Italian National Seismic Network Research Articles

The optical, seismic, and infrasound signature of the March 5 2022, bolide over Central Italy

On March 5, 2022, a 12 kg meteoroid crossed the sky above Central Italy and was observed by three different observational systems: the PRISMA all-sky camera network (10 stations), the Italian national seismic network (61 stations), and a 4-element infrasound array. The corresponding datasets, each with its own resolution, provided three independent assessments of the trajectory, size and speed of the meteoroid. The bolide traveled across central Italy with an azimuth of 102 degrees, becoming visible at about 91 km above sea level with a velocity of about 15.4 km/s. Its visible trajectory lasted about 15 s. Reasonably, the residual portion of the ablated bolide terminated its path in the Adriatic Sea and could not be recovered. Seismic and infrasound data well match optical observations detecting the bolide Mach cone at 68 km above sea level with a back azimuth of 25 degrees with respect to the array. By comparing results from the three different systems, discrepancies are within the estimated uncertainties, thus confirming the mutual consistency of the adopted methodologies. Therefore, this study shows that different approaches can be integrated to improve the detection capability for bolide crossing the sky in monitored regions.

CRISP: an archive for the site characterization of permanent Italian seismic stations

In this paper we describe an advanced database for the site characterization of seismic stations, named “CRISP—Caratterizzazione della RIsposta sismica dei Siti Permanenti della rete sismica” (http://crisp.ingv.it, quoted with https://doi.org/10.13127/crisp), designed for the Italian National Seismic Network (Rete Sismica Nazionale, RSN, operated by Istituto Nazionale di Geofisica e Vulcanologia). For each site, CRISP collects easily accessible station information, such as position, type(s) of instrumentation, instrument housing, thematic map(s) and descriptive attributes (e.g., geological characteristics, etc.), seismic analysis of recordings, and available geophysical investigations (shear-wave velocity [VS] profile, non-linear decay curve). The archive also provides key proxy indicators derived from the available data, such as the time-averaged shear-wave velocity of the upper 30 m from the surface (VS30) and site and topographic classes according to the different seismic codes. Standardized procedures have been applied as motivated by the need for a homogenous set of information for all the stations. According to European Plate Observing System infrastructural objectives for the standardization of seismological data, CRISP is integrated into pre-existing INGV instrument infrastructures, shares content with the Italian Accelerometric Archive, and complies map information about the stations, as well as local geology, through web services managed by Istituto Superiore per la Protezione e la Ricerca Ambientale. The design of the CRISP archive allows the database to be continually updated and expanded whenever new data are available from the scientific community, such as the ones related to new seismic stations, map information, geophysical surveys, and seismological analyses.

An updated view of the Italian seismicity from probabilistic location in 3D velocity models: The 1981–2018 Italian catalog of absolute earthquake locations (CLASS)

We have built a complete catalog of three-dimensional (3D) hypocenter locations of earthquakes recorded in Italy from 1981 to 2018. Our catalog includes more than 420,000 events relocated by inverting a newly integrated dataset of ∼5.0 million P and ∼ 3.5 million S wave arrival times recorded by the Italian National Seismic Network and other permanent seismological networks operating in Italy. Available magnitudes are associated with earthquake locations from the most recent datasets and bulletins of Italian seismicity. Earthquakes are located in an updated 3D tomographic model of Italy obtained by including the Moho discontinuity and the seismic velocities of the Ionian subduction zone. We used a probabilistic, non-linear earthquake location code which provides complete information of the hypocenter solution uncertainties. Quality estimators of earthquake locations are analyzed a posteriori with an original criterion for a quantitative classification of the results, allowing users to select seismic events belonging to consistent quality classes and giving a more controlled image of the Italian instrumental seismicity for tectonic and geodynamical studies. The resulting catalog gives a new, coherent view of the spatial and temporal distribution of Italian seismicity. By selecting well constrained located events we construct the new Italian Crustal Seismogenic Layer (CSL) with a good spatial resolution, allowing us to show a comparison between seismogenic thicknesses and Moho geometry distribution. We finally present some examples of seismicity distribution in selected areas of Italy at regional and local scale relating the relocated events of our catalog with available multidisciplinary information from geology, geochemistry, geodynamical models, and historical seismicity.

