ALBERT

Description: Following the M 6.0 of August 24 2016 Amatrice earthquake, a temporary seismic network was installed in the village of Amatrice, under the umbrella of the Italian Center for Seismic Microzonation (http://www.centromicrozonazionesismica.it), who conducted a preparatory survey to seismic microzonation with other Italian Institutions. This work focuses on data analysis of 7 stations installed in the Amatrice terrace which is representative of the geological condition of the town, with the aim of studying the possible presence of secondary effects during the seismic sequence caused by site conditions. Stiff bedrock outcroppings were also sampled with 2 reference stations. Preliminary analyses carried out on several earthquakes with Ml 〉 4 highlight the presence of a low frequency phase with a high energy content at 6 -7 seconds after the first P-wave arrival in almost all the recordings. This wave package was observed for the Mw 6.5 of October 30 earthquake and it has the highest amplitude of the entire recordings, having PGA values of 0.5 g and frequency between 2 and 3 Hz. Frequency-wavenumber analysis performed for the 7 stations array gives an important contribution for the interpretation of this phenomenon, showing that the low frequency wave-package for the examined events does not always show back-azimuth and slowness values compatible with the very first portion of the seismic record. It could be associated to the presence of reflected or refracted waves generated by secondary effects as geological or morphological heterogeneities at local or larger scale.

Description: Published

Description: Vienna

Description: 5T. Sismologia, geofisica e geologia per l'ingegneria sismica

Description: Several months of ambient seismic noise recordings are used for investigating the distribution of elastic properties in the Fucino Plain, one of the largest intermontane tectonic depressions of the Italian Apennine chain (Central Italy). The Plain is characterized by a low level of seismicity but the presence of several active faults makes it an Italian area of high seismic hazard. The most recent and strongest seismic event in Fucino Plain occurred in the 1915 (Avezzano earthquake) and it represents one of the most energetic events (Ms = 7.0) happened in central Apennines. Inter-stations Green’s functions are reconstructed by the cross-correlation of continuous ambient noise data recorded from twelve seismic velocimeters deployed around the Avezzano city, and organized in two different temporally sub-networks. The aim of cross-correlation analysis is to extract surface waves from Green’s functions for investigating the dispersive response of the structure. We analyzed the temporal stability of the cross-correlated signals that is used as an indicator of reliability of measurements and as criteria to select the Green’s functions to analyze

Description: Published

Description: 1173-1176

Description: 5T. Sismologia, geofisica e geologia per l'ingegneria sismica

Description: Normal fault systems, outcropping or hidden below Quaternary covers in intermountain basins, are the expression of the Neogene-Quaternary evolution of central Italy, characterized by an extensional tectonic regime following the fold and thrust structuring of the Appenninic orogen. The presence of these features plays an important role in seismic risk evaluation of an area. In this work we deal with the use of single-station seismic noise measurements to detect sudden lateral variations of the geometries and/or properties of subsoil connected to the presence of tectonic elements (fault zones). Ambient noise data were collected along transects perpendicular to the strike of hypothetical fault lines for 3 test sites within the Abruzzi Region. The proposed approach is suitable for detecting in a fast and simple way local lateral changes in the subsoil characteristics close to geological structures and can be very effective to properly address more expensive and time consuming classical geophysical and paleoseismological approaches.

Description: Published

Description: Pescina, Fucino Basin, Italy

Description: 5T. Sismologia, geofisica e geologia per l'ingegneria sismica

Description: Buildings close to each other can perform different behaviour despite its similar seismic vulnerability. This effect is mainly due to the local seismic response connected to the characteristics of the shallow soil layers, especially when we move away from the epicentral area and the near field motion reduces its importance among the total amount of shaking. In this paper we show some results of the microzonation project of the Avezzano municipality, a town located in the southwestern portion of the Abruzzi region, which experienced the severe effects of the January 13th, 1915 M 7.0 earthquake. Starting from a particularly detailed knowledge of the geological characteristics of outcropping lithologies and inferring the trend of subsoil geometries, we explored the role played by the near-surface geology in causing variability of the ground motion by analysing a large database of earthquakes and microtremor recordings acquired by temporary seismological networks using classical site-reference and non-reference spectral techniques. Based on the obtained results we can seismically characterize all the municipal territory not only in terms of fundamental resonance frequency, useful in drawing maps of seismic microzonation and design geological sections, but also of amplification factors helpful in verifying numerical modelling of seismic response as required by national microzonation guidelines. We have also found many criticisms that need a more detailed analysis in order to establish the cause of these anomalies.

