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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 Mid-Adriatic 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.

The Istituto Nazionale di Geofisica e Vulcanologia (INGV) monitors the Italian peninsula seismicity by using the data recorded by the Italian National Seismic Network (RSN) together with the ones gathered by other permanent regional networks (PRN). Earthquakes are real-time located by the INGV surveillance system and manually revised by the Italian Seismic Bulletin (BSI) group analysts.Starting from a catalog composed by homogeneous absolute locations (CLASS; Latorre et al., 2023), obtained by using a 3D regional-scale velocity model, we generated a catalog of relative seismic locations (CARS) of about 310,000 events occurred in Italy during 1981-2018.We inverted absolute P- and S- waves arrival times derived from data collected by RSN plus PRN for the period 1981-2008 and only by RSN for 2009-2018 to apply the double-difference relocation algorithm (Waldhauser and Ellsworth, 2000).For the second period, we combined the absolute travel times with relative ones obtained by waveforms cross-correlations analysis performed on pairs of similar events. The time domain cross‐correlation method proposed by Schaff et al., 2004 and Schaff & Waldhauser, 2005 was applied to seismograms of all pairs of events separated by 10 km or less and recorded at common stations. Seismograms were filtered in the 1–15 Hz frequency range using a four pole, zero phase band‐pass Butterworth filter. The correlation measurements were performed on 1.0 s long window for P- and S-waves. We collected a total of ~17 million P- and ~23 million S-wave delay times, retaining all measurements with correlation coefficients greater than 0.7.1D velocity models characterising 18 different (geologically, seismically and tectonically homogeneous) Italian macroareas (after Pastori et al. B2-2019-2021, Wp1-task4) were used in the location procedure.To cope with the memory limits (15,000 events with less than 200 readings) of the HypoDD code so to use it in a steady operational mode, we first subdivided the study region in the 18 macroareas related to the velocity models, then we additional discretize each in 100x100km2 cells, overlapped by the 80% in longitude and latitude. We repeatedly produced hypocentral locations of the same events that we merged by computing a final weighted mean location.We present the double-difference catalog of Italian seismicity, allowing to depict alignments clearer with respect to the starting catalog (CLASS), to be related to seismogenic faults and/or to regional structures along the whole Italian peninsula.

The analysis of microseismicity has a fundamental role in understanding earthquakes, giving insights on the long- and short-term driving forces and processes preparing and generating the seismicity occurrence and its evolution in space and time.Recent advances in detection and location algorithms, paired with dense seismic networks, and supported by higher computing capacity, allow dramatic increase in the quality and quantity of low magnitude earthquakes recorded resulting in high resolution earthquakes catalogs in terms of both location attributes and completeness.Such catalogs enable us to analyze small magnitude (M

We re-located 70 earthquakes belonging to the seismic sequence started on 2021 March 27th, with a mainshock of Mw 5.2 at 13:47 UTC, in the Central Adriatic region (Italy off-shore) by using the on-purpose designed code by Zhao et al. (2007, 2011), modeling the sP converted phases.The mainshock of the 2021 seismic sequence occurred about 20 km north of the Palagruza island, 80 km from the Gargano promontory and about 40 km from the Croatian island of Lastovo. It was felt in many central-southern Italian regions, from Ancona to Foggia, and in Central Dalmatia. All the epicenters of this seismic sequence lie in the open sea, about 100km to the SE and about 50 km to the NW of the 2003 Jabuka seismic sequence, and the 1988 Palagruza seismic sequence, respectively.Though the seismicity in the central Adriatic Sea has been recorded by improving seismic networks, especially in recent decades, the precise location of the Adriatic offshore earthquakes was hampered mainly by the large distance of the closest stations, and by the large gap in the distribution of seismic stations.The possibility to model the sP depth phases enables us to estimate the epicentral parameters and focal depths of these offshore earthquakes more accurately, thanks to the peculiar ray-path that mimics the presence of a receiver approximately on top of the hypocenter. The refined earthquake locations allow us to make inferences on the structure responsible for the seismicity of the 2021 seismic sequence, a thrust fault NW-SE striking and ~35° NE-dipping, and on its seismotectonic context.The use of depth-phase arrival times to constrain the off-network events' locations is of particular interest to Italy due to both the peculiar shape of the peninsula and the extreme scarcity of seafloor stations, whose cost and management are very expensive and complex.We present the first attempt to apply this off-network location technique to the Italian offshore seismicity with the aim of improving the hazard estimation of these hard-to-monitor regions.

