Ciaccio, Maria Grazia

La revisione da parte degli analisti del BSI della sismicità registrata in Italia dal 1 settembre al 31 dicembre 2022 ha riguardato tutti i terremoti di magnitudo M≥1.5, mentre i parametri dei terremoti di magnitudo inferiore a tale soglia sono quelli calcolati in tempo reale, nella SALA DI SORVEGLIANZA SISMICA DI ROMA. I terremoti più forti (M≥3.5) e pochi altri di particolare interesse [vedi Marchetti et al., 2016, DOI: 10.4401/ag-6116], sono stati revisionati dagli analisti del BSI, mediamente nelle 24 ore successive al loro accadimento.

La revisione da parte degli analisti del BSI della sismicità registrata in Italia dal 1 gennaio al 30 aprile 2023 ha riguardato tutti i terremoti di magnitudo M≥1.5, mentre i parametri dei terremoti di magnitudo inferiore a tale soglia sono quelli calcolati in tempo reale, nella SALA DI SORVEGLIANZA SISMICA DI ROMA. I terremoti più forti (M≥3.5) e pochi altri di particolare interesse [vedi Marchetti et al., 2016, DOI: 10.4401/ag- 6116], sono stati revisionati dagli analisti del BSI, mediamente nelle 24 ore successive al loro accadimento.

This study presents data and preliminary analysis from a temporary seismic network (SPQR), which was deployed in the urban area of Rome (Italy) for three months in early 2021. The network was designed to investigate the city’s subsurface while evaluating the feasibility of a permanent urban seismic network, and consisted of 24 seismic stations. Despite significant anthropogenic noise, the SPQR network well recorded earthquake signals, revealing clear spatial variability referable to site effects. In addition, the network’s continuous recordings allowed the use of seismic noise and earthquake signals to derive spectral ratios at sites located in different geological and lithological settings. During the experiment, there were periods of activity restrictions imposed on citizens to limit the spread of COVID‐19. Although the observed power spectral density levels at stations may not show visible noise reductions, they do cause variations in calculated spectral ratios across measurement sites. Finally, a statistical noise analysis was conducted on continuous seismic station data to evaluate their performance in terms of detection threshold for earthquakes. The results indicate that all network stations can effectively record earthquakes with a good signal‐to‐noise ratio (≥5 for P and S phases) in the magnitude range of 1.9–3.3 at distances of 10 km and 80 km, respectively. In addition, the network has the potential to record earthquakes of magnitude 4 up to 200 km, covering areas in Central Italy that are far from the city. This analysis shows that it is possible to establish urban observatories in noisy cities such as Rome, where hazard studies are of particular importance due to the high vulnerability (inherent fragility of its monumental heritage) and exposure.

High-resolution adjoint tomography has emerged as a powerful tool for unraveling the complexities of the Earth's lithosphere. We present an overview of the analysis conducted on the seismic images generated by the application of high-resolution adjoint tomography for the lithosphere beneath Southern Italy and the Adriatic region. Recently, we have proposed IMAGINE_IT, a reference 3D high-resolution seismic tomography model for tectonic and geological structures of the Italian lithosphere. Enhanced accuracy is enabled by state-of-the-art methods, including three-dimensional wavefield simulations based on SPECFEM3D in combination with an adjoint-state method. Adria plate plays a peculiar role in the geodynamics of the Central Mediterranean. It is the foreland of non-coeval mountain ranges and its margins are consumed in the process by subduction systems under the Alps to the north, the Apennines to the west and the Dinarides to the east. The complex behavior of this system and the large geographical heterogeneity in data availability lead to a fragmented understanding of the Adria plate. In particular, its lithospheric structure, in terms of Vp and Vs profiles, is poorly known due to a lack of seismic stations, poor earthquake location quality (large observational gaps), and the consequent lack of coverage by classical seismic tomography methods. The uncertainties increase the difficulty of correctly assessing the seismic hazard along the Adriatic coasts (including tsunami hazard evaluation). Here, we present additional details of this region, such as the mid-Adriatic ridge, and a preliminary set of iterations that exploit 7 years of additional data (IMAGINE_IT was limited to data until 2015) and the recent deployment of very dense regional arrays of broadband seismic stations– the 2016-2019 AlpArray and the AdriaArray Seismic Network currently under installation – which provide a new opportunity to improve our comprehension of the area. Furthermore, we focus on southern Italy, starting from L’Aquila region up to Calabrian Arc. The analysis of the images produced by high-resolution adjoint tomography IMAGINE_IT reveals intricate details of the lithospheric architecture, including crustal thickness variations, seismic velocity anomalies, and (lack of) subduction-related features.

