CRUSTAL FRACTURING FIELD AND PRESENCE OF FLUID AS REVEALED BY SEISMIC ANISOTROPY: CASE HISTORY FROM SEISMOGENIC AREAS IN THE APENNINES

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-satu- rated rocks. In fact seismic anisotropy is related to stress-aligned, fluid-filled micro-cracks (Fig. 1, EDA model, Crampin et al., 1984; Barkved et al., 2004). Università di Perugia and Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy Seismic anisotropy is an almost ubiquitous property of the Earth. The Shear Wave Splitting is the most unambiguous indicator of anisotropy but the automatic estimation of the splitting parame- ters presents difficulties because the effect of the anisotropy on the seismogram is a second order effect not very easily detectable. Various researchers developed automated techniques for the study of Shear Wave Splitting. In the last three years, it was developed, tested and improved an automatic analysis code“Ani- somat_plus”, to calculate the anisotropic parameters, fast polarization direction (f) and delay time (∂t). “Anisomat_plus” is a set of MatLab scripts able to retrieve crustal anisotropy parameters from three-component seismic recording of local earthquakes. The code uses the horizontal component cross-correlation method, a mathematical operation that allows measuring the similarity of the pul- se shape between two S-waves (Fig. 2). These two waves have similar shape, mutually orthogonal oscillation directions and travel with different velocities. The analysis procedure consists in choo- sing an appropriate frequency range, that better highlights the signal containing the shear waves, and a time window on the seismogram centred on the S arrival (the temporal window contains at least one cycle of S wave). The code was tested on three key areas selected because of their peculiar geological setting. For each area I used the anisotropic parameters resulting from the automatic computation, in order to determine the fracture field geometries in the portion of crust sampled by S waves. This led to defi- ne the strain field of the three areas. In detail, the three study areas show the following geological features: 1) Val d’Agri basin: I investigated the upper crust trying to relate the anisotropy to the active structures and the stress field (there is still an open discussion about the location of the seismoge- nic source of 1857 earthquake) and to the changes in seismicity rate probably related to temporal evolution of pore pressure caused by fluid migration in the oil reservoir and by the water level oscil- lations of the Pertusillo artificial lake. It is important keep in mind that the Val d’Agri basin is the most important Mediterranean oil reservoir. Fig. 3 shows the rose diagrams of the fast polarization directions at station having more than 10 measurements. These plots consider only no-null events and the length of rose petal is proportional to the number of measurements in the correspondent 10th interval, at each station. The lower inset shows the total fast directions at all stations. I note a NW–SE dominant fast direction at most of the stations (AG04, AG09, AG13, AG14, AG18) whereas other measurements are slightly rotated in a more N100E direction (AG11 and AG17) or strikes E–W (AG05). The remaining station, AG01, doesn’t show a preferentially orien- tation. In the Val d’Agri I observe a dominant fast polarization direction striking NW–SE, perpen- dicular to the Shmin active stress indicators available for the region, such as borehole breakout data and T-axis of focal mechanisms (Cucci et al., 2004).This also agrees with those recently estimated by Valoroso et al. (2009).I estimate an average normalized delay times for the region of 0.009 s km-1. The estimated values for single station strongly vary, ranging from 0.002 to 0.012 s km-1. I found greater values (above 0.01 s km-1, at stations AG09, AG14, AG18) characterize stations located along the SW mar- gin of the Val d’Agri basin, whereas lower values (below 0.01 s km-1) characterize both the NE mar- gin of the basin (sites AG04 and AG11) and station AG05 located to the SE of the Vallo di Diano basin, as shown by the colours in Fig. 3 representing the interpolated values of normalized delay times. Furthermore I analysed the temporal variations of anisotropic parameters in order to explain if fluids play an important role in the earthquake generation process. The close association between anisotropic variations and Vp/Vs changes with the increase of seismicity rate in the shallow portion of the fault system supports the hypothesis that the background seismicity is influenced by the fluc- tuation of pore fluid pressure at depth (Valoroso et al., submitted). An increase in the pore pressure leads to a decrease in the strength on the fault due to lower values of effective normal stress over the fault surface (Bell and Nur, 1978). In this framework, fluid flow and pore pressure relaxation, moulding the seismicity at the southern termination of the western fault system, where we also found the highest values of delay time, might facilitate future ruptures along pre-existing zones of weakness. 