We computed moment (Mw) and local magnitude (ML) of about 250,000 earthquakes occurred in Italy from 2009 to 2018 and recorded at seismic stations of the Italian National Network managed by INGV.For moment magnitude computation, we start from raw velocity waveforms and invert the displacement spectra of more than 2,000,000 S-waves manually picked. We use the probabilistic method of Supino et al. [2019] to estimate the a-posteriori joint probability density function of the source parameters: seismic moment M0, corner frequency fc and high-frequency decay γ. Mw is obtained from M0 using the Kanamori [1977] equation.We start from the same waveforms to compute local magnitude using two designed on purpose codes, PyAmp and PyML [Di Stefano et al., 2023], and an attenuation law specific for the Italian region, Di Bona et al. [2016], obtaining ML values characterized by quality and homogeneity.Both magnitude catalogs can be reproduced due to the availability in open databases of all the input and output parameters used for processing.We observe a self-similar scaling between fc and M0 for Mw larger than ~2.0. For smaller magnitudes, S-wave spectra show an almost constant corner frequency (~10 Hz), which does not scale with the earthquake source (seismic moment). We interpret this as the constant cut-off frequency of the anelastic attenuation, which acts as a low-pass filter and produces an apparent corner frequency. The latter is lower than expected, and corresponds to an apparent larger source duration.Because of the conservation of total displacement integral after a low-pass filtering, signals must exhibit a maximum amplitude lower than expected to “compensate” the apparent larger source duration. ML values are therefore expected to be underestimated while moment magnitudes, by definition, are not affected by this as they are proportional to the displacement integral.Coherently, the comparison of our Mw and ML estimates shows the systematic underestimation of ML with respect to Mw for small magnitude events. The deviation from a 1:1 scaling relationship between ML and Mw overlaps the magnitude range where the constant apparent corner frequency arises in the M0-fc scaling (ML

One important aspect of the seismicity is the spatiotemporal clustering; hence, the distinction between independent and triggered events is a critical part of the analysis of seismic catalogs. Stochastic declustering of seismicity allows a probabilistic distinction between these two kinds of events. Such an approach, usually performed with the epidemic-type aftershock sequence (ETAS) model, avoids the bias in the estimation of the frequency–magnitude distribution parameters if we consider a subset of the catalog, that is, only the independent or the triggered events. In this article, we present a framework to properly include the probabilities of any event to be independent (or triggered) both in the temporal variation of the seismic rates and in the estimation of the b-value of the Gutenberg–Richter law. This framework is then applied to a high-definition seismic catalog in the central part of Italy covering the period from April 2010 to December 2015. The results of our analysis show that the seismic activity from the beginning of the catalog to March 2013 is characterized by a low degree of clustering and a relatively high b-value, whereas the following period exhibits a higher degree of clustering and a smaller b-value.

The Istituto Nazionale di Geofisica e Vulcanologia (INGV) is an Italian research institution with focus on earth sciences. Moreover, the INGV is the operational center for seismic surveillance and earthquake monitoring in Italy and is a part of the civil protection system as a center of expertise on seismic, volcanic, and tsunami risks.INGV operates the Italian National Seismic Network and other networks at national scale and is a primary node of the European Integrated Data Archive for archiving and distributing strong-motion and weak-motion seismic recordings. In the control room in Rome, INGV staff performs seismic surveillance and tsunami warning services; in Catania and Naples, the control rooms are devoted to volcanic surveillance. Volcano monitoring includes locating earthquakes in the regions around the Sicilian (Etna, Eolian Islands, and Pantelleria) and the Campanian (Vesuvius, Campi Fregrei, and Ischia) active volcanoes. The tsunami warning is based on earthquake location and magnitude (M) evaluation for moderate to large events in the Mediterranean region and also around the world. The technologists of the institute tuned the data acquisition system to accomplish, in near real time, automatic earthquake detection, hypocenter and magnitude determination, and evaluation of several seismological products (e.g., moment tensors and ShakeMaps). Database archiving of all parametric results is closely linked to the existing procedures of the INGV seismic surveillance environment and surveillance procedures. Earthquake information is routinely revised by the analysts of the Italian seismic bulletin. INGV provides earthquake information to the Department of Civil Protection (Dipartimento di Protezione Civile) to the scientific community and to the public through the web and social media. We aim at illustrating different aspects of earthquake monitoring at INGV: (1) network operations; (2) organizational structure and the hardware and software used; and (3) communication, including recent developments and planned improvements.