Description: Published

Description: 1153-1157

Description: 5T. Sismologia, geofisica e geologia per l'ingegneria sismica

Description: In this work we tested the capability of single station ambient noise spectral ratios (HVNSR) technique to be used as a proxy for detecting sharp variations in the subsoil characteristics in sedimentary basins. In sites characterized by 1D response HVNSR is able to detect the resonance frequency of sediments (f0) to be related with their thickness and velocity. In quasi-1D geological situation, a smooth variation in the resonance frequency suggests some variation in the thickness of the soft sedimentary layers or in their velocity. In the case of sharp lateral variation of the subsoil, as in presence of morphological steps or buried faults in the bedrock, the noise wave-field becomes more complex and spectral ratios show some directional effect strictly related to the presence of the lateral heterogeneity (Matsushima et al. 2014). In this work we focus our attention on the detection of a buried shallow tectonic element known in bibliography (Galadini, Galli 1999) as the Luco dei Marsi fault, located in the western margin of the Fucino basin (central Appenines, Italy). We performed simultaneous ambient noise measurements using MarsLite digitizers equipped with Lennarts 3d-5s velocimeters along a transect, with inter-station distance between 30 and 50 meters, which crosses perpendicularly the surface projection of fault line. The collected data, analyzed with classical HVNSR technique, show some complexities in the f0 distribution compatible with the presence of the buried fault. In detail, our results highlight a 1D response with a clear resonance peak for the stations located away from the fault. Moving closer to the fault line, where the 1D condition is not verified, the resonance peak becomes broader. Directional analysis of spectral ratios suggests the dependence of the noise wave-field to the presence of the fault line. Close to the geological feature, results of directional analysis show a doubling in the resonance frequency with relative amplitudes changing for fault-parallel and fault-normal direction of polarization. To support with an independent approach our observations, an electrical resistivity tomography was performed along the transect whose results show the presence of a clear step in a high resistivity shallow layer that can be assumed as the bedrock. After this analysis we decided to extend the ambient noise transect towards the centre of the basin where no fault line is mapped and where we expected a smooth lowering of f0 values due to the deepening of the bedrock. This feature was really observed for a while but at a distance between 400 and 500 meters from the Luco fault we detected again some anomaly in HVNSR data with a doubling of resonance frequency and relative amplitudes depending from the direction of polarization. This observation suggests the presence of deeper tectonic element parallel to the Luco fault. As a conclusion of our work we believe that HVNSR and polarization analysis can represent a quick and fast method to hypothesize the presence of buried discontinuity in the subsoil and can be usefully used to guide the positioning of more sophisticated geophysical analysis aimed at mapping their geometries.

Description: Published

Description: Trieste, Italy

Description: 5T. Sismologia, geofisica e geologia per l'ingegneria sismica

Description: We present a summary of seismological and geophysical investigations at Amatrice (Central Italy), a village seated on an alluvial terrace and severely stroke by the Mw 6.0 event of August 24th 2016. The high vulnerability alone could not explain the heavy damage (X-XI MCS), whereas the vicinity of the seismic source and the peculiar site effects should be claimed to understand the ground motion variability. After the first mainshock, we investigated the Amatrice terrace for microzonation purposes together with several Italian institutions (Priolo et al., Bull. Earthquake Eng. 2019). In particular: (i) we installed 7 seismic stations as a part of the 3A network (DOI: 10.13127/SD/ku7Xm12Yy9; Cara et al., Sci. Data 2019); we performed (ii) an extensive campaign of 60 single-station ambient noise measurements (downtown stations recorded also few earthquakes), and (iii) several 2D passive seismic arrays aimed at obtaining Vs profiles down to a depth of few tens of meters (Milana et al., Bull. Earthquake Eng. 2019). Earthquake recordings were used to empirically evaluate ground motion amplification effects through spectral ratio approaches, and noise data were collected for defining the spatial distribution of the resonance frequencies. Data analysis reveals a diffuse amplification effect that reaches its maximum values in downtown area with a resonant frequency (f0) of about 2 Hz. Seismic amplification is also characterized by spatial variation and directional amplification, mainly in downtown to the west side of the alluvial terrace, and related to both stratigraphic and topographic effects. This effect tends to decrease and almost vanishes in the central part of the terrace, and it increases again moving towards its eastern edge with a clear shift of f0 towards higher frequencies. Empirical transfer functions were then used to recover the ground motion that could have hit the historical center of Amatrice during the August 24th mainshock, through the convolution with the only record in the vicinity (IT.AMT station experienced a PGA of 0.87 g). The reconstructed peak values are much greater than expected from ground motion models, showing that detailed studies on local site response can largely modify the seismic hazard assessment.