An alternative method to evaluate earthquake detection from synthetic Wood–Anderson seismograms: an application in Italy

SUMMARY Defining the regional variability of minimum magnitude for earthquake detection is crucial for planning seismic networks. Knowing the earthquake detection magnitude values is fundamental for the optimal location of new stations and to select the priority for reactivating the stations of a seismic network in case of a breakdown. In general, the assessment of earthquake detection is performed by analysing seismic noise with spectral or more sophisticated methods. Further, to simulate amplitude values at the recording sites, spectral methods require knowledge of several geophysical parameters including rock density, S-wave velocity, corner frequency, quality factor, site specific decay parameter and so on, as well as a velocity model for the Earth's interior. The simulation results are generally expressed in terms of Mw and therefore a further conversion must be done to obtain the values of local magnitude (ML), which is the parameter commonly used for moderate and small earthquakes in seismic catalogues. Here, the relationship utilized by a seismic network to determine ML is directly applied to obtain the expected amplitude [in mm, as if it were recorded by a Wood–Anderson (WA) seismometer] at the recording site, without any additional assumptions. The station detection estimates are obtained by simply considering the ratio of the expected amplitude with respect to the background noise, also measured in mm. The seismic noise level for the station is estimated starting from four waveforms (each signal lasting 1 min) sampled at various times of the day for a period of one week. The proposed method is tested on Italian seismic events occurring in 2019 by using the locations of 16.879 earthquakes recorded by 374 stations. The first results indicate that by evaluating the station noise level with 5-s windows, a representative sample of the variability in expected noise level is generated for every station, even if only 4 min of signal per day over a week of recordings is used. The method was applied to define the detection level of the Italian National Seismic Network (RSN). The RSN detection level represents a reference for the definition and application of guidelines in the field of monitoring of subsurface industrial activities in Italy. The proposed approach can be successfully applied to define the current performance of a local seismic network (managed by private companies) and to estimate the expected further improvements, requested to fulfil the guidelines with the installation of new seismic stations. This method has been tested in Italy and can be reproduced wherever the local magnitude ML, based on synthetic WA records, is used.

Minimum 1D VP and VP/VS Models and Hypocentral Determinations in the Central Mediterranean Area

Abstract Minimum 1D velocity models and station corrections have been computed for the central Mediterranean area using two main data sets. The first one consists of accurate first arrival-time readings from 103 seismic events with magnitude (ML)≥3.5 recorded by the Italian National Seismic Network (RSN) and the AlpArray Seismic Network (AASN) in the period 2014–2021. Earthquakes were selected on the basis of their spatial distribution, epicentral distance to the nearest seismic station, and maximum distance traveled by Pn and Sn phases. This fine selection of high-quality data combined with the spatial density of the AlpArray seismic stations was decisive in obtaining high resolution for upper mantle velocity, especially in the Alpine belt. To obtain a denser coverage of crustal rays, we extended the first data set with P and S arrivals of local earthquakes from Istituto Nazionale di Geofisica e Vulcanologia (INGV) bulletin data (2016–2018). A total of 75,807 seismic phases (47,183 P phases and 28,264 S phases) have been inverted to calculate best-fit 1D velocity models, at regional and local scales. We then test the performance of the optimized velocity models by relocating the last four years of seismicity recorded by INGV (period 2017–2020). The computed velocity models are very effective for routine earthquake location, seismic monitoring, source parameter modeling, and future 3D seismic tomography.