The Near Fault Observatories (NFOs) community is one of the European Plate Observing System (EPOS, http://www.epos-eu.org) Thematic Communities, today consisting of six research infrastructures that operate in regions characterised by high seismic hazard originating from different tectonic regimes. Earthquakes respond to complex natural systems whose mechanical properties evolve over time. Thus, in order to understand the multi-scale, physical/chemical processes responsible for the faulting that earthquakes occur on, it is required to consider phenomena that intersect different research fields, i.e., to put in place multidisciplinary monitoring. Hence, NFOs are grounded on modern and multidisciplinary infrastructures, collecting near fault high resolution raw data that allows generation of innovative scientific products. The NFOs usually complement regional backbone networks with a higher density distribution of seismic, geodetic, geochemical and other geophysical sensors, at surface and sometimes below grade. These dense and modern networks of multi-parametric sensors are sited at and around active faults, where moderate to large earthquakes have occurred in the past and are expected in the future. They continuously monitor the underlying Earth instability processes over a broad time interval. Data collected at each NFO results in an exceptionally high degree of knowledge of the geometry and parameters characterizing the local geological faults and their deformation pattern. The novel data produced by the NFO community is aggregated in EPOS and is made available to a diverse set of stake-holders through the NFO Federated Specific Data Gateway (FRIDGE). In the broader domain of the Solid Earth sciences, NFOs meet the growing expectations of the learning and communication sectors by hosting a large variety of scientific information about earthquakes as a natural phenomenon and a societal issue. It represents the EPOS concept and objective of aggregating and harmonising the European research infrastructures capabilities to facilitate broader scientific opportunity. The NFOs are at the cutting edge of network monitoring. They conduct multidisciplinary experiments for testing multi-sensor stations, as well as realise robust and ultra-low latency, transmis-sion systems that can routinely accommodate temporary monitoring densification. The effort to continuously upgrade the technological efficiency of monitoring systems positions the NFO at the centre of marketing opportunities for the European enterprises devoted to new sensor technology. The NFOs constitute ideal test beds for generating expertise on data integration, creating tools for the next generation of multidisciplinary research, routine data analysis and data visualization. In particular focus is often on near-real time tools and triggering alarms at different levels are tested and implemented, strengthening the cooperation with the Agencies for risk management. NFOs have developed innovative operational actions such as the Testing Centre for Earthquake Early Warning and Source Characterisation (CREW) and detailed fast ground shaking and damage characterization. Complementing the recent growth of modern laboratory and computational models, the NFOs can provide interdisciplinary observations of comparable high resolution to describe the behaviour of fault slip over a vast range of spatial and temporal scales and aiding to provide more accurate earthquake hazard characterizations., Annals of Geophysics, 65 (3), ISSN:1593-5213

Three devastating earthquakes ofMW≥5.9 activated a complex system of high-angle normal, antithetic, and sub-horizontal detachment faults during the 2016–2017 central Italy seismic sequence. Waveform cross-correlation based double-difference location of nearly 400,000 aftershocks illuminate complex, fine-scale structures of interacting fault zones. The Mt. Vettore–Mt. Bove (VB) normal fault exhibits wide and complex damage zones, including a system of bookshelf faults that intersects the detachment zone. In the Laga domain, a comparatively narrow, shallow dipping segment of the deep Mt. Gorzano fault progressively ruptures through the detachment zone in four subsequentMW∼ 5.4 events. Reconstructed fault planes show that the detachment zone is fragmented in four sub-horizontal, partly overlaying shear planes that correlated with the extent of the mainshock ruptures. We find a new, deep reaching seismic barrier that coincides with a bend in the VB fault and may play a role in controlling rupture evolution.