Seismology is the study of earthquakes and of the propagation of seismic waves within the Earth. Seismologists study the Earth’s—and other planets’ interiors; provide detailed information on the shallow subsurface composition, where they help find resources (e.g., oil, gas, and geothermal) or estimate the ground stability, an information that is nowadays widely used in building codes. Seismology is a relatively young science that profited enormously from the technological and computational improvements of the past 2 decades. The first analogue seismographs, weighing several tons, appeared in the late 19th century. It was not before the mid 20th century that seismometers were fully digital and of portable sizes, which resulted in much denser deployments and recordings and an explosion in research of various aspects of our Earth (Agnew, 1989; Shearer, 2019).

The evolution and state of geological structure at Earth’s surface is best understood with an accurate characterization of the subsurface, where fluid distribution plays a key role. We present high-resolution seismic tomographic images of tectonic and geological features of the Italian lithosphere based on ground motion recordings and obtained through an iterative procedure. Enhanced accuracy is enabled by state-of-the-art three-dimensional wavefield simulations in combination with an adjoint-state method. The resulting tomographic model characterizes the subsurface structure in terms of compressional and shear wavespeed values at remarkable resolution, corresponding to a minimum period of ~10 s. As primary findings of our work, images of the lithospheric structure in Central Italy are consistent with recent studies on the distribution of fluids and gas (CO2) within the Italian subsurface, allowing us to infer the presence of deep melted material that induces shallow gas fluxes, or traps and deep storage of gas that can be correlated with seismicity. We illuminate Mt. Etna volcano and support the hypothesis of a deep reservoir (~30 km) feeding an intermediate-depth magma-filled intrusive body, which in turn is connected to a shallow chamber. We also investigate the intriguing features of the Adriatic plate offshore of the eastern Italian coast. Tomographic evidence reveals a structure of the plate made of two distinct microplates with different fabric and behavior, and separated by the Gargano deformation zone, indicating a complex lithosphere and tectonic evolution.

