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Estimation of local seismic response plays a key role in assessing local seismic hazard and particularly in the design of shaking scenarios. Modelling local seismic response involves knowing of the shear wave velocity (VS) and quality factor (QS) profiles for the site in question. The many techniques that have been developed to assess VS in surface deposits produce reliable measurements of VS , but these rarely correspond to direct measurements of QS. The latter is often considered through damping measures from laboratory tests on small-scale soil samples, which can provide information primarily on intrinsic attenuation, neglecting the contribution of scattering effects. In this paper, using seismic recordings obtained at the surface and in boreholes at 100 m depth, we estimate an average value of QS of some characteristic alluvial deposits of the Po Plain (northern Italy). Data come from a microseismic network which sampled an almost uniform lithology in the central Po Plain and consisted of three surface and four borehole stations with an interstation distance of about 2 km. The average value of QS of the shallowest 100 m of the sedimentary strata, Q S ,100, is estimated by considering: (1) the high-frequency attenuation of seismic waves due to propagation through the corresponding stratigraphy and (2) the interference between incident and surface-reflected waves observed at borehole stations. We parametrize the first through |$\Delta k_{0_{ {100}}}$|⁠ , the difference between the values of the spectral decay parameter kappa (k) estimated at the surface and at the boreholes depth, respectively. We use the second in order to compute V S ,100, the time-averaged VS referred to the uppermost 100 m stratigraphy. We obtain: |$\Delta k_{0_{ {100}}} = (11 \pm 3)\,$|  ms, |${V_{S,100}} \, =\, (309\, \pm \, 11)\,$| m s−1 and |${Q_{S,100}} \, =\, 31 \pm 10$|⁠. At the surface, the estimated values of the site-specific kappa, k 0, are found to range from 75 to 79 ms. As expected, these results are in good agreement with studies performed in other sites characterized by sandy or clayey lithologies, and can be usefully used in site response analysis at sites where the rigidity is mainly controlled by lithostatic pressure. [ABSTRACT FROM AUTHOR]

Stimulated by the recent advances in computational tools for the simulation of seismic wave propagation problems in realistic geological configurations, this paper presents a 3D physicsbased numerical analysis of near-source ground motion during the MW 6.0 2012 May 29 earthquake in the Po Plain, Northern Italy. To reproduce with sufficient accuracy some of the most peculiar features of the near-source strong-motion records and of the spatial variability of damage distribution, this study required a sequence of investigations, starting from the analysis of a wide set of near-source records, to the calibration of an improved kinematic seismic source model, up to the development of a 3D numerical model of the portion of the Po Plain interested by the earthquake. The latter includes the basin geometry, characterized by sediment thickness sharply varying from few tens of metres to some kilometres. The spatial resolution of the numerical model is suitable to propagate frequencies up to about 1.5 Hz. Numerical simulations were performed using the open-source high-performance code SPEED, based on the Discontinuous Galerkin Spectral Elements method. The 3D numerical model, coupled with the updated slip distribution along the rupturing fault, proved successful to reproduce with good agreement, measured through quantitative goodness-of-fit criteria, the most relevant features of the observed ground motion. These include: (i) the large fault normal velocity peaks at the near-source stations driven by updip directivity effects; (ii) the small-scale variability at short distance from the source, resulting in the out-of-phase motion at stations separated by only 3 km distance; (iii) the propagation of prominent trains of surface waves, especially in the Northern direction; (iv) the map of earthquake-induced ground uplift with maximum values of about 10 cm, in substantial agreement with geodetic measurements and (v) the two-lobed pattern of the peak ground velocity map, well correlated with the distribution of macroseismic intensity. [ABSTRACT FROM AUTHOR]