Comparison of Deep Learning Techniques for the Investigation of a Seismic Sequence: An Application to the 2019, Mw 4.5 Mugello (Italy) Earthquake

AbstractThe increase of available seismic data prompts the need for automatic processing procedures to fully exploit them. A good example is aftershock sequences recorded by temporary seismic networks, whose thorough analysis is challenging because of the high seismicity rate and station density. Here, we test the performance of two recent Deep Learning algorithms, the Generalized Phase Detection and Earthquake Transformer, for automatic seismic phases identification. We use data from the December 2019 Mugello basin (Northern Apennines, Italy) swarm, recorded on 13 permanent and nine temporary stations, applying these automatic procedures under different network configurations. As a benchmark, we use a catalog of 279 manually repicked earthquakes reported by the Italian National Seismic Network. Due to the ability of deep learning techniques to identify earthquakes under poor signal‐to‐noise‐ratio (SNR) conditions, we obtain: (a) a factor 3 increase in the number of locations with respect to INGV bulletin and (b) a factor 4 increase when stations from the temporary network are added. Comparison between deep learning and manually picked arrival times shows a mean difference of 0.02–0.04 s and a variance in the range 0.02–0.07 s. The improvement in magnitude completeness is ∼0.5 units. The deep learning algorithms were originally trained using data sets from different regions of the world: our results indicate that these can be successfully applied in our case, without any significant modification. Deep learning algorithms are efficient and accurate tools for data reprocessing in order to better understand the space‐time evolution of earthquake sequences.

INSTANCE – the Italian seismic dataset for machine learning

Abstract. The Italian earthquake waveform data are collected here in a dataset suited for machine learning analysis (ML) applications. The dataset consists of nearly 1.2 million three-component (3C) waveform traces from about 50 000 earthquakes and more than 130 000 noise 3C waveform traces, for a total of about 43 000 h of data and an average of 21 3C traces provided per event. The earthquake list is based on the Italian Seismic Bulletin (http://terremoti.ingv.it/bsi, last access: 15 February 2020​​​​​​​) of the Istituto Nazionale di Geofisica e Vulcanologia between January 2005 and January 2020, and it includes events in the magnitude range between 0.0 and 6.5. The waveform data have been recorded primarily by the Italian National Seismic Network (network code IV) and include both weak- (HH, EH channels) and strong-motion (HN channels) recordings. All the waveform traces have a length of 120 s, are sampled at 100 Hz, and are provided both in counts and ground motion physical units after deconvolution of the instrument transfer functions. The waveform dataset is accompanied by metadata consisting of more than 100 parameters providing comprehensive information on the earthquake source, the recording stations, the trace features, and other derived quantities. This rich set of metadata allows the users to target the data selection for their own purposes. Much of these metadata can be used as labels in ML analysis or for other studies. The dataset, assembled in HDF5 format, is available at http://doi.org/10.13127/instance (Michelini et al., 2021).

Using dense seismo-acoustic network to provide timely warning of the 2019 paroxysmal Stromboli eruptions

Stromboli Volcano is well known for its persistent explosive activity. On July 3rd and August 28th 2019, two paroxysmal explosions occurred, generating an eruptive column that quickly rose up to 5 km above sea level. Both events were detected by advanced local monitoring networks operated by Istituto Nazionale di Geofisica e Vulcanologia (INGV) and Laboratorio di Geofisica Sperimentale of the University of Firenze (LGS-UNIFI). Signals were also recorded by the Italian national seismic network at a range of hundreds of kilometres and by infrasonic arrays up to distances of 3700 km. Using state-of-the-art propagation modeling, we identify the various seismic and infrasound phases that are used for precise timing of the eruptions. We highlight the advantage of dense regional seismo-acoustic networks to enhance volcanic signal detection in poorly monitored regions, to provide timely warning of eruptions and reliable source amplitude estimate to Volcanic Ash Advisory Centres (VAAC).