Introduction In November 2022 a seismic sequence occurred in the Marche offshore, about 29 km from the coast and the city of Fano. The sequence started on November 9 (06:07:25 UTC) with a ML=5.7 earthquake (Mw=5.5 from TDMD computation, Scognamiglio et al., 2006), immediately followed by a ML=5.2 earthquake (06:08:29 UTC) located about 8 km to the south. The two mainshocks activated a seismic sequence with about 400 aftershocks lasting the first week, 13 of them with ML>= 3.5 (Fig. 1). Few hours after the occurrence of the mainshock, the BSI (“Bollettino Sismico Italiano”) working group started to manually analyze P and S phase arrival times and seismogram amplitudes of earthquakes with magnitude ML>= 3.5 recorded by the the Italian National Seismic Network (Rete Sismica Nazionale, hereafter RSN) in order to better constrain hypocenter locations previously provided by the seismic surveillance room of the INGV in Rome for rapid communication to the Italian Civil Protection (Dipartimento Protezione Civile, DPC). Later, the BSI working group analyzed the seismicity of the sequence of the first weeks of seismic activity by revising hypocentral parameters of more than 500 events. The 2022 Marche offshore sequence took place along the Adriatic outer front of the northern Apennines in central Italy. Offshore seismic reflection profiles image a shallow thrust-and-fold system striking WNW–ESE to NNW–SSE. Along the coastal Adriatic area, active blind thrusts deform Plio-Quaternary siliciclastic turbidites that are few hundreds of meters to more than 2 km thick in correspondence of ramp anticlines and synclines, respectively. In a recent work, through the analysis of high-quality background seismicity data, De Nardis et al. (2022) identified two lithospheric-scale active thrusts deepening westward under the Adriatic outer front from upper- to lower-crustal depths. These new data support previous thick-skinned interpretations of seismic commercial profiles and CROP03 deep reflection data (Lavecchia et al., 2003). Focal mechanisms of weak to moderate (ML < 4.8) local earthquakes occurred between 2009-2017 at upper- to deep-crustal depths show prevailing reverse and reverse/oblique solutions (De Nardis et al., 2022) and subordinate strike-slip faulting (Mazzoli et al., 2014). The analysis of the 2022 Marche offshore sequence opens again the discussion on the uncertainties related to the hypocenter locations of earthquakes that occur in the Adriatic offshore domain (e.g., Di Stefano et al., 2022) and the limits of our present capability to provide an accurate seismotectonic interpretation of the instrumental seismicity in this region. Actually, the 2022 sequence area is only covered on land by RSN, with the closest seismic station located at about 28 km from the epicentral location of the mainshock. The particular geometry of the network along the Italian coast makes it difficult to correctly constrain hypocenter locations compared with other regions of Italy. Taking into account this configuration, although the INGV is able to obtain coherent earthquake information for Civil Protection purposes into the limits of the communication threshold, we note that data provided by the seismic surveillance room in terms of both seismic phase readings of arrival times for hypocenter location and waveform amplitudes for magnitude computation need to a more accurate analysis if the main goal is the correct reconstruction of the active structures involved in the sequence. This analysis should include a) a careful revision of the arrival time pickings to reduce the errors due to seismic phase misinterpretations, b) an accurate study to constrain earthquake locations with appropriate velocity models, and c) the hypocenter solution assessment through adequate tests that define which information can be inferred from earthquake location results. Data analysis and phases interpretation Through the interpretation of the seismic records, the BSI analysts have identified refracted first arrivals of P and S phases at epicentral distances of about 60 km, smaller than those expected for Pn/Sn refracted phases at the Moho discontinuity (e.g., Di Stefano and Ciaccio, 2014) whose arrivals should be observed at distances of about 90-100 km in this area. Since possible systematic misinterpretation of P and S arrivals can strongly affect the correct hypocenter locations, we have carefully revised the phase pickings provided by the INGV surveillance room by discriminating direct from refracted phases at stations located at distances greater than 60 km. This is mainly important for interpretation of weak S refracted phases that are often hidden into the arrivals after the P phase. We have taken into account these characteristics in the earthquake location process by only using clear direct/refracted S phases in our inversion procedure. The comparison of the ML>= 3.5 hypocenter locations performed by the BSI and the INGV surveillance room (Figs. 1 and 2) shows how an accurate analysis of the pickings is necessary to obtain robust earthquake locations for seismotectonic interpretation: even using the same hypocenter location code and velocity model, we observe that the mislocation of the hypocenters in this area can range from few to about 10 kilometers (Fig. 1) while the formal errors are strongly reduced after the BSI picking revision (Fig. 2) The velocity model issue Events location in the Adriatic Sea suffers from the lack of a specific velocity model for the seismic sequence area. The use of inadequate velocity parameters during the location process can introduce systematic errors, which may result in incorrect seismotectonic interpretations. We therefore built and tested different velocity models from both available geophysical data and our inversion of the velocity structure using the arrival time readings revised by the BSI working group. In order to define deterministic 1D models suitable for earthquake location (Vp and Vp/Vs), we integrated sonic logs from local deep wells (ViDEPI Project, 2005) with literature data that include: seismic commercial profiles, deep seismic refraction surveys, the CROP03 crustal profile, Receiver Function and regional seismic tomography models, Vp/Vs reference values for mid- and lower-crustal crystalline rocks (Coward et al., 1999; Ponziani et al., 1995; Lavecchia et al., 2003; Spada et al., 2013; Di Stefano et al., 2009, Christiansen and Mooney, 1993). In order to obtain the velocity structure from our revised dataset, we first determined the Vp/Vs ratio by using the arrival time pickings of selected P and S phases. The mean velocity ratio Vp/Vs was computed through the cumulative Wadati diagram. Then, by collecting all the a priori available information regarding the structure of Adriatic Sea (velocities, layer thicknesses and Moho depth), we applied the VELEST software (Kissling, 1995) to compute a new 1D velocity model for earthquake location. Conclusions In this work we present our first analyses of the sequence and the accurate study of the velocity models that we obtained from both a revision of available data and the inversion of arrival time pickings analyzed by the BSI analists. Moreover, we will discuss our preliminary earthquake locations with a particular attention to resolution analysis and hypocenter location assessment.

