Soldati, Gaia

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).

Da venerdì 4 novembre a domenica 6 novembre 2022, si è tenuta una esercitazione nazionale denominata “Exe Sisma dello Stretto 2022” in un'area del territorio della Regione Calabria e della Regione Sicilia caratterizzata da una elevatissima pericolosità sismica. L’esercitazione è stata indetta e coordinata dal Dipartimento della Protezione Civile e aveva l’obiettivo di verificare la risposta operativa a un evento sismico significativo del Servizio Nazionale della Protezione Civile, di cui anche l’Istituto Nazionale di Geofisica e Vulcanologia fa parte. Durante le tre giornate, l’INGV ha avuto modo di testare tutte le procedure che l’Istituto ha codificato a partire da quelle del “Protocollo di Ente per le emergenze sismiche e da maremoto”. Dopo che INGV ha dato l’avvio all’intera esercitazione simulando il terremoto di magnitudo MW 6.2 (ML 6.0) alle ore 09:00 UTC in provincia di Reggio Calabria (5 km a SW dal comune di Laganadi), e ha, quindi, inviato il messaggio per il potenziale maremoto con un livello di allerta arancione; inoltre, il Presidente INGV ha prontamente convocato l’Unità di Crisi e attivato tutti Gruppi Operativi. Questi ultimi, nell’ambito dello scenario esercitativo, hanno verificato che i flussi di comunicazione interna e tutte le attività necessarie in emergenza sismica, presenti nei relativi protocolli operativi, risultassero rispettati. L’obiettivo primario dell’esercitazione è stato quindi quello di validare le attività previste e di aggiornare il personale afferente ai Gruppi Operativi stessi. Tra di essi, SISMIKO, che rappresenta il GO dedicato al coordinamento delle reti sismiche mobili INGV in emergenza, nelle settimane precedenti l’esercitazione ha predisposto tutte le attività che intendeva testare, descrivendole brevemente nel Documento d’impianto INGV e con maggior dettaglio in quello del Gruppo Operativo. A pochi giorni dalla chiusura dell’esercitazione, un terremoto di magnitudo ML 5.7 (MW 5.5) registrato alle ore 06:07 UTC del 09 novembre 2022 ha spostato l’attenzione dalla simulazione alla realtà.

Il 09 novembre 2022 alle ore 06:07 UTC (07:07 ora italiana) un terremoto di magnitudo ML 5.7 (MW 5.5) è stato localizzato dal sistema di sorveglianza sismica dell’Istituto Nazionale di Geofisica e Vulcanologia (INGV). L'epicentro è stato localizzato nel Mar Adriatico ad una distanza di circa 30 km dalla costa marchigiana in provincia di Pesaro e Urbino, a circa 31 km dalla città di Fano e 35 km dal capoluogo di provincia Pesaro. Il mainshock è stato seguito da una replica di ML 5.2 a un minuto di distanza. I due terremoti sono stati ben avvertiti in tutta la regione Marche, e anche in tutto il centro Italia fino a Roma e nelle regioni del nord Italia. Il Presidente dell’INGV, come previsto nel Protocollo di Ente per le emergenze sismiche e da maremoto, ha prontamente convocato l’Unità di Crisi e attivato tutti i Gruppi Operativi. Tra questi SISMIKO, che coordina le reti sismiche mobili INGV in emergenza, si è attivato immediatamente preparando la strumentazione necessaria per l’installazione di una rete sismica temporanea e per l’integrazione dei dati in acquisizione nel sistema di monitoraggio e sorveglianza sismica dell’INGV. Parallelamente alle attività di coordinamento e gestione dell’emergenza sono state attivate tutte le procedure inerenti la divulgazione (report, siti web, ecc) e l’analisi dei dati preliminari. La rete temporanea in emergenza è stata installata nelle prime 24 ore dalla scossa principale ad integrazione della rete sismica permanente dell’INGV in area epicentrale. La rete temporanea di SISMIKO, costituita da 8 stazioni sismiche trasmesse in tempo reale, ha permesso di migliorare il monitoraggio dell’evoluzione della sequenza, abbassando la soglia di detezione degli eventi sismici in area epicentrale e consentendo quindi una migliore localizzazione da parte del servizio di sorveglianza sismica nazionale. La gestione dell’emergenza sismica è avvenuta a pochi giorni di distanza dall’ esercitazione nazionale denominata “EXE Sisma dello Stretto 2022” svoltasi dal 4 al 6 Novembre 2022 nel territorio della Regione Calabria e della Regione Sicilia. L’esercitazione è stata coordinata dal Dipartimento della Protezione Civile. Le attività svolte durante EXE 2022 sono state per l’istituto, e in particolare per SISMIKO, propedeutiche per il buon esito di tutte le azioni messe in campo dall’INGV sin dai primi minuti dall’accadimento del mainshock del 9 Novembre.