Description: Published

Description: San Francisco, California (USA)

Description: 4T. Sismicità dell'Italia

Description: 5T. Sismologia, geofisica e geologia per l'ingegneria sismica

Description: The Amphiteatrum Flavium in Rome (Italy) is one of the most known monument in the world. With the aim of understanding the seismic response of the area where the Amphiteatrum Flavium is built and in- vestigating possible soil-structure interactions, we performed a seismic experiment in 2014, based on ambient vibration (AMV) recordings. The measurements were performed at the original ground level, on the foundation and at different oors of the monument. Data were analyzed in terms of standard Fourier analysis (FAS) and horizon- tal-to-vertical spectral ratio technique (H/V). Moreover, we performed an active P-wave seismic tomography analysis of the foundation ma- terials to better understand their in uence on the recorded signals. Our results point out that there is a strong temporal and spatial sta- bility of the H/V curves, suggesting a uniform seismic response at the monument site. Conversely, spectral amplitudes of AMV show rele- vant temporal and spatial variability at the investigated site, due to the daily variations of AMV levels and to the low-pass ltering effect of the stiff Amphiteatrum Flavium foundation that strongly attenu- ates the signals for frequencies above 4 Hz, i.e. those mostly originated by traf c vibrations. Moreover, we observe that the main vibration frequencies of the super-structure are not present as energetic peaks in the spectra of the ground-motion recorded at its base.

Description: Published

Description: S0438

Description: 4T. Sismologia, geofisica e geologia per l'ingegneria sismica

Description: JCR Journal

Description: Seismic resonance inside sedimentary basins severely influences ground shaking at the free surface in case of earthquakes. Starting from few observations of a low-frequency resonance in the historical center of Rome, Italy, we performed several single-station ambient vibration measures to verify and estimate the resonance frequency in a wide area of the city by Horizontal-to-Vertical spectral ratio method. We verified a stable low-frequency peak in the range 0.3–0.4 Hz. Recordings of August 24th 2016, Mw 6.0 Amatrice earthquake, available both inside and outside the basin of Rome, confirm the presence of high-energy components at frequencies of 0.2–0.4 Hz within the basin. These observations support the hypothesis of a deep seismic impedance contrast responsible for the low frequency resonance. To infer the depth range of subsoil deposits related to this impedance contrast, we analyzed ambient vibration data recorded by 2-D seismic arrays aiming at retrieving the shear-wave velocity structure up to relevant depths. To increase the investigation depth (up to 2000 m), we jointly inverted for Rayleigh-waves dispersion and ellipticity curves and resonance frequency. The shear-wave velocity profile shows two main discontinuities at depths of about 500 m and 1800 m that can be related to the bottom of the Plio-Pleistocene filling of the Rome basin and to the top of the basal limestone formation, respectively. These results fill a gap of knowledge about the deep velocity structure in the city that may be helpful for ground-motion scenario studies.

Description: Published

Description: 2395–2410

Description: 4T. Sismicità dell'Italia

Description: JCR Journal

Description: We present the results of seismological and geophysical investigations performed by the “Istituto Nazionale di Geofisica e Vulcanologia” team operating in Amatrice village (Central Italy), in the emergency phases following the Mw 6.0 event of August 24th 2016, that caused severe damage in downtown and surrounding areas. Data from seven seismic stations equipped with both weak and strong motion sensors are analyzed in terms of standard spectral ratio to empirically define amplification function using a bedrock reference site. Ambient vibration spectral ratios between horizontal and vertical component of motion are also evaluated in a large number of sites, spread out in the investigated area, to recover the resonance frequency of the soft soil outcropping layers and to generalize the results obtained by earthquake data. Ambient noise vibration are also used for applying a 2D array approach based on surface waves techniques in order to define the near-surface velocity model and to verify its lateral variation. The results allows to better understand the amplification factors in the investigated area, showing spatial variation of site effects despite of the homogeneous shallow geological condition indicated by the microzonation studies available at moment of the described field campaign. The analysis reveals a diffuse amplification effect which reaches its maximum values in downtown area with a resonant frequency of about 2 Hz. The obtained results were used to integrate the microzonation studies and they can be used for urban planning and reconstruction activities.

Description: Published

Description: 5713–5739

Description: 4T. Sismicità dell'Italia

Description: JCR Journal

Description: The presence of normal fault systems in central Italy, outcropping or hidden below Quaternary covers in intra-mountain basins, is the expression of the Neogene–Quaternary evolution of the area, characterized by an extensional tectonic regime following the fold and thrust structuring of the Apennine orogen. Italian urban settlements of central Italy are developed on hills or mountains but also in lowland areas, which are often set up in sedimentary basins. In this framework, urban centers found close to fault lines are common, with strong implications on the seismic risk of the area. In this work, we performed a dense seismological passive survey (88 single-station ambient noise measurements) and used the horizontal-to-vertical spectral ratio (HVNSR) technique to investigate hidden faults in the Trasacco municipality located in the southern part of the Fucino Basin (central Italy), where microzonation studies pointed out hypothetical fault lines crossing the urban area with the Apennine orientation. These hidden structures were only suggested by previous studies based on commercial seismic lines and aerial photogrammetry; their presence in the basin area is confirmed by our measurements. This case study shows the potentiality of using the HVNSR technique in fault areas to have a preliminary indication of anomalous behaviors, to be investigated later with specific geophysical techniques. Our approach can support microzonation studies whenever fault zones are involved, especially in urban areas or in places designated for future developments.

Description: Published

Description: 937848

Description: 5T. Sismologia, geofisica e geologia per l'ingegneria sismica

Description: JCR Journal