Il giorno 9 novembre 2022, alle 06:07:24 UTC (07:07:24 ora locale) un terremoto di magnitudo momento (MW) pari a 5.5 ha interessato la Costa Marchigiana Pesarese (Pesaro Urbino). A causa della magnitudo del mainshock e del livello di danneggiamento riscontrato, l’INGV ha attivato il gruppo operativo EMERSITO (http://emersitoweb.rm.ingv.it/index.php/it/), il cui obiettivo è di svolgere e coordinare le campagne di monitoraggio per studi di effetti di sito e di microzonazione sismica. Il gruppo operativo ha provveduto all’installazione di una rete sismica temporanea nel territorio del comune di Ancona; molte delle stazioni sismiche sono state installate in corrispondenza di edifici pubblici (scuole, Tribunale, Marina Militare, strutture religiose), grazie alla collaborazione con la sede INGV di Ancona, con la Protezione Civile Regione Marche, la Marina militare e la Capitaneria di Porto. Nel presente Report vengono brevemente riassunte le attività già svolte (si vedano i Report precedenti), discusse le analisi dei dati raccolti e mostrati alcuni risultati preliminari riguardanti la rete sismica temporanea. Sono state effettuate le seguente analisi preliminari: qualità delle registrazioni; rapporti spettrali su rumore sismico ambientale e su una selezione di terremoti registrati; analisi della dipendenza dei risultati dei rapporti spettrali dalla direzione del moto sismico (polarizzazione del segnale); calcolo dei meccanismi focali su alcuni eventi selezionati. Infine è stato prodotto un modello geologico semplificato, inclusivo delle informazioni derivanti dalle indagini geologiche e geofisiche preesistenti, che fornisce una chiave interpretativa dei risultati ottenuti.

An earthquake sequence occurred in the Central Adriatic region during March–June 2021. This sequence started on 27 March with a mainshock of moment magnitude (Mw) 5.2 occurring at 13:47 coordinated universal time (UTC). No foreshock was observed before this mainshock. The sequence lasted approximately three months, until the end of June 2021. Approximately 200 seismic events were recorded by the regional seismic network during this time, including four M ≥ 4.0 earthquakes. The 27 March 2021 earthquake was one of the strongest instrumentally recorded events in the area bounded approximately by the Ancona–Zadar line to the north and the Gargano–Dubrovnik line to the south. The mainshock originated at a focal depth of 9.9 km. The seismicity spread from the mainshock up-dip and down-dip along a northeast-dipping plane. Here, we investigate the geometry of the fault activated by this seismic sequence by using sP depth phases. We aim to significantly reduce the large uncertainties associated with the hypocentral locations of offshore earthquakes beneath the Adriatic Sea—an area that plays a fundamental role in the geodynamics of the Mediterranean. These refined earthquake locations also allow us to make inferences with regards to the seismotectonic context responsible for the analyzed seismicity, thus identifying a structure (here referred to as the MidAdriatic fault) consisting of a northwest–southeast-striking thrust fault with a ∼ 35° northeast-dipping plane. The use of depth-phase arrival times to constrain off-network event locations is of particular interest in Italy due to both the peculiar shape of the peninsula and the extreme scarcity of seafloor stations, the cost and management of which are very expensive and complex. Here, we present the first attempt to apply this off-network locating technique to the Italian offshore seismicity research with the aim of improving hazard estimations in these hard-to-monitor regions.

La revisione da parte degli analisti del BSI della sismicità registrata in Italia dal 1 maggio al 31 agosto 2022 ha riguardato tutti i terremoti di magnitudo ML≥1.5, mentre i parametri dei terremoti di magnitudo inferiore a tale soglia sono quelli calcolati in tempo reale, nella SALA DI SORVEGLIANZA SISMICA DI ROMA. I terremoti più forti (ML≥3.5), e pochi altri di particolare interesse [vedi Marchetti et al., 2016, DOI: 10.4401/ag- 6116], sono stati revisionati dagli analisti del BSI, mediamente nelle 24 ore successive al loro accadimento.