We present an overview of the potential of active monitoring techniques to investigate the many factors affecting the concentration of radon in houses. We conducted two experiments measuring radon concentration in 25 apartments in Rome and suburban areas for two weeks and in three apartments in the historic center for several months. The reference levels of 300 and 100 Bq/m3 are overcome in 17% and 60% of the cases, respectively, and these percentages rise to 20% and 76% for average overnight radon (more relevant for residents' exposure). Active detectors allowed us to identify seasonal radon fluctuations, dependent on indoor-to-outdoor temperature, and how radon travels from the ground to upper floors. High levels of radon are not limited to the lowest floors when the use of heating and ventilation produces massive convection of air. Lifestyle habits also reflect in the different values of gas concentration measured on different floors of the same building or in distinct rooms of the same apartment, which cannot be ascribed to the characteristics of the premises. However, the finding that high residential radon levels tend to concentrate in the historic center proves the influence of factors such as building age, construction materials, and geogenic radon.

We present the results of an experiment taking place inside the geophysical museum of Rocca di Papa (Rome, Italy), where the high radon levels detected might pose a risk to the health of workers and of the public audience. As a first step towards the mitigation of potential exposure risk, four active sensors were installed at different floors of the building, in order to continuously monitor not only radon exhalation from the soil but also its transport from the ground up to elevated floors. Collecting more than three years of data of radon concentration enables us to identify fluctuations over both short and seasonal scales and to elucidate the relation between radon variations and changes of internal temperature and relative humidity. The analysis of such dataset reveals how the healthiness of indoor environments in terms of radon concentration is controlled by a number of factors, including the environmental conditions and the use of heating and ventilation systems. Finally, the continuous radon monitoring at different levels of the building provides a unique chance to trace the vertical radon diffusion, allowing to make a first-order estimate of upward radon velocity.

The radioactive nature of radon makes it a powerful tracer for fluid movements in the crust, and a potentially effective marker to study processes connected with earthquakes preparatory phase. To explore the feasibility of using soil radon variations as earthquakes precursor, we analyse the radon concentration data recorded by two stations located close to the epicentre of the strongest mainshock (Mw 6.5 on October 30, 2016) of the seismic sequence which affected central Italy from August 2016. The two stations CTTR and NRCA operate in the framework of the permanent Italian Radon monitoring Network and recorded almost continuously since 2012 and 2016, respectively, the latter being installed just after the first mainshock of the sequence (Mw 6.0 on August 24, 2016). An increase of radon emanation is clearly visible about 2 weeks before the Mw 6.5 event on both the time series, more pronounced on NRCA, nearer to the epicentre, suggesting the possibility of a direct association with the earthquake occurrence. An independently developed detection algorithm aimed at highlighting the connections between radon emission variations and major earthquakes occurrence succeeds in forecasting the Mw 6.5 mainshock on NRCA time series. The resulting time advance of the alarm is consistent with that obtained using a Bayesian approach to compute the a posteriori probability of multiple change points on the radon time series of NRCA. Moreover, it is in agreement with the delay time which maximizes the correlation between radon and seismic anomalies. Applying the detection algorithm to CTTR time series returns alarms for both the Mw 6.0 event, with epicentre closer to this station, and the stronger Mw 6.5 event, but with a higher number of false detections. Finally, we found that a preliminary correction of the bias introduced by variations of meteorological parameters does not affect our main finding of an increase in radon concentration before the major mainshocks. Our study confirms that, although much work is still needed, a monitoring approach based on a permanent dense network is crucial for making radon time series analysis an effective complement to traditional seismological tools.

All’interno dei percorsi forma vi di Alternanza Scuola­Lavoro un gruppo di ricercatori dell’Istituto Nazionale di Geofisica e Vulcanologia ha proposto e sviluppato con una terza classe del Liceo Scientifico Cavour di Roma un progetto riguardante l’avvicinamento degli studenti al mondo della ricerca scientifica, allo scopo di affrontarla e svilupparla in tu e le sue fasi. Dopo aver spiegato e condiviso l’obiettivo della ricerca con gli alunni, la prima fase del progetto ha riguardato la raccolta dei da tramite l’utilizzo di strumenti di misura; si è in seguito proceduto all’analisi dei dati ottenuti , attraverso un foglio elettronico di calcolo, la produzione di grafici e mappe tema che e infine la scrittura di un articolo scientifico (parte essenziale del lavoro del ricercatore) con la descrizione della ricerca svolta e dei risultati ottenuti . La ricerca ha riguardato la distribuzione del radon indoor in alcuni rioni di Roma. Gli studenti hanno avuto la possibilità di misurare il radon indoor tramite strumenti specifici nelle loro abitazioni e nella loro scuola, di analizzarne la distribuzione spaziale e temporale e di individuare dei casi particolarmente anomali. Sono stati quindi evidenziati i legami tra i valori misura , la geologia del sottosuolo e le tipologie abitative, illustrate per mezzo di mappe tematiche, facendo confluire il tutto o nella scrittura di un articolo scientifico. Il presente rapporto tecnico è stato introdotto dai ricercatori e sviluppato dagli studenti .