In this study, we investigate the correlation between the residuals of a neGMM (non-ergodic Ground Motion Model) and the physics-based parameters obtained using a non-parametric GIT approach (Generalized Inversion Technique) to lay the groundwork for the implementation of an ad-hoc FAS (Fourier Amplitude Spectra) neGMM for the Central Italy region. This region is particularly suitable for data-driven methodologies as those applied in this work because of the large amount of available data due to the recent multiple main shock–aftershock sequences occurred in this area. Both neGMM and GIT models are developed for Fourier spectra in the frequency range between 0.5 and 25 Hz and using the same reference sites. The comparison of the non-ergodic terms with the source, path and site spectral parameters provides interesting results. First, we find a strong correlation between the source parameters, stress drop Δσ and decay |${k}_{\mathrm{ source}}$| and the source neGMM corrective terms (the combination of the between-event δBe and the location-to-location terms δL2L). This correlation is frequency dependent and, at high frequency, is remarkably positive for Δσ and negative for |${k}_{\mathrm{ source}}$|⁠. Concerning the attenuation terms, the path-to-path residuals (δP2P) are clearly associated with the deviations from the regional Q estimates obtained from GIT analysis. This indicates that the neGMM properly captures the properties of the anelastic attenuation and that the corrective terms δP2P can be used to account for differences in travel paths across different crustal domains. Finally, adopting the same reference sites for neGMM and non-parametric GIT, we observe that the systematic site terms (δS2Ss) and the GIT-derived amplification functions are in good agreement. The next step for an appropriate modelling is to identify the physical parameters (e.g. VS,30 and |${k}_0$|⁠) describing the empirical amplification curves to be introduced as explanatory variables in the ground motion model. [ABSTRACT FROM AUTHOR]

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 M w and therefore a further conversion must be done to obtain the values of local magnitude (M L), which is the parameter commonly used for moderate and small earthquakes in seismic catalogues. Here, the relationship utilized by a seismic network to determine M L 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 M L, based on synthetic WA records, is used. [ABSTRACT FROM AUTHOR]

SUMMARY: We derived new, reversible relationships between macroseismic intensity (I), expressed in either the European Macroseismic (EMS-98) or the Mercalli–Cancani–Sieberg (MCS) scales and peak ground acceleration (PGA), peak ground velocity (PGV) and the spectral acceleration (SA) at 0.3, 1.0 and 3.0 s [SA(0.3), SA(1.0) and SA(3.0)] for Italy. We adopted the orthogonal distance regression technique to fit a quadratic function. This research aims to improve ground motion and intensity estimates for earthquake hazard applications, and for the calculation of shakemaps in Italy. To this end, the recently published INGe data set was used (https://doi.org/10.13127/inge.2). The new relations are: where PGA and SAs are expressed in cm s−2 and PGV is expressed in cm s−1. Tests performed to assess the robustness and the accuracy of the results demonstrate that adoption of quadratic relationships for this regression problem is a suitable choice within the range of values of the available data set. Comparison with similar published regressions for Italy evidences that the proposed relations provide statistically significant improved fits to the data. The new relations are also tested by inserting them in the ShakeMap system of the Italian configuration evidencing a significant improvement when compared to those implemented. [ABSTRACT FROM AUTHOR]

High-rate global navigation satellite system (GNSS) has emerged as an effective method to recover seismic waveforms without saturation and drifts, but it has the limitation of relatively lower sampling rate and higher noise level compared to seismic instruments. In this study, we present a new seismogeodetic method by integrating GNSS and accelerometer data to obtain optimal real-time seismic waveforms. Unlike traditional integration methods based on GNSS techniques of relative positioning or precise point positioning, the new method uses a GNSS time difference technique and inherits its unique advantage in real-time and high-accuracy velocity solutions. Furthermore, by incorporating the tightly coupled structure, it can overcome the cascading problem and provide more accurate and robust waveforms compared to its loosely coupled counterpart. The performance of this method is first compared with the traditional loosely coupled approach in challenging environments through a set of shake table experiments. With three GNSS satellites, this approach method can improve the accuracy of velocities and displacements by 42 and 87 per cent, respectively. With four or more GNSS satellites, the average improvements of the method reach 25 and 41 per cent for the velocities and displacements, respectively. We then validate the full performances of the method through simulated shake table experiments and collocated GNSS and accelerometer data during the 2016 M w 6.6 central Italy earthquake. The simulated and real-event analyses demonstrate that the new integration method can take full advantage of the complementary characteristics of GNSS and accelerometer sensors. By providing more accurate and broad-band velocity and displacement waveforms in a real-time or near-real-time manner, this method is quite promising in earthquake early warning and rapid source inversion. [ABSTRACT FROM AUTHOR]