Time-series of VP/VS ratio have been used to track local changes in elastic properties of rock volumes. Identifying such variations can provide information on the geophysical processes taking place inside a rock volume during the seismic cycle. A value of VP/VS ratio can be computed from traveltime of P and S waves generated from a single local event and it is representative of the value of the VP/VS ratio for the rocks traversed by the seismic ray, between the source and the receiver. It is straightforward, during a seismic sequence, to generate time-series of VP/VS ratio for events located close together and a single station. Such time-series should be able to monitor temporal variations of elastic parameters in the rock volume. Due to the very small nature of the expected changes in P- and S-wave velocity, the evaluation of VP/VS ratio time-series has been problematic in the past, and subjective choices about, for example the time-averaging scheme applied or event selection for constructing the time-series, have been proven to strongly affect the outcomes of the analysis. In this contribution, we present the application of a new methodology for a statistical evaluation of changes in VP/VS ratio time-series. The new methodology belongs to the wide class of 'change-point analysis' algorithms and is developed in the framework of Bayesian inference. The posterior probability distribution (PPD) of the change-point locations is obtained using a trans-dimensional Markov chain Monte Carlo (trans-D McMC) algorithm, where the existence and number of change-points is directly dictated by the data themselves. We apply the new algorithm to the seismic catalogue produced by the Alto Tiberina Near Fault Observatory seismic network (Northern Apennines, Italy). Here the high rate of background seismic release and the dense seismic network allow for a robust statistical analysis. The occurrence of change-points in VP/VS time-series identified with the proposed procedure is represented in space and time. The space-time distributions of change-points in the study area shows a clear peak of change-points following the occurrence of local main events, clustered along the main fault system activated. The robustness of the proposed approach makes it appropriate as an automatic, real-time tool for monitoring rock property changes related to seismic activity.

At 01:36 UTC (03:36 local time) on August 24th 2016, an earthquake Mw 6.0 struck an extensive sector of the central Apennines (coordinates: latitude 42.70° N, longitude 13.23° E, 8.0 km depth). The earthquake caused about 300 casualties and severe damage to the historical buildings and economic activity in an area located near the borders of the Umbria, Lazio, Abruzzo and Marche regions. The Istituto Nazionale di Geofisica e Vulcanologia (INGV) located in few minutes the hypocenter near Accumoli, a small town in the province of Rieti. In the hours after the quake, dozens of events were recorded by the National Seismic Network (Rete Sismica Nazionale, RSN) of the INGV, many of which had a ML > 3.0. The density and coverage of the RSN in the epicentral area meant the epicenter and magnitude of the main event and subsequent shocks that followed it in the early hours of the seismic sequence were well constrained. However, in order to better constrain the localizations of the aftershock hypocenters, especially the depths, a denser seismic monitoring network was needed. Just after the mainshock, SISMIKO, the coordinating body of the emergency seismic network at INGV, was activated in order to install a temporary seismic network integrated with the existing permanent network in the epicentral area. From August the 24th to the 30th, SISMIKO deployed eighteen seismic stations, generally six components (equipped with both velocimeter and accelerometer), with thirteen of the seismic station transmitting in real-time to the INGV seismic monitoring room in Rome. The design and geometry of the temporary network was decided in consolation with other groups who were deploying seismic stations in the region, namely EMERSITO (a group studying site-effects), and the emergency Italian strong motion network (RAN) managed by the National Civil Protection Department (DPC). Further 25 BB temporary seismic stations were deployed by colleagues of the British Geological Survey (BGS) and the School of Geosciences, University of Edinburgh in collaboration with INGV. All data acquired from SISMIKO stations, are quickly available at the European Integrated Data Archive (EIDA). The data acquired by the SISMIKO stations were included in the preliminary analysis that was performed by the Bollettino Sismico Italiano (BSI), the Centro Nazionale Terremoti (CNT) staff working in Ancona, and the INGV-MI, described below.

We relocated the aftershocks of the MW 6.0 Amatrice 2016 mainshock by inverting with a non-linear probabilitstic method P- and S-arrival time readings produced and released in near realtime by the analyst seismologists of IGNV on duty in the seismic monitoring room. Earthquakes distribution shows the activation of a normal fault system with a main SW-dipping fault extending from Amatrice to NW of Accumoli village for a total length of 40 km. On the northern portion of the main fault hanging-wall volume, the structure become more complex activating an antithetic fault below the Norcia basin. It is worth nothing that below 8-9 km of depth, the whole fault system has an almost continuous sub-horizontal layer interested by an intense seismic activity, about 2 km thick.

