SEISMIC ANISOTROPY AND MICRO-SEISMICITY IN THE UPPER CRUST AT NORTH OF GUBBIO BASIN (CENTRAL ITALY): RELATION WITH THE SUBSURFACE GEOLOGICAL STRUCTURES AND THE ACTIVE STRESS FIELD

During the months of April and May 2010, a seismic sequence (here named “Pietralunga seismic sequence”) took place in the northeastern part of the Gubbio basin (Northern Apennines); this area is well known to be interested by a continuous background micro-seismic activity. The sequence was recorded both by the INGV National Seismic Network, and by the stations installed by the Project “AIRPLANE” (financially supported by MIUR-Italian Ministry of Education and Research) with the aim of investigating the seismogenetic processes in the Alto Tiberina Fault (ATF) system region. In this work we present the anisotropic results at four stations: ATFO, ATPC, ATPI, ATVO located around the northern termination of the Gubbio basin that well delimit both the seismic se- quence and the whole 2010 seismicity (about 2500 events). The study of seismic anisotropy has provided useful information for the interpretation and evaluation of the stress field and active crustal deformation. Seismic anisotropy can yield valuable information on upper crustal structure, fracture field, and presence of fluid-saturated rocks. Moreover, the large number of seismic waveforms recorded especially during the Pietralunga sequence allows us also to study the spatio-temporal changes of anisotropic parameters to better understand its evolution and the possible correlation to the presence and migration of fluids

IMAGING THE ACTIVE STRESS FIELD OF THREE SEISMOGENIC AREAS ALONG THE APENNINES AS REVEALED BY CRUSTAL ANISOTROPY

During the last decades, the study of seismic anisotropy has provided useful information for the interpretation and evaluation of the stress field and active crustal deformation. Seismic anisotropy can yield valuable information on upper crustal structure, fracture field, and presence of fluid-saturated rocks crossed by shear waves. Several studies worldwide demonstrate that seismic anisotropy is related to stress-aligned, filled-fluid micro-cracks (EDA model). An automatic analysis code, “Anisomat+”, was developed, tested and improved to calculate the anisotropic parameters: fast polarization direction (φ) and delay time (∂t). Anisomat+ has been compared to other two automatic analysis codes (SPY and SHEBA) and tested on three zones of the Apennines (Val d’Agri, Tiber Valley and L’Aquila surroundings). The anisotropic parameters, resulting from the automatic computation, have been interpreted to determine the fracture field geometries; for each area, we defined the dominant fast direction and the intensity of the anisotropy, interpreting these results in the light of the geological and structural setting and of two anisotropic interpretative models, proposed in the literature. In the first one, proposed by Zinke and Zoback, the local stress field and cracks are aligned by tectonics phases and are not necessarily related to the presently active stress field. Therefore the anisotropic parameters variations are only space-dependent. In the second, EDA model, and its development in the APE model fluid-filled micro-cracks are aligned or ‘opened’ by the active stress field and the variation of the stress field might be related to the evolution of the pore pressure in time; therefore in this case the variation of the anisotropic parameters are both space- and time- dependent. We recognized that the average of fast directions, in the three selected areas, are oriented NW-SE, in agreement with the orientation of the active stress field, as suggested by the EDA model, but also, by the proposed by Zinke and Zoback model; in fact, NW-SE direction corresponds also to the strike of the main fault structures in the three study regions. The mean values of the magnitude of the normalized delay time range from 0.005 s/km to 0.007 s/km and to 0.009 s/km, respectively for the L'Aquila (AQU) area, the High Tiber Valley (ATF) and the Val d'Agri (VA), suggesting a 3-4% of crustal anisotropy. In each area are also examined the spatial and temporal distribution of anisotropic parameters, which lead to some innovative observations, listed below. 1) The higher values of normalized delay times have been observed in those zones where most of the seismic events occur. This aspect was further investigated, by evaluating the average seismic rate, in a time period, between years 2005 and 2010, longer than the lapse of time, analyzed in the anisotropic studies. This comparison has highlighted that the value of the normalised delay time is larger where the seismicity rate is higher. 2) In the Alto Tiberina Fault area the higher values of normalised delay time are not only related to the presence of a high seismicity rate but also to the presence of a tectonically doubled carbonate succession. Therefore, also the lithology, plays a important role in hosting and preserving the micro-fracture network responsible for the anisotropic field. 3) The observed temporal variations of anisotropic parameters, have been observed and related to the fluctuation of pore fluid pressure at depth possibly induced by different mechanisms in the different regions, for instance, changes in the water table level in Val D’Agri, occurrence of the April 6th Mw=6.1 earthquake in L’Aquila.Since these variations have been recognized, it is possible to affirm that the models that better fit the results, both in term of fast directions and of delay times, seems to be EDA and APE models