2) Alto Tiberina Fault (ATF) area: I focused my attention in this area because the geometry of ATF is well known and earthquakes with hypocenters down to 30 km were recorded in the area so I might be able to characterize anisotropic behaviour of volume of rock above and below the ATF, by using anisotropic parameters and their variations. I would also compare the characteristics of ear- thquakes related to the ATF activity from the ones related to antithetic normal faults. These infor- mation are relevant for assessing seismic hazard and for accurately constraining possible ground shaking scenarios. Fig. 4 shows the rose diagrams of the fast polarization directions (as in Fig. 3); the spatial varia- tion of delay time is also reported (coloured background area). In the total fast plot, lower inset, I observe a dominant direction NW-SE, as well as the major faults in the area and perpendicular to the Shmin of extensional stress field (Boncio & Lavecchia, 2000). Looking the spatial variation of the delay time I see an higher delay times below the stations N006 and A002, but these values are related to few measurements (4-5) connected to deep events, so if the delay times are normalized to length of ray-travel the values are more homogeneous with the others. In detail the means of fast directions, at the selected 13 stations, are roughly parallel to the main geological structures, even if there are stations with rotated directions (i.e. C002, B4, B5, A003). If I consider only normalized delay times, I observe higher values, 0.01 s/km, located at stations C3 and C004, both situated in the footwall of the Gubbio Fault (GuF) and where most of the seismici- ty is located. These values suggest a percentage of anisotropy A=3% (A= Vsmean *dtn*100, Wüstefeld et al., 2010, Vsmean is assumed 3.3 km/s, see Piccinini et al., 2003): this anisotropic volume seems to be related to the deep junction between GuF and ATF. I also studied deep and temporal variations of anisotropic parameters, and I can conclude that 3D variations exist, probably related to overpressured fluids, and temporal changes in delay time e fast direction are to refers to state changes of fluid-filled stress-oriented micro-cracks. 3) On April 2009 the Mw=6.3 L’Aquila earthquake struck central Italy, so I decided to investi- gate this region of central Apennines. Some authors suggest that observations of shear-wave split- ting at seismic stations above a seismic sequence can be used to monitor the build up of stress befo- re earthquakes and the stress release as earthquakes occur (Gao & Crampin, 2004). For this area I analysed the seismic sequence recorded at 4 stations belonging to RSNC (AQU, CAMP, FAGN and FIAM). Preliminary results Fig. 5) of shear wave splitting show a dominant fast orientation striking NW-SE, in agreement with the active stress field in the area. Furthermore I reco- gnize the same situation at singular stations except FAGN. Delay time values are about 0.01 s/km, but higher values are recorded during the days around the mainshock, with a maximum of about 0.02 s/km. Looking at temporal variations of anisotropic parameters approaching the earthquake, I observe some variations of the seismic wave propagation properties. The elastic characteristics of rocks in the fault region underwent certainly a change in the days before the earthquake and these conditions remained for some days after the main event. From a posteriori observations, which show very scattered measurements, I might suppose that a complex sequence of dilatancy-diffusion processes took place in the rock volume and that fluids play a key role in the fault failure process. Variation of elastic and anisotropic parameters during L’Aquila seismic sequence indicates that a complex sequence of dilatancy and fluid diffusion processes affected the rock volume surrounding the nucleation area (Lucente et al., in press). There are evidences for a major role played by fluids over the seismogenesis, in the only past comparable case in Italy (Chiarabba et al., 2009b; Miller et al., 2004), and over-pressurization in fault structures has been suggested as a primary trigger of nor- mal faulting earthquakes (Sibson, 2000; Chiodini et al., 2004). The three studies suggest that aniso- tropic fast directions can be used to define the active stress field, finding a general consistence bet- ween fast direction and main stress indicators (focal mechanisms and borehole break-outs); and that the magnitude of delay times can be used to define the fracture field intensity, higher values being found in the volume where micro-seismicity occurs. The prevalent carbonatic nature of the seismo- genic crust in Italy makes it a favourable candidate for the formation of fluids reservoir at depth: in fact active tectonics produces the fracture field and the presence of deep thrusts and low angle nor- mal faults could act as traps (structural seal) where fluids may accumulate and generate reservoirs (Chiodini et al., 2004)