Seismogenic Structure Orientation and Stress Field of the Gargano Promontory (Southern Italy) From Microseismicity Analysis

Historical seismic catalogs report that the Gargano Promontory (southern Italy) was affected in the past by earthquakes with medium to high estimated magnitude. From the instrumental seismicity, it can be identified that the most energetic Apulian sequence occurred in 1995 with a main shock of MW = 5.2 followed by about 200 aftershocks with a maximum magnitude of 3.7. The most energetic earthquakes of the past are attributed to right-lateral strike-slip faults, while there is evidence that the present-day seismicity occur on thrust or thrust-strike faults. In this article, we show a detailed study on focal mechanisms and stress field obtained by micro-seismicity recorded from April 2013 until the present time in the Gargano Promontory and surrounding regions. Seismic waveforms are collected from the OTRIONS Seismic Network (OSN), from the Italian National Seismic Network (RSN), and integrated with data from the Italian National Accelerometric Network (RAN) in order to provide a robust dataset of earthquake localizations and focal mechanisms. The effect of uncertainties of the velocity model on fault plane solutions (FPS) has been also evaluated indicating the robustness of the results. The computed stress field indicates a deep compressive faulting with maximum horizontal compressive stress, SHmax, trending NW-SE. The seismicity pattern analysis indicates that the whole crust is seismically involved up to a depth of 40 km and indicates the presence of a low-angle seismogenic surface trending SW-NE and dipping SE-NW, similar to the Gargano–Dubrovnik lineament. Shallower events, along the eastern sector of the Mattinata Fault (MF), are W-E dextral strike-slip fault. Therefore, we hypothesized that the seismicity is locally facilitated by preexisting multidirectional fractures, confirmed by the heterogeneity of focal mechanisms, and explained by the different reactivation processes in opposite directions over the time, involving the Mattinata shear zone.

Minor earthquake sequences in the Amatrice-Norcia epicentral area (Central Italy)

This study presents a detailed analysis of minor seismic sequences recorded in the Amatrice-Norcia area (central Italy) before 2016 when the most important seismic sequence of the last 40 years struck the region with the Mw 6.0 Amatrice and the Mw 6.5 Norcia earthquakes. We observe that, in the four decades before the 2016–2018 Amatrice-Visso-Norcia sequence, the instrumental seismicity rate is low, with maximum magnitudes lower than Mw 4.0, and is characterized by different types of behaviours as single shock events, swarms and minor sequences. For the first time, we relocate the minor seismicity recorded before 2016 by the Italian National Seismic Network of the Istituto Nazionale di Geofisica e Vulcanologia by applying the nonlinear inversion code NonLinLoc in a local velocity model. Revised earthquake locations of the past seismic sequences are compared to the recent 2016–2018 seismicity in order to investigate some possible correlations with the seismogenic structures reactivated in 2016. With this goal, we also integrated our new hypocentral locations with fault plane solutions and geological data to interpret our results with respect to the 2016–2018 seismicity. Our results show how some of the structures identified by the minor seismicity before 2016 were reactivated during the recent sequence, while others seismic structures remained silent. We therefore highlight how the study of minor seismic sequences provides important information about the seismogenic attitude of less active or less known seismogenic structures with consequent impact on the evaluation of the seismic hazard.

Seismic Surveillance and Earthquake Monitoring in Italy

Abstract 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.

An automatically generated high-resolution earthquake catalogue for the 2016–2017 Central Italy seismic sequence, includingPandSphase arrival times