We investigate the temporal changes of crustal velocity associated to the seismic sequence of 2016–2017, which struck central Italy with a series of moderate to large earthquakes. We cross- correlate continuous recordings of 2 yr of ambient seismic noise from a network of 28 stations within a radius of 90 km around Amatrice town. We then map the spatio-temporal evolution of the velocity perturbations under the effect of subsequent earthquakes. Coinciding with each of the three main shocks of the sequence we observe a sudden drop of seismic velocity which tends to quickly recover in the short term. After the end of the strongest activity of the sequence, the coseismic velocity changes display gradual healing towards pre-earthquake conditions following a quasi-linear trend, such that by the end of 2017 about 75 per cent of the perturbation is recovered. The spatial distribution of the velocity drop fluctuates with time, and the area that shows the most intense variations beyond the ruptured fault system elongates in the NE direction. This zone roughly corresponds to a region of foredeep sedimentary deposits consisting of highly hydrated and porous sandstones, which respond to the passage of seismic waves with increased pore pressure and crack number, leading to a reduction of the effective relative velocity.

The relative seismic velocity variations possibly associated to large earthquakes can be readily monitored via cross-correlation of seismic noise. In a recently published study, more than 2 yr of continuous seismic records have been analysed from three stations surrounding the epicentre of the 2009 April 6, Mw 6.1 L’Aquila earthquake, observing a clear decrease of seismic velocities likely corresponding to the co-seismic shaking. Here, we extend the analysis in space, including seismic stations within a radius of 60 km from the main shock epicentre, and in time, collecting 5 yr of data for the six stations within 40 km of it. Our aim is to investigate how far the crustal damage is visible through this technique, and to detect a potential post-seismic recovery of velocity variations. We find that the co-seismic drop in velocity variations extends up to 40 km from the epicentre, with spatial distribution (maximum around the fault and in the north– east direction from it) in agreement with the horizontal co-seismic displacement detected by global positioning system (GPS). In the first few months after L’Aquila earthquake, the crust’s perturbation in terms of velocity variations displays a very unstable behaviour, followed by a slow linear recovery towards pre-earthquake conditions; by almost 4 yr after the event, the co-seismic drop of seismic velocity is not yet fully recovered. The strong oscillations of the velocity changes in the first months after the earthquake prevent to detect the fast exponential recovery seen by GPS data. A test of differently parametrized fitting curves demonstrate that the post-seismic recovery is best explained by a sum of a logarithmic and a linear term, suggesting that processes like viscoelastic relaxation, frictional afterlip and poroelastic rebound may be acting concurrently.

Since the late 1960s - early 1970s, seismologists started studying the elastic properties of the Earth crust looking for signals from the Earth interior indicating that a large earthquake is coming. To be useful for prediction a signal needs to: 1) occur before most large earthquakes and 2) occur only before large earthquakes. Up to now, no one has ever found such a signal, but since the beginning of the search, seismologists developed theories that included variations of the elastic property of the Earth crust prior to the occurrence of a large earthquake. The most popular is the theory of the dilatancy: when a rock is subject to stress, the rock grains are shifted generating micro-cracks, thus the rock itself increases its volume. Inside the fractured rock, fluid saturation and pore pressure play an important role in earthquake nucleation, by modulating the effective stress. Thus measuring the variations of wave speed and of anisotropic parameter in time can be highly informative on how the stress leading to a major fault failure builds up. In 1980s and 1990s such kind of research on earthquake precursors slowed down and the priority was given to seismic hazard and ground motions studies, which are very important since these are the basis for the building codes in many countries. Today we have dense and sophisticated seismic networks to measure wave-fields characteristics: we archive continuous waveform data recorded at three components broad-band seismometers, we almost routinely obtain highresolution earthquake locations. Therefore we are ready to start to systematically look at seismic-wave propagation properties to possibly reveal short-term variations in the elastic properties of the Earth crust. One seismological quantity which, since the beginning, is recognized to be diagnostic of the level of fracturation and/or of the pore pressure in the rock, hence of its state of stress, is the ratio between the compressional (P-wave) and the shear (S-wave) seismic velocities: Vp/Vs. Variations of this ratio have been recently observed and measured during the preparatory phase of a major earthquake. In active fault areas and volcanoes, tectonic stress variation influences fracture field orientation and fluid migration processes, whose evolution with time can be monitored through the measurement of the anisotropic parameters. Through the study of S waves anisotropy it is therefore potentially possible to measure the presence, migration and state of the fluid in the rock traveled by seismic waves, thus providing a valuable route to understand the seismogenic phenomena and their precursors. On the other hand, only in the very recent times with the availability of the continuous seismic records, many authors have shown how it is possible to estimate the relative variations in the wave speed through the analysis of the crosscorrelation of the ambient seismic noise. In this paper we first analyze in detail these two seismological methods: shear wave splitting and seismic noise cross correlation, presenting a short historical review, their theoretical bases, the problems, learning, limitations and perspectives. We, then, compare the main results in terms of temporal trends of the observables retrieved applying both methods to the Pollino area (southern Apennines, Italy) case study.