S wave Splitting in Central Apennines (Italy): anisotropic parameters in the crust during seismic sequences

In this work, we reviewed the main anisotropic results obtained in the last two decades along the Central Apennines. Moreover, we improved this database, with new results coming from the seismicity that occurred in the Montereale area, between 2009 and 2017, which corresponds to a spatio-temporal gap in the previously analyzed datasets. The examined papers concerned both seismic sequences (as Colfiorito in 1997, Pietralunga in 2010, L’Aquila in 2009, Amatrice in 2016) and background seismicity (as the 2000-2001 Città di Castello experiment). The whole of the collected results shows a general NW-SE fast shear wave direction consistent with both the orientation of the extensional active Quaternary and inherited compressive fault systems, focal mechanisms and local stress field. Also, we observed a more intense anisotropy strength (normalized delay time > 0.006 s/km) nearby the strongest events (M > 5), all concentrated in the hanging-wall of the activated fault systems. In fact, this area is deeply affected by the surrounding rock volume perturbations that, in turn, have altered both the local stress field and crustal fracturing network. The most common anisotropic interpretative models that could explain our results are 1) the stress-induced anisotropy according to the Extensive-Dilatancy Anisotropy (EDA) model where the anisotropic pattern is related to the local stress variation and most of the variability is visible in time; 2) the tectonic-controlled anisotropy according to the Structural-Induced Anisotropy (SIA) model where the anisotropic pattern is related to the major structural features and most of the variability is visible only in space. As reported by the examined studies in Central Apennines the possibility to discriminate between stress and structural anisotropy is quite complex in a region where the directions of the extensional regime, the in situ horizontal maximum stress, the strike of major faults, both active and inherited coincide. Generally, in this review, we noted an overlap and mixture of the two aforementioned mechanisms and, just through a temporal analysis, made in the Montereale area, we supposed a predominant stressinduced anisotropy only in rock volumes where anisotropic parameter variations have been detected

The Pollino seismic sequence: Can shear wave anisotropy monitoring help earthquakes forecast?

Since the late the late ’60s-early ’70s era seismologists started developed theories that included variations of the elastic property of the Earth crust and the state of stress and its evolution crust prior to the oc- currence of a large earthquake. Among the others the theory of the dilatancy (Scholz et al., 1973): when a rock is subject to stress, the rock grains are shifted generating micro-cracks, thus the rock itself in- creases 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 80s and 90s such kind of research on earthquake precursor 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 high resolution ear- thquake 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. 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 pa- rameters ( Piccinini et al., 2006). 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 understanding the seismogenic phenomena and their precursors (Crampin & Gao, 2010). Variations of anisotropic parameter and of the ratio between the compressional (P-wave) and the shear (S-wave) seismic velocities, the Vp/Vs (Nur, 1972) have been recently observed and measured during the preparatory phase of a major earthquake (Lucente et al. 2010). Here we show the anisotropic parameters at station MMN during the Pollino seismic sequence 2010-2013

CRUSTAL FRACTURING FIELD AS REVEALED BY SEISMIC ANISOTROPY IN THREE SEISMOGENIC AREAS OF THE APENNINIC CHAIN

In the last three years, we developed, tested and improved an automatic analysis code to calculate the shear wave splitting parameters, fast polarization direction (φ) and delay time (∂t). The code is a set of MatLab scripts able to retrieve crustal anisotropy parameters from three-component seismic recording of local earthquakes using horizontal component cross-correlation method. The analysis procedure consists in choosing an appropriate frequency range, that better highlights the signal containing the shear waves, and a length of time window on the seismogram centred on the S arrival (the temporal window contains at least one cycle of S wave). The code was compared to other two automatic analysis code (SPY and SHEBA) and tested on three Italian areas (Val d’Agri, Tiber Valley and L’Aquila surrounding) along the Apennine mountains. For each region we used the anisotropic parameters resulting from the automatic computation as a tool to determine the fracture field geometries connected with the active stress field. The anisotropic fast directions are used to define the active stress field (EDA model), finding a general consistence between fast direction and main stress indicators (focal mechanism and borehole break-out). The magnitude of delay time is used to define the fracture field intensity finding higher value in the volume where micro-seismicity occurs. Furthermore we studied temporal variations of anisotropic parameters in order to explain if fluids play an important role in the earthquake generation process. The close association of anisotropic parameters variations and seismicity rate changes supports the hypothesis that the background seismicity is influenced by the fluctuation of pore fluid pressure in the rocks

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

SEISMIC ANISOTROPY AND ITS RELATION WITH FAULTS AND STRESS FIELD IN THEVAL D'AGRI (SOUTHERN ITALY).