SUMMARYThe 2016–2017 central Italy earthquake sequence began with the first main shock near the town of Amatrice on August 24 (Mw 6.0), and was followed by two subsequent large events near Visso on October 26 (Mw 5.9) and Norcia on October 30 (Mw 6.5), plus a cluster of four events with Mw > 5.0 within few hours on 18 January 2017. The affected area had been monitored before the sequence started by the permanent Italian National Seismic Network (RSNC), and was enhanced during the sequence by temporary stations deployed by the National Institute of Geophysics and Volcanology and the British Geological Survey. By the middle of September, there was a dense network of 155 stations, with a mean separation in the epicentral area of 6–10 km, comparable to the most likely earthquake depth range in the region. This network configuration was kept stable for an entire year, producing 2.5 TB of continuous waveform recordings.Here we describe how this data was used to develop a large and comprehensive earthquake catalogue using the Complete Automatic Seismic Processor (CASP) procedure. This procedure detected more than 450 000 events in the year following the first main shock, and determined their phase arrival times through an advanced picker engine (RSNI-Picker2), producing a set of about 7 million P- and 10 million S-wave arrival times. These were then used to locate the events using a non-linear location (NLL) algorithm, a 1-D velocity model calibrated for the area, and station corrections and then to compute their local magnitudes (ML). The procedure was validated by comparison of the derived data for phase picks and earthquake parameters with a handpicked reference catalogue (hereinafter referred to as ‘RefCat’). The automated procedure takes less than 12 hr on an Intel Core-i7 workstation to analyse the primary waveform data and to detect and locate 3000 events on the most seismically active day of the sequence. This proves the concept that the CASP algorithm can provide effectively real-time data for input into daily operational earthquake forecasts,The results show that there have been significant improvements compared to RefCat obtained in the same period using manual phase picks. The number of detected and located events is higher (from 84 401 to 450 000), the magnitude of completeness is lower (from ML 1.4 to 0.6), and also the number of phase picks is greater with an average number of 72 picked arrival for a ML = 1.4 compared with 30 phases for RefCat using manual phase picking. These propagate into formal uncertainties of ±0.9 km in epicentral location and ±1.5 km in depth for the enhanced catalogue for the vast majority of the events. Together, these provide a significant improvement in the resolution of fine structures such as local planar structures and clusters, in particular the identification of shallow events occurring in parts of the crust previously thought to be inactive. The lower completeness magnitude provides a rich data set for development and testing of analysis techniques of seismic sequences evolution, including real-time, operational monitoring of b-value, time-dependent hazard evaluation and aftershock forecasting.

Spatial analysis for an evaluation of monitoring networks: examples from the Italian seismic and accelerometric networks

In this work, we propose a statistical approach to evaluate the coverage of a network based on the spatial distribution of its nodes and the target information, including all those data related to the final objectives of the network itself. This statistical approach encompasses descriptive spatial statistics in combination with point pattern techniques. As case studies, we evaluate the spatial arrangements of the stations within the Italian National Seismic Network and the Italian Strong Motion Network. Seismic networks are essential tools for observing earthquakes and assessing seismic hazards, while strong motion (accelerometric) networks allow us to describe seismic shaking and to measure the expected effects on buildings and infrastructures. The capability of both networks is a function of an adequate number of optimally distributed stations. We compare the seismic network with the spatial distributions of historical and instrument seismicity and with the distribution of well-known seismogenic sources, and we compare the strong motion station distribution with seismic hazard maps and the population distribution. This simple and reliable methodological approach is able to provide quantitative information on the coverage of any type of network and is able to identify critical areas that require optimization and therefore address areas of future development.

Long‐Period Amplification in Deep Alluvial Basins and Consequences for Site‐Specific Probabilistic Seismic‐Hazard Analysis: An Example from the Po Plain (Northern Italy)

This study deals with the impact of deep‐soil discontinuities on ground‐motion amplification in the Po alluvial basin (northern Italy), and consequently, on long‐period probabilistic seismic‐hazard analysis estimates. The focus is on Castelleone, where a seismic station of the Italian National Seismic Network has been deployed since 2009. To define a detailed shear‐wave velocity ( V S ) profile, extensive active and passive geophysical surveys were carried out. The active measurements included seismic profiles examined through standard refraction techniques and multichannel analysis of surface waves. The passive methods included ambient vibration measurements in both single and array configurations. In particular, three microtremor arrays with increasing apertures were performed to capture the experimental dispersion curve down to 0.2 Hz. The horizontal‐to‐vertical spectral ratio (HVSR) showed two main peaks, at 0.17 and 0.70 Hz. A joint inversion of the experimental phase‐velocity dispersion and HVSR curves was performed to obtain the V S profile. The data show two main discontinuities at ∼160 and 1350 m in depth. According to the Italian and European seismic codes, the shallow V S discontinuity can be ascribed to the seismic bedrock (i.e., V S >800 m/s), whereas the deeper one is observed at the transition between Pliocene–Quaternary deposits and Miocene marls. Preliminary site‐specific seismic‐hazard analysis in terms of displacement response spectra for periods up to 10 s shows that neglecting the effects of the deeper discontinuity implies underestimation in hazard estimates of up to about 49% for mean return period (MRP) of 475 years and about 57% for MRP of 2475 years, with possible consequences regarding the design of very tall buildings and large bridges.

