acquisition of multibeam and sidescan sonar, multichannel seismic reflection, magnetic and gravimetric data is planned. Nearly 2500 km of multichannel seismic reflection lines will be acquired during the project in the Marsili Basin (Tyrrhenian Sea) and Mt. Etna offshore. This large amount of data will allow to: better understand the relationship between tectonics and evolution of volcanism; identify active faults and volcanic bodies; better constrain the seismostratigraphic and structural setting of the study areas, and investigate the eventual occurrence of unstable volcanic slopes which could lead to landslide and tsunami. Finally, the deployment offshore southeastern Sicily of a temporary Ocean Bottom Seismometer (OBS) network will carry out for monitoring the natural seismicity in the area of VEGA platform, the largest oil extraction site in Italian seas. Data collected will be used to study the eventual correlation between local seismicity and oil extractive activities.PublishedRome2T. Deformazione crostale attiv

Seismological constraints for the dyke emplacement of the July-August 2001 lateral eruption at Mt. Etna volcano, Italy

In this paper we report seismological evidence regarding the emplacement of the dike that fed the July 18 - August 9, 2001 lateral eruption at Mt. Etna volcano. The shallow intrusion and the opening of the eruptive fracture system, which mostly occurred during July 12, and July 18, were accompanied by one of the most intense seismic swarms of the last 20 years. A total of 2694 earthquakes (1 £ Md £ 3.9) were recorded from the beginning of the swarm (July 12) to the end of the eruption (August 9). Seismicity shows the upward migration of the dike from the basement to the relatively thin volcanic pile. A clear hypocentral migration was observed, well constraining the upwards propagation of a near-vertical dike, oriented roughly N-S, and located a few kilometers south of the summit region. Earthquake distribution and orientation of the P-axes from focal mechanisms indicate that the swarm was caused by the local stress source related to the dike intrusion

Velocity structures and kinematics in the Ionian Sea (Italy) from seismological data recorded by NEMO-SN1 seafloor observatory

In the Western Ionian basin high magnitude earthquakes have occurred in the past and in recent times (e.g. 1193, M 6.6; 1693, M 7.4; 1908, M 7.2; 1990, M 5.7), sometimes followed by violent tsunamis [Boschi et al., 1997; Bianca et al., 1999]. The sources and mechanism related to the generation of these events are still either unknown or debated due to the inadequacy of the monitoring network to detect offshore medium low magnitude earthquakes and to the lack of a satisfactory velocity model to locate these events. In the periods October 2002February 2003 and June 2012May 2013 the NEMOSN1 seafloor observatory operated about 25 km from the eastern coast of Sicily at 2100 m of depth. During the two periods, NEMOSN1 recorded several hundreds of local events. Thanks to the good signal to noise ratio [Monna et al., 2005] of the seismological signals, the seafloor observatory recorded medium low magnitude earthquakes linked to the explosive eruption of Mt. Etna volcano occurred between October 2002 and January 2003, and, in addition, an intense microseismicity not recorded by any land station [Sgroi et al., 2007]. We integrated the travel times of about 1000 earthquakes recorded by NEMOSN1 and by the land stations with the aim of improving the location of these events. Moreover, thanks to the seismological data recorded by NEMOSN1 we were able to calculate a new 1D velocity model for the Western Ionian Sea. The first step was the preliminary relocation of the whole dataset (Figure 1a). From this dataset, we selected 108 best quality hypocentres (GAP ≤ 220°; rms ≤ 0.5 s; P and S phases number ≥ 8) and 33 seismic stations to ensure the best possible coverage of earthquakes and stations around NEMOSN1. Since the inversion involves the use of a starting velocity model [Kissling et al., 1994], we considered five initial reference models [Steinmetz et al., 1983; Hirn et al., 1991; De Voogdt et al., 1992; Continisio et al., 1997; Polonia et al., 2016] to better represent the structural heterogeneity and velocities in the offshore area of Sicily and south Calabria. Then, we computed the new 1D velocity model using the VELEST software [Kissling, 1995]. We performed many trials, adjusting the layer thickness of the initial model to better estimate the depth of the main discontinuities and the Moho. The minimum misfit of the traveltime residuals was achieved after many inversions and the output model was considered to be stable. The new 1D velocity model for the Ionian Sea consists of six layers above the Moho, located at a depth of 21 km (Figure 1b). The thickness of the layers and velocities are in agreement with the lithostratigraphic interpretation proposed by Polonia et al., [2016]. We used this new model to relocate the earthquakes of the whole dataset. The final relocations show a major concentration of earthquakes in the Mt. Etna volcano sector (in relation to its volcanic activity); a minor and more dispersed seismicity is evident in the Ionian Sea. Although events originated in the depth range 080 km, most of the earthquakes have hypocentral depths less than 30 km. An important achievement was the improvement in the location of earthquakes using the new velocity model, particularly in the Ionian offshore area, in terms of RMS, GAP, and horizontal and vertical errors. To infer the kinematics of earthquakes occurred in the Ionian basin, we computed 66 new fault plane solutions, by applying the standard FPFIT procedure [Reasenberg & Oppenheimer, 1985]. The events had a minimum number of eight clear polarities and most of the selected 66 faults plane solutions did not have discrepant polarities. The map of distribution of focal mechanisms and the EW and NS sections (Figure 2) show that earthquake kinematics are rather homogeneous. Normal, normal to strike-slip and strike-slip faulting mechanisms prevail, showing a depth distribution in good agreement with the regional structural model.PublishedIstituto Nazionale di Geofisica e Vulcanologia, Rome3A. Geofisica marina e osservazioni multiparametriche a fondo mar