The M w 6.0 earthquake that hit central Italy on 2016 August 24 caused an abnormally high level of destruction in the town of Amatrice. In order to clarify the role of site response in causing such a disaster, a series of ambient noise recordings acquired in the aftermath of the event are analysed here to identify site resonance properties from the ratios H/V between horizontal and vertical amplitudes of ground motion. Although the noise data acquisition was limited by the emergency management activities, the use of a new analysis technique, which isolates the contribution of Rayleigh waves to the noise wavefield and averages instantaneous estimates of H/V ratios, provided more stable results compared to the standard Nakamura's technique based on mean spectral ratios. The results demonstrated the occurrence of significant resonance phenomena, but without an obvious correlation with the spatial distribution of damage severity. It is apparent that the damage severity was also influenced by some additional local factors related to building vulnerability. Moreover, the time-series analysis revealed seasonal variations in the Rayleigh wave ellipticity curves likely related to the water content changes in the surface deposits and their influence on the Poisson coefficient. Finally, the new method proved capable of recognizing time-varying directions of Rayleigh wave propagation. This capability could be exploited to support other passive seismic methods (e.g. ReMi), whose results' reliability is limited by the lack of control on wave direction origin. [ABSTRACT FROM AUTHOR]

To evaluate the site response using both empirical approaches (e.g. standard spectral ratio, ground motion models (GMMs), generalized inversion techniques, etc.) and numerical 1-D/2-D analyses, the definition of the reference motion, that is the ground motion recorded at stations unaffected by site-effects due to topographic, stratigraphic or basin effects, is needed. The main objective of this work is to define a robust strategy to identify the seismic stations that can be considered as reference rock sites, using six proxies for the site response: three proxies are related to the analysis of geophysical and seismological data (the repeatable site term from the residual analysis, the resonance frequencies from horizontal-to-vertical spectral ratios on noise or earthquake signals, the average shear wave velocity in the first 30 m); the remaining ones concern geomorphological and installation features (outcropping rocks or stiff soils, flat topography and absence of interaction with structures). We introduce a weighting scheme to take into account the availability and the quality of the site information, as well as the fulfillment of the criterion associated to each proxy. We also introduce a hierarchical index, to take into account the relevance of the proposed proxies in the description of the site effects, and an acceptance threshold for reference rock sites identification. The procedure is applied on a very large data set, composed by accelerometric and velocimetric waveforms, recorded in Central Italy in the period 2008–2018. This data set is composed by more than 30 000 waveforms relative to 450 earthquakes in the magnitude range 3.2–6.5 and recorded by more than 450 stations. A total of 36 out of 133 candidate stations are identified as reference sites: the majority of them are installed on rock with flat topography, but this condition is not sufficient to guarantee the absence of amplifications, especially at high frequencies. Seismological analyses are necessary to exclude stations affected by resonances. We test the impact of using these sites by calibrating a GMMs. The results show that for reference rock sites the median predictions are reduced down to about 45 per cent at short periods in comparison to the generic rock motions. [ABSTRACT FROM AUTHOR]

This study describes a deep convolutional neural network (CNN) based technique to predict intensity measurements (IMs) of earthquake ground shaking. The input data to the CNN model consists of multistation, 3C acceleration waveforms recorded during the 2016 Central Italy earthquake sequence for M  ≥ 3.0 events. Using a 10 s window starting at the earthquake origin time, we find that the CNN is capable of accurately predicting IMs at stations far from the epicentre which have not yet recorded the maximum ground shaking. The CNN IM predictions do not require previous knowledge of the earthquake source (location and magnitude). Comparison between the CNN model predictions and those obtained with the Bindi et al. GMPE (which requires location and magnitude) shows that the CNN model features similar error variance but smaller bias. Although the technique is not strictly designed for earthquake early warning, we find that it can provide useful estimates of ground motions within 15–20 s after earthquake origin time depending on various setup elements (e.g. times for data transmission, computation, latencies). The technique has been tested on raw data without any initial data pre-selection in order to closely replicate real-time data streaming. When noise examples were included with the earthquake data the CNN was found to be stable, accurately predicting the ground shaking intensity corresponding to the noise amplitude. [ABSTRACT FROM AUTHOR]

Dense strong motion and high-rate Global Navigation Satellite Systems (GNSS) networks have been deployed in central Italy for rapid seismic source determination and corresponding hazard mitigation. Different from previous studies for the consistency between two kinds of sensor at collocated stations, here we focus on the combination of high-rate GNSS displacement waveforms with collocated seismic strong motion accelerators, and investigate its application to image rupture history. Taking the 2016 August 24 Mw 6.1 Central Italy earthquake as a case study, we first generate more accurate and longer period seismogeodetic displacement waveforms by a Kalman filter, then model the rupture behaviour through a joint inversion including seismogeodetic waveforms and InSAR observations. Our results reveal that strong motion data alone can overestimate the magnitude and mismatch the GNSS observations, while 1 Hz sampling rate GNSS is insufficient and the displacement is too noisy to depict rupture process. By contrast, seismogeodetic data enhances temporal resolution and maintains the static offsets that provide vital constraint to the reliable estimation of earthquake magnitude. The obtained model is close to the jointly inverted one. Our work demonstrates the unique usefulness of seismogeodesy for fast seismic hazard response. [ABSTRACT FROM AUTHOR]