A microseismic study in a low seismicityarea: the 2001 site-response experimentin the Città di Castello Basin (Italy)

A site response experiment was performed in the basin of Città di Castello (a small town in Central Italy) in May 2001. This study is part of a project on the evaluation of seismic hazard in seismogenic areas funded by the Gruppo Nazionale Difesa dai Terremoti (GNDT). The experiment consisted of a dense fixed transect configuration with most of the stations recording in continuous mode, and several ambient noise measurements both in single station and in array configuration spread over the investigated area. The dense transect was composed of 26 seismic stations in a crosswise configuration with a maximum inter-station distance of 250 m. The stations were deployed in the southern part of the basin, from the eastern bedrock outcrop to the western edge, across the town. About 70 earthquakes were recorded during 10 days of deployment, generally low magnitude or regional events. We located 23 earthquakes and 17 of them were located using the waveform similarity approach at 4 stations outside the target area. These 4 stations were part of a dense temporary seismic network involved in a previous experiment of the same project, aimed at performing a high-resolution picture of the local seismicity. Delay analysis on the recorded waveforms allowed us to infer the basin geometry at depth and estimate the S-wave velocity of sediments. Moreover, we evaluated relative site response along the E-W transect by performing a standard spectral ratio. Amplification factors up to 9 are found inside the basin; at frequencies above 5 Hz stations closer to the edges show higher amplification, whereas stations located in the middle of the basin, where the alluvial sediments are thicker (CD11-CD14), show higher amplification below 5 Hz. We considered the average amplification in two frequency bands (1-5 Hz and 5-10 Hz), representative of the resonance frequency for 2-3 storey buildings and 1 storey houses,respectively. Our results suggest that the potential hazard for 2-3 storey buildings is higher in the center of the basin (amplification factor up to 6), and for 1 storey houses is higher at the edges (amplification factor up to 5)

Open File Report Task5, Characterization of site effects for the Colfiorito, Città di Castello and Val d’Agri areas: predictability and site transfer functions.

The effect of local site amplification has been recognized as an important factor in ground motion assessment and is nowadays frequently studied. A common approach to evaluate ground shaking is to first estimate ground motion parameters at rock sites and then to correct them introducing site transfer functions derived from experimental data and from numerical modelling. The site transfer functions to be used to modify ground motion evaluated at rock sites can be evaluated starting from strong motion, weak motion and microtremor data. According to the amount and quality of the available data the transfer functions are evaluated for specific sites, in order to be used as a punctual information, or as representative of an average local condition in selected areas. All the available geological and geotechnical data must be collected to put some constraints on the obtained results and to permit numerical modeling to be compared with experimental results. The obtained transfer functions can be introduced in scenario studies convolving rock seismograms by the pulse response of the upper layers for different situations considered as representative of the geology of the studied areas. The capability of describing local site effects is strongly affected by the amount of seismological, geophysical and geotechnical data available. This is particularly true if numerical modelling needs to be performed and if the contribution of non linear soil behaviour has to be taken into account. For the three areas investigated in the framework of the project, the different amount of available data and information guided the performed studies and the obtained results. For Colfiorito test site, the availability of strong motion data recorded during the largest events of the Umbria Marche sequence (1997-98) yields well constrained information for specific sites. For Città di Castello the collection of weak motion and microtremor data allowed to reconstruct the geometry of the sedimentary basin underlying the city and to define zones with homogenous site response where to evaluate site transfer functions in a 1D approximation, including non linear behaviour. For Val D’Agri area, the lack of seismic and geotechnical data did not allow to describe in detail the site response in the sedimentary basin. In this case some sample sites with a known uppermost geological structure were selected as representative of the seismic response of the basin. For them, microtremor data were collected to put some constraint on the transfer functions computed in a 1D approximation. In this case it was not possible to consider the non linear soil behaviour.Gruppo Nazionale per la Difesa dei TerremotiUnpublished1.1. TTC - Monitoraggio sismico del territorio nazionaleope