Earthquakes recorded and located in the Mt. Etna area in the period 2002-2021.

Earthquakes recorded and located in the Mt. Etna area in the period 2002-2021. The table shows the main parameters of the analysed earthquakes. Specifically: No= identification number; Date, in format day-month-year; origin time (hour, minute and second); latitude north and longitude east; depth in km b.s.l.; magnitude.</p

Selection of daily distance variations (2002-2021) at Mount Etna

Selection of daily distance variations between two pairs of GPS benchmarks (EMGL-EMAL and EMEG-ESLN) located in the western flank of Mt. Etna during 2002-2021. (Format: two GPS daily distance variations separated by comma in meters) The authors acknowledge the Technicians and Technologists of the INGV - Osservatorio Etneo (GPS Permanent Network) for enabling and improving the acquisition of raw GNSS data. We are grateful to F. Cannavò that provided distance daily variations between two pairs of GPS benchmarks during the time periods when the solution is not available in Literature. </p

When probabilistic seismic hazard climbs volcanoes: the Mt. Etna case, Italy – Part 1: Model components for sources parameterization

The volcanic region of Mt. Etna (Sicily, Italy) represents a perfect lab for testing innovative approaches to seismic hazard assessment. This is largely due to the long record of historical and recent observations of seismic and tectonic phenomena, the high quality of various geophysical monitoring and particularly the rapid geodynamics clearly demonstrate some seismotectonic processes. We present here the model components and the procedures adopted for defining seismic sources to be used in a new generation of probabilistic seismic hazard assessment (PSHA), the first results and maps of which are presented in a companion paper, Peruzza et al. (2017). The sources include, with increasing complexity, seismic zones, individual faults and gridded point sources that are obtained by integrating geological field data with long and short earthquake datasets (the historical macroseismic catalogue, which covers about 3 centuries, and a highquality instrumental location database for the last decades). The analysis of the frequency–magnitude distribution identi- fies two main fault systems within the volcanic complex featuring different seismic rates that are controlled essentially by volcano-tectonic processes. We discuss the variability of the mean occurrence times of major earthquakes along the main Etnean faults by using an historical approach and a purely geologic method. We derive a magnitude–size scaling relationship specifically for this volcanic area, which has been implemented into a recently developed software tool – FiSH (Pace et al., 2016) – that we use to calculate the characteristic magnitudes and the related mean recurrence times expected for each fault. Results suggest that for the Mt. Etna area, the traditional assumptions of uniform and Poissonian seismicity can be relaxed; a time-dependent fault-based modeling, joined with a 3-D imaging of volcano-tectonic sources depicted by the recent instrumental seismicity, can therefore be implemented in PSHA maps. They can be relevant for the retrofitting of the existing building stock and for driving risk reduction interventions. These analyses do not account for regional M >6 seismogenic sources which dominate the hazard over long return times ( 500 years).Published1981–19985T. Modelli di pericolosità sismica e da maremotoJCR Journa