The Italian National Seismic Network and the earthquake and tsunami monitoring and surveillance systems

Abstract. The Istituto Nazionale di Geofisica e Vulcanologia (INGV) is an Italian research institution, with focus on Earth Sciences. INGV runs the Italian National Seismic Network (Rete Sismica Nazionale, RSN) and other networks at national scale for monitoring earthquakes and tsunami as a part of the National Civil Protection System coordinated by the Italian Department of Civil Protection (Dipartimento di Protezione Civile, DPC). RSN is composed of about 400 stations, mainly broadband, installed in the Country and in the surrounding regions; about 110 stations feature also co-located strong motion instruments, and about 180 have GPS receivers and belong to the National GPS network (Rete Integrata Nazionale GPS, RING). The data acquisition system was designed to accomplish, in near-real-time, automatic earthquake detection, hypocenter and magnitude determination, moment tensors, shake maps and other products of interest for DPC. Database archiving of all parametric results are closely linked to the existing procedures of the INGV seismic monitoring environment and surveillance procedures. INGV is one of the primary nodes of ORFEUS (Observatories & Research Facilities for European Seismology) EIDA (European Integrated Data Archive) for the archiving and distribution of continuous, quality checked seismic data. The strong motion network data are archived and distributed both in EIDA and in event based archives; GPS data, from the RING network are also archived, analyzed and distributed at INGV. Overall, the Italian earthquake surveillance service provides, in quasi real-time, hypocenter parameters to the DPC. These are then revised routinely by the analysts of the Italian Seismic Bulletin (Bollettino Sismico Italiano, BSI). The results are published on the web, these are available to both the scientific community and the general public. The INGV surveillance includes a pre-operational tsunami alert service since INGV is one of the Tsunami Service providers of the North-eastern Atlantic and Mediterranean Tsunami warning System (NEAMTWS).

Man‐induced low‐frequency seismic events in Italy

AbstractUnconventional seismic events in Italy are detected by scanning three years of continuous waveforms recorded by the Italian National Seismic Network. Cross correlation of signal templates with continuous seismic records has evidenced unusual events with similar low‐frequency characteristics in several Italian regions. Spectral analysis and spatiotemporal distribution of these events, some of which are previously interpreted as tectonic long‐period transients, suggest that they are not natural, but produced by huge cement factories. Since there are at least 57 full‐cycle cement plants operating in Italy, each affecting areas of about 1250 to 2800 km2, we argue that significant portions of the Italian territory (23% to 51%) can be affected by this man‐made noise. Seismic noise analyses, such as those used for microzonation or crustal structure investigations, as well as data mining techniques used to retrieve anomalous transient signals, should thus take into account this peculiar and pervasive source of seismic waves.

Seismic moment tensors and regional stress in the area of the December 2013–January 2014, Matese earthquake sequence (Italy)

The main goal of this study is to provide moment tensor solutions for small and moderate earthquakes of the Matese seismic sequence in southern Italy for the period of December 2013–January 2014. We estimate the focal mechanisms of 31 earthquakes with local magnitudes related to the Matese earthquake seismic sequence (December 2013–January 2014) in Southern-Central Italy which are recorded by the broadband stations of the Italian National Seismic Network and the Mediterranean Very Broadband Seismographic Network (MedNet) run by the Istituto Nazionale di Geofisica e Vulcanologia (INGV). The solutions show that normal faulting is the prevailing style of seismic deformation in agreement with the local faults mapped out in the area. Comparisons with already published solutions and with seismological and geological information available allowed us to properly interpret the moment tensor solutions in the frame of the seismic sequence evolution and also to furnish additional information about less energetic seismic phases. Focal data were inverted to obtain the seismogenic stress in the study area. The results are compatible with the major tectonic domain of the area.