Shear-wave splitting is measured at 17 seismic stations deployed in the Val DAgri by INGV, which recorded local back-ground seismicity from May 2005 to June 2006 . The splitting results suggest the presence of an anisotropic upper crust (max hypocentral depth 15.5 km). The dominant fast polarisation direction strikes NW-SE parallel to the Apennines orogen and is approximately parallel to the maximum horizontal stress in the region and also parallel to the strike of the main normal faults in the Val DAgri. The size of the delay times, average is 0.1 second suggests 4.5% shear-wave velocity anisotropy. At stations located at the North West portion of the deployment average delay times are larger on the order of 0.2s. These parameters agree with an interpretation of seismic anisotropy in terms of the Extensive-Dilatancy Anisotropy model which considers the rock volume to be pervaded by fluid-saturated microcracks aligned by the active stress field. We cannot completely rule out the contribution of aligned macroscopic fractures as the cause of the shear wave anisotropy even if the parallel shear-wave polarisations we found are diagnostic of transverse isotropy with a horizontal axis of symmetry. This symmetry is commonly explained by parallel stress-aligned microcracks

Seismic measurements to reveal short-term variations in the elastic properties of the Earth crust

Since the late the late ’60s-early ’70s era seismologists started developed theories that included variations of the elastic property of the Earth crust and the state of stress and its evolution crust prior to the occurrence of a large earthquake. Among the others the theory of the dilatancy (Scholz et al., 1973): 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 80s and 90s such kind of research on earthquake precursor 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 high resolution 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 ‘70s, 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, the Vp/Vs (Nur, 1972; Kisslinger and Engdahl, 1973). Variations of this ratio have been recently observed and measured during the preparatory phase of a major earthquake (Lucente et al. 2010). 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 pa- rameters (Miller and Savage, 2001; Piccinini et al., 2006). 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 understanding the seismogenic phenomena and their precursors (Crampin & Gao, 2010). In terms of determination of Earth crust elastic properties, recent studies (Brenguier et al., 2008; Chen et al., 2010; Zaccarelli et al., 2011) have shown how it is possible to estimate the relative variations in the wave speed through the analysis of the crosscorrelation of ambient seismic noise. In this paper we analyze in detail two seismological methods dealing with shear wave splitting and seismic noise cross correlation: a short historical review, their theoretical bases, the problems, learnings, limitations and perspec- tives. Moreover we discuss the results of these methods already applied on the data recorded in the L’Aquila region, before and after the destructive earthquake of April 6th 2009, represent their self an interesting case study

The contribution of the Istituto Nazionale di Geofisica 1 e Vulcanologia (INGV) to 2 “Adria LithosPHere investigAtion (ALPHA)” active seismic experiment

During the winter 2012, from 20 January to 4 February, the German oceanographic FS METEOR cruise (M86/3) took place in the central-southern Adriatic Sea in the frame of “Adria LithosPHere InvestigAtion” (ALPHA [Kopp et al., 2013]). The primary goal of the project was high-resolution tomographic imaging of the crust and lithospheric mantle underneath the southern Adriatic Sea, the Apulia eastern margin and the external zone of the Dinaric thrust-belt by collecting offshore-onshore seismic data along three multi-fold wide-aperture profiles. The definition of reliable velocity models of the Adriatic lithosphere was considered crucial for a better understanding of the structure, fragmentation, geodynamic evolution, and seismotectonics of the Adria-Apulia microplates. The ALPHA Project was coordinated by Helmholtz Centre for Ocean Research Kiel, Germany (GEOMAR), former Leibniz Institute of Marine Sciences (German: Leibniz-Institut für Meereswissenschaften, IFM-GEOMAR) and conducted in close cooperation with different European institutions of Germany, Albania, Croatia, Italy and Montenegro. The Istituto Nazionale di Geofisica Vulcanologia (INGV) participated by deploying land stations along two transects in the Apulia and Gargano Promontory to extend westwards the seismic profiles. The primary goal was to record shallow-to-deep seismic phases travelling along the transition between the Adriatic basin and the Apulia foreland. In this paper we present the field work related to the two Italian onshore transects, the recorded data, and the processing flow developed to highlight crustal and mantle refractions and wide-angle reflections

Le attività del gruppo operativo INGV "SISMIKO" durante la sequenza sismica "Amatrice 2016",

SISMIKO è un gruppo operativo dell’Istituto Nazionale di Geofisica e Vulcanologia (INGV) che coordina tutte le Reti Sismiche Mobili INGVPublishedLecce3T. Sorgente sismica4T. Sismicità dell'Italia8T. Sismologia in tempo reale1SR TERREMOTI - Sorveglianza Sismica e Allerta Tsunami2SR TERREMOTI - Gestione delle emergenze sismiche e da maremoto3SR TERREMOTI - Attività dei Centr

SISMIKO:emergency network deployment and data sharing for the 2016 central Italy seismic sequence

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