Integrated SEED data archive for temporary seismic experiments

One of the most valuable results achieved during the work on S5 project is the implementation of a new temporary network data management that allows the integration in the National Data Center together with all other seismological data produced by INGV. This makes all data gathered during temporary experiments immediately available from the same source and in the same data format (SEED) increasing the availability for processing and analysis. Moreover the data are distributed to the scientific community using the EIDA (European Integrated Data Archive http://eida.rm.ingv.it/). The first application has been carried out for the Messina 1908-2008 experiment (WP2.2) http://dpc-s5.rm.ingv.it/en/Database-MessinaFault.html where has been achieved the complete integration of permanent networks (National Seismic Network, MedNet and Peloritani Local Network), temporary deployments (INGV-CNT and INGVCT mobile networks) and OBS data. All the procedures were used and further improved during the L'Aquila sequence (Task 4) where data was available for processing together with permanent network data as soon as it was gathered from the field giving to the scientific community the opportunity to study the evolution of the seismic sequence with higher density of stations (WP4.2) ( h t t p : / / d p c - s 5 . r m . i n g v . i t / e n / D a t a b a s e - AquilaFaultSystem.html).UnpublishedSede Ispra | Via Curtatone 7, Roma1.1. TTC - Monitoraggio sismico del territorio nazionaleope

11 maggio 2011: il terremoto previsto e l’Open Day all’INGV

Fin dal 2010, un terremoto devastante era stato previsto per l’11 maggio 2011 a Roma. La previsione era stata erroneamente attribuita a Raffaele Bendandi, uno studioso autodidatta di scienze naturali, originario di Faenza e vissuto fra il 1893 e il 1979. Nei mesi precedenti, l’Istituto Nazionale di Geofisica e Vulcanologia (INGV) aveva ricevuto un notevole numero di richieste d’informazioni non solo da parte dei residenti a Roma, ma anche da parte di turisti e pendolari. Con l’approssimarsi del mese di maggio, cresceva l’attenzione della popolazione e dei media. L’INGV ha quindi deciso di organizzare un Open Day presso la propria sede di Roma per consentire al pubblico di approfondire la conoscenza del terremoto come fenomeno naturale e di avere informazioni sulla sismicità e pericolosità sismica italiana. L’Open Day è stato preceduto da una conferenza stampa, con lo scopo di presentare l’iniziativa e di avviare una discussione scientifica con i giornalisti sulla previsione dei terremoti e sul rischio sismico in Italia. Più di 30 giornalisti di quotidiani nazionali e locali, tv, agenzie di stampa e testate web hanno partecipato alla conferenza stampa e centinaia di articoli sono apparsi nei giorni successivi, pubblicizzando l’Open Day dell’11 maggio. L’INGV ha aperto la propria sede al pubblico per tutto il giorno e ha organizzato incontri con i ricercatori, visite guidate della Sala di Monitoraggio Sismico e delle mostre interattive sui terremoti e sul campo magnetico terrestre, conferenze su temi di carattere generale, quale l’impatto sociale della diffusione di voci incontrollate e la riduzione del rischio sismico. Durante la giornata sono stati inoltre inseriti sul canale YouTube/INGVterremoti 13 nuovi video per spiegare come e perché avviene un terremoto e per fornire aggiornamenti periodici sulla sismicità in Italia dalla Sala di Monitoraggio Sismico. L’11 maggio, dalle 10 del mattino alle 9 di sera, la sede INGV è stata pacificamente invasa da oltre 3000 visitatori: famiglie, scolaresche con e senza insegnanti, gruppi di protezione civile e molti giornalisti. L’iniziativa, costruita in poche settimane, ha avuto notevole risonanza ed è stata un’importante occasione per fare informazione capillare sul rischio sismico

SEISMIC SWARM vs MAINSHOCK‐AFTERSHOCKS SEQUENCE: REFINED HYPOCENTERS LOCATIONS AT THE APENNINES‐CALABRIAN ARC BOUNDARY (SOUTHERN ITALY)