The availability of multidisciplinary and high-resolution data is a fundamental requirement to understand the physics of earthquakes and faulting. We present the Alto Tiberina Near Fault Observatory (TABOO), a research infrastructure devoted to studying preparatory processes, slow and fast deformation along a fault system located in the upper Tiber Valley (northern Apennines), dominated by a 60 km long low-angle normal fault (Alto Tiberina, ATF) active since the Quaternary. TABOO consists of 50 permanent seismic stations covering an area of 120 × 120 km2. The surface seismic stations are equipped with 3-components seismometers, one third of them hosting accelerometers. We instrumented three shallow (250 m) boreholes with seismometers, creating a 3-dimensional antenna for studying micro-earthquakes sources (detection threshold is ML 0.5) and detecting transient signals. 24 of these sites are equipped with continuous geodetic GPS, forming two transects across the fault system. Geochemical and electromagnetic stations have been also deployed in the study area. In 36 months TABOO recorded 19,422 events with ML ≤ 3.8 corresponding to 23.36e-04 events per day per squared kilometres; one of the highest seismicity rate value observed in Italy. Seismicity distribution images the geometry of the ATF and its antithetic/synthetic structures located in the hanging-wall. TABOO can allow us to understand the seismogenic potential of the ATF and therefore contribute to the seismic hazard assessment of the area. The collected information on the geometry and deformation style of the fault will be used to elaborate ground shaking scenarios adopting diverse slip distributions and rupture directivity models.

n February 2010, we launched an experimental scientific video channel on YouTube (http://www.youtube.com/ingvterremoti), to improve our communication strategy for earthquake risk and preparedness. The main goals of this initiative were to inform people of the ongoing seismic activity in Italy and around the World, to communicate the results of scientific research in seismology, and to increase the knowledge of seismic hazard of the people. To date, after almost two years from the start, we have published 52 original videos on YouTube, through the collaboration of many researchers at the Istituto Nazionale di Geofisica e Vulcanologia (INGV). The videos are organized into eight play lists: (i) earthquakes in Italy; (ii) earthquakes worldwide; (iii) the 2009 L’Aquila, Italy, earthquake; (iv) ongoing seismic activity; (v) tsunami; (vi) earthquake prediction; (vii) seismic hazard; and (viii) May 11 2011 (when a major earthquake was predicted to hit Rome). To date, the total number of views is over 466,000, with two peaks of more than 20,000/day after the Tohoku, Japan, earthquake (March 2011) and before the presumed prediction of a major earthquake to hit Rome on May 11, 2011. The most popular videos (6) have been viewed more than 20,000 times, with a maximum of over 67,000. We think that this initiative has increased people’s knowledge and awareness of seismic risk, although at the moment the outreach is limited only to a specific (but growing) target of citizens. In the future, we will try to improve the technical aspects of our video communication, and we will try to broaden our audience. We have learned that when specific earthquakes occur (or even when there are unfounded predictions about upcoming seismic events) this is when the attention is highest, which represent the best occasions to move forward towards improved risk communication strategies.

The largest dataset ever recorded during a normal fault seismic sequence was acquired during the 2009 seismic emergency triggered by the damaging earthquake in L'Aquila (Italy). This was possible through the coordination of different rapid-response seismic networks in Italy, France and Germany. A seismic network of more than 60 stations recorded up to 70,000 earthquakes. Here, we describe the different open-data archives where it is possible to find this unique set of data for studies related to hazard, seismotectonics and earthquake physics. Moreover, we briefly describe some immediate and direct applications of emergency seismic networks. At the same time, we note the absence of communication platforms between the different European networks. Rapid-response networks need to agree on common strategies for network operations. Hopefully, over the next few years, the European Rapid-Response Seismic Network will became a reality., Annals of Geophysics, 54 (4), ISSN:1593-5213

[1] Recently, Mt. Vesuvius has been the subject of renewed seismological studies designed to allow the definition of the subsurface structure, focusing on the location and geometry of a presumed shallow magma chamber. The high volcanic risk associated with the activity of Mt. Vesuvius motivates detailed studies to trace a possible magmatic supply at shallow depth that could feed future eruptions. In 1994 and 1996 two active seismic surveys were carried out by an international team of scientists. Body waves from 17 shots executed around the volcano were recorded by dense arrays of seismic stations located across the volcano and extending to the surrounding Apenninic belt. These experiments encountered logistic difficulties due to the highly populated area; nevertheless, a good-quality data set of waveforms was collected. In this work we present the three-dimensional P wave velocity structure of the upper crust beneath Mt. Vesuvius, obtained through tomographic inversion of data collected during the TomoVes active seismic surveys. Due to the geometry of the problem, we used a tomographic technique that allows ray tracing in a complex model with both sharp velocity discontinuities and velocity gradients. The shot and station distribution and the presence of a very shallow velocity discontinuity, producing head wave first arrivals, limited the analysis to the upper 3 km of the crust. The images we obtained show a well-resolved, main axial high-Vp body interpreted as a magmatic intrusion remnant of recent eruptions from the central crater. Lateral branches depart from the central anomaly, possibly indicating the path followed by the magma during lateral eruptions that occurred in the past.