Large-scale coherent anisotropy of upper mantle beneath the Italian peninsula comparing quasi-Love waves and SKS splitting

We document quantitatively observations of quasi-Love waves obtained at permanent (Italian National Seismic Network) and temporary seismic stations deployed in Italy between 2003 and 2006 (Retreat, CAT/SCAN projects). We analyzed large earthquakes with source parameters that favor quasi-Love wave generation within this time-span, including the Sumatra–Andaman earthquake of 12/26/04. The presence or the absence of the quasi-Love phase is compared to the smoothed anisotropic pattern defined by the numerous SKS splitting measurements obtained in peninsular Italy, and to the Italian upper mantle structure as defined by seismic tomography. The large-scale anisotropic features, responsible for shear-wave splitting and documented also by Pn and surface-wave anisotropy, generally display the correct geometry to explain the scattered quasi-Love waves. Quasi-Love observations do not demand a tilted-axis anisotropic geometry. We argue instead for anisotropy with laterally-variable horizontal symmetry axis in the upper mantle below the Italian peninsula.

High frequency seismic waves and slab structures beneath Italy

Tomographic images indicate a complicated subducted slab structure beneath the central Mediterranean where gaps in fast velocity anomalies in the upper mantle are interpreted as slab tears. The detailed shape and location of these tears are important for kinematic reconstructions and understanding the evolution of the subduction system. However, tomographic images, which are produced by smoothed, damped inversions, will underestimate the sharpness of the structures. Here, we use the records from the Italian National Seismic Network (IV) to study the detailed slab structure. The waveform records for stations in Calabria show large amplitude, high frequency (f>5 Hz) late arrivals with long coda after a relatively low-frequency onset for both P and S waves. In contrast, the stations in the southern and central Apennines lack such high frequency arrivals, which correlate spatially with the central Apennines slab window inferred from tomography and receiver function studies. Thus, studying the high frequency arrivals provides an effective way to investigate the structure of slab and detect possible slab tears. The observed high frequency arrivals in the southern Italy are the strongest for events from 300 km depth and greater whose hypocenters are located within the slab inferred from fast P-wave velocity perturbations. This characteristic behavior agrees with previous studies from other tectonic regions, suggesting the high frequency energy is generated by small scale heterogeneities within the slab which act as scatterers. Furthermore, using a 2-D finite difference (FD) code, we calculate synthetic seismograms to search for the scale, shape and velocity perturbations of the heterogeneities that may explain features observed in the data. Our preferred model of the slab heterogeneities beneath the Tyrrhenian Sea has laminar structure parallel to the slab dip and can be described by a von Kármán function with a down-dip correlation length of 10 km and 0.5 km in thickness with ∼2.5% Vp fluctuations within the slab. This suggests that the heterogeneities are inherited from the melt shear bands formed during the original formation of the oceanic lithosphere near the mid-ocean ridge.

Evolution and strengthening of the Calabrian Regional Seismic Network

Abstract. The Calabrian Arc is an area of high seismic hazard, in the past often affected by destructive earthquakes. The seismicity of the Calabrian region is monitored by the Italian National Seismic Network integrated by the Calabrian Regional one and, in the last three years, by the Pollino temporary array. We have applied the Seismic Network Evaluation through Simulation to assess the individual contribution of each network in locating earthquakes with epicentres in the Calabrian region and surrounding. We shows that the Calabrian Regional Seismic Network greatly improves the quality of the coverage in almost the Calabria territory except in the Crotone Basin, in the Serre and in the offshore areas. We show that the contribution of the Pollino temporary array is instead restricted to a very small area centred on the Pollino Chain. Due to the presence in the Serre of important seismogenic volumes, which in the past have generated destructive earthquakes, it would be opportune to add at least several seismic stations in this area and surrounding to improve the seismic monitoring.