In the last years the Apennines-Calabrian arc boundary has been affected by intense seismicity concentrated in the Pollino mountain region. The Pollino is located at the northernmost edge of the Calabrian Arc, the last remnant of subduction along the Africa- Eurasian boundary. The area is subject to Northeast- Southwest extension, which results in a complex system of normal faults striking Northwest-Southeast, nearly parallel to the Apenninic mountain range. The Italian Seismic Network between 2010 and 2014 detected more than 5500 earthquakes in the area (Italian Seismological Instrumental and Parametric Data- Base; http:// iside .rm .ingv .it). In 2010 and 2011 the earthquake rate has been variable, with increasing and decreasing phases and maximum magnitudes below M=4. On May 28th 2012, a shallow event with local magnitude of 4.3 struck, about 5 kilometers east of the previous swarm. The seismic activity remained concentrated in the M=4.3 source region until early August. At that time seismicity jumped back westward to the previous area, with several earthquakes of magnitude larger than 3, culminating with a M=5.0 earthquake on 25 October 2012. The seismic rate remained high for some months, but aftershock magnitudes did not exceed magnitude 3.7. The seismic rate then suddenly decreased at the beginning of 2013 and stayed quite low for the rest of the year up to the beginning of 2014. During these years several temporary seismic stations were deployed in the area, improving the detecting threshold of the Italian Seismic Network and giving us the opportunity to refine the location of the earthquakes hypocenters. A combined dataset, including three-component seismic waveforms recorded by both permanent and temporary stations, has been analyzed in order to obtain an appropriate 1-D and 3D velocity model for earthquake location in the study area. Here we describe the main seismological characteristics of this seismic sequence and, relying on refined earthquakes location, we make inferences on the geometry of the fault system responsible for the two strongest shocks. Swarm activity seems to occur on a diffuse crustal volume more than on fault planes. To yield a better understanding of the origin of the ongoing seismic activity in the Pollino area, using thousand of seismograms, we analyze vp and vp/vs models and anisotropic parameters in the crust. The main goal of this study is to increase the understanding of the physical mechanisms behind the seismic swarm and its influence on the seismic hazard of the Apennines- Calabrian arc boundary region.EAEE - ESCPublishedIstanbul - August 24-29 20142T. Tettonica attivaope

The 2012 Emilia seismic sequence (Northern Italy): Imaging the thrust fault system by accurate aftershock location

Starting from late May 2012, the Emilia region (Northern Italy) was severely shaken by an intense seismic sequence, originated from a ML 5.9 earthquake on May 20th, at a hypocentral depth of 6.3 km, with thrusttype focal mechanism. In the following days, the seismic rate remained high, counting 50 ML ≥ 2.0 earthquakes a day, on average. Seismicity spreads along a 30 km east–west elongated area, in the Po river alluvial plain, in the nearby of the cities Ferrara and Modena. Nine days after the first shock, another destructive thrust-type earthquake (ML 5.8) hit the area to the west, causing further damage and fatalities. Aftershocks following this second destructive event extended along the same east-westerly trend for further 20 km to the west, thus illuminating an area of about 50 km in length, on thewhole. After the first shock struck, on May 20th, a dense network of temporary seismic stations, in addition to the permanent ones, was deployed in the meizoseismal area, leading to a sensible improvement of the earthquake monitoring capability there. A combined dataset, including threecomponent seismic waveforms recorded by both permanent and temporary stations, has been analyzed in order to obtain an appropriate 1-D velocity model for earthquake location in the study area. Here we describe the main seismological characteristics of this seismic sequence and, relying on refined earthquakes location, we make inferences on the geometry of the thrust system responsible for the two strongest shocks

The 2012 Emilia seismic sequence (Northern Italy): Imaging the thrust fault system by accurate aftershock location

Starting from late May 2012, the Emilia region (Northern Italy) was severely shaken by an intense seismic sequence, originated from a ML 5.9 earthquake on May 20th, at a hypocentral depth of 6.3 km, with thrusttype focal mechanism. In the following days, the seismic rate remained high, counting 50 ML ≥ 2.0 earthquakes a day, on average. Seismicity spreads along a 30 km east–west elongated area, in the Po river alluvial plain, in the nearby of the cities Ferrara and Modena. Nine days after the first shock, another destructive thrust-type earthquake (ML 5.8) hit the area to the west, causing further damage and fatalities. Aftershocks following this second destructive event extended along the same east-westerly trend for further 20 km to the west, thus illuminating an area of about 50 km in length, on thewhole. After the first shock struck, on May 20th, a dense network of temporary seismic stations, in addition to the permanent ones, was deployed in the meizoseismal area, leading to a sensible improvement of the earthquake monitoring capability there. A combined dataset, including threecomponent seismic waveforms recorded by both permanent and temporary stations, has been analyzed in order to obtain an appropriate 1-D velocity model for earthquake location in the study area. Here we describe the main seismological characteristics of this seismic sequence and, relying on refined earthquakes location, we make inferences on the geometry of the thrust system responsible for the two strongest shocks.Published44-552T. Tettonica attivaJCR Journalope