Macroseismic intensity is assessed on the basis of the effects caused by an earthquake. These effects reflect the expression of both the intensity and frequency of the ground motion, thus complicating prediction equation modelling. Here we analysed data of several macroseismic transitory effects caused by recent Italian earthquakes in order to study their attenuation as a function of magnitude and hypocentral distance and to obtain a specific prediction equation, of simple functional form, that could be applied to each of the effects under analysis. We found that the different attenuation behaviours could be clearly defined by the values of the specially formulated magnitude-distance scaling ratio (S), thus allowing to group the effects on the basis of the S value. The oscillation of hanging objects and liquids, together with the feeling of dizziness, were separated from most other variables, such as the effects of the earthquake on small objects, china and windows, which were caused by a vibration of higher frequency. Besides, the greater value of S, associated with the perception of the seismic sound, explained the peculiarity of this phenomenon. As a result, we recognized the frequency range associated with each effect through comparisons with the ground motion prediction equations and, in particular, with the 5 per cent damped horizontal response spectra. Here we show the importance of appropriately selecting the diagnostic elements to be used for intensity assessment in order to improve the correlation with ground motion. [ABSTRACT FROM AUTHOR]

The Sulmona plain (central Italy) is an intramontane basin of the Abruzzi Apennines that is known in the literature for its high seismic hazard. We use extensive measurements of ambient noise to map the fundamental frequency and to detect the presence of geological heterogeneities in the basin. We perform noise measurements along two basin-scale orthogonal transects, in conjunction with 2-D array experiments in specific key areas. The key areas are located in different positions with respect to the basin margins: one at the eastern boundary (fault-controlled basin margin) and one in the deepest part of the basin. We also collect independent data by using active seismic experiments (MASW), down-hole and geological surveys to characterize the near-surface geology of the investigated sites. In detail, the H/V noise spectral ratios and 2-D array techniques indicate a fundamental resonance (f0) in the low-frequency range (0.35-0.4 Hz) in the Sulmona Basin. Additionally, our results highlight the important role that is played by the alluvial fans near the edge-sectors of the basin, which are responsible for a velocity inversion in the uppermost layering of the soil profile. The H/V ratios and the dispersion curves of adjacent measurements strongly vary over a few dozens of meters in the alluvial fan area. Furthermore, we perform 1-D numerical simulations that are based on a linear-equivalent approach to estimate the site response in the key areas, using realistic seismic inputs. Finally, we perform a 2-D simulation that is based on the spectral element method to propagate surface waves in a simple model with an uppermost stiff layer, which is responsible for the velocity inversion. The results from the 2-D modelling agree with the experimental curves, showing deamplified H/V curves and typical shapes of dispersion curves of a not normally dispersive site. [ABSTRACT FROM AUTHOR]

We perform a systematic investigation of along-strike rupture directivity of 70 earthquakes (3.0 ≤ Mw ≤ 6.1) of the 2009 L'Aquila seismic sequence by analysing azimuthal variations of broad-band seismograms recorded in the distance range 60 km < R < 230 km. We use reference spectra of events with little directivity (similar to the empirical Green's function method) to deconvolve propagation-site effects and focus on source properties. A directivity index (0 ≤ IDIR ≤ 1) calculated for each earthquake quantifies the spectral separation above the corner frequency of the target event at opposite along-strike directions. A large number (73 per cent) of events including the Mw 6.1 main shock showhigh (>0.7) IDIR values indicating predominantly unilateral rupture propagation. The preferred rupture propagation direction is generally to the southeast with no dependence on the earthquake magnitude or occurrence time. Events on two main faults (L'Aquila and Campotosto) show somewhat different behaviour. Almost all earthquakes on the L'Aquila fault have strong unilateral directivity to the southeast, whereas earthquakes on the Campotosto fault show more diverse behaviour. However, there is a predominance of unilateral ruptures (14 out of 22) also on the Campotosto fault, and the few (five) earthquakes with ruptures to the northwest are limited to the most northwestern segment of the fault. The spectral results are consistent with time-domain analysis when the latter samples adequately the frequency band above corner frequency. The preferred rupture direction may be produced at least in part by a velocity contrast across the fault. The results provide important input for estimates of seismic motion and physics of earthquake ruptures. [ABSTRACT FROM AUTHOR]