ALBERT

Description: This article describes the Engineering Strong-Motion Database (ESM), developed in the framework of the European project Network of European Research Infrastructures for Earthquake Risk Assessment and Mitigation (NERA, see Data and Resources ). ESM is specifically designed to provide end users only with quality-checked, uniformly processed strong-motion data and relevant parameters and has done so since 1969 in the Euro-Mediterranean region. The database was designed for a large variety of stakeholders (expert seismologists, earthquake engineers, students, and professionals) with a user-friendly and straightforward web interface. Users can access earthquake and station information and download waveforms of events with magnitude≥4.0 (unprocessed and processed acceleration, velocity, and displacement, and acceleration and displacement response spectra at 5% damping). Specific tools are also available to users to process strong-motion data and select ground-motion suites for code-based seismic structural analyses.

Description: We used strong-motion records from the 2012 May 20 and 29 Emilia-Romagna earthquakes ( M w 6.1 and 5.9, respectively) and four aftershocks with magnitudes ranging between 4.9 and 5.5 to analyse the S -wave spectral amplitude decay with distance and estimate acceleration source functions and site effects. The data set consists of six earthquakes, 44 stations and 248 records with hypocentral distances in the range 10 〈 r  〈 100 km. We rotated the accelerograms to calculate transverse and radial components of the acceleration spectrum. We found non-parametric attenuation functions that describe the spectral amplitude decay of SH and SV waves with distance at 60 different frequencies between 0.1 and 40 Hz. These attenuation functions provide an estimate of the quality factor Q at each frequency analysed. Assuming that geometrical spreading is 1/ r for r  ≤ r x and 1/( r x r ) 0.5 for r  〉 r x with r x  = 60 km and normalizing at 15 km (the recording distance where the attenuation functions start to decay), we find that the average Q for SH waves can be approximated by Q SH  = 82 ± 1 f  1.2±0.02 and by Q SV  = 79 ± 1 f  1.24±0.03 for SV waves in the frequency range 0.10 ≤ f  ≤ 10.7 Hz. At higher frequencies, 11.8 ≤ f  ≤ 40 Hz, the frequency dependence of Q weakens and is approximated by Q SH  = 301 ± 1 f   0.36±0.04 and Q SV  = 384 ± 1 f  0.28±0.04 . These results indicate that the S -wave attenuation is radially isotropic at local distances in the epicentral area. Nevertheless, we used these attenuation parameters separately to correct the radial (with Q SV ) and transverse (with Q SH ) components of the acceleration spectra and to separate source and site effects using a non-parametric spectral inversion scheme. We found that the source function of the main event and the bigger aftershocks show enhanced low frequency radiation between 0.4 and 3.0 Hz. We converted the source functions into far-field source acceleration spectra and interpreted the resulting source spectra in terms of Brune's model. The stress drops obtained range between approximately 0.9 and 2.9 MPa. Although all the recording stations used are located in the Po Plain, the site functions obtained from the spectral inversion show important amplification variability between the sites. We compared these site functions with the average horizontal to vertical spectral ratios calculated for each station, and we found consistent results for most stations.

Description: In this study we derive a spectral model describing the source, propagation and site characteristics of S waves recorded in central Italy. To this end, we compile and analyse a high-quality data set composed of more than 9000 acceleration and velocity waveforms in the local magnitude ( M l ) range 3.0–5.8 recorded at epicentral distances smaller than 120 km. The data set spans the time period from 2008 January 1 to 2013 May 31, and includes also the 2009 L'Aquila (moment magnitude M w 6.1, M l = 5.8) sequence. This data set is suitable for the application of data-driven approaches to derive the empirical functions for source, attenuation and site terms. Therefore, we apply a non-parametric inversion scheme to the acceleration Fourier spectra of the S waves of 261 earthquakes recorded at 129 stations. In a second step, with the aim of defining spectral models suitable for the implementation in numerical simulation codes, we represent the obtained non-parametric source and propagation terms by fitting standard parametric models. The frequency-dependent attenuation with distance r shows a complex trend that we parametrize in terms of geometrical spreading, anelastic attenuation and high-frequency decay parameter k. The geometrical spreading term is described by a piecewise linear model with crossover distances at 10 and 70 km: in the first segment, the spectral ordinates decay as 〈 tex – mathid = " IM 0001" 〉 r – 1.01 while in the second as 〈 tex – mathid = " IM 0002" 〉 r – 1.68 . Beyond 70 km, the attenuation decreases and the spectral amplitude attenuate as 〈 tex – mathid = " IM 0003" 〉 r – 0.64 . The quality factor Q ( f ) and the high-frequency attenuation parameter k , are 〈 tex – mathid = " IM 0004" 〉 Q ( f ) = 290 f 0.16 and k = 0.012 s, respectively, the latter being applied only for frequencies higher than 10 Hz. The source spectra are well described by 2 models, from which seismic moment and stress drops of 231 earthquakes are estimated. We calibrate a new regional relationship between seismic moment and local magnitude that improves the existing ones and extends the validity range to 3.0–5.8. We find a significant stress drop increase with seismic moment for events with M w larger than 3.75, with so-called scaling parameter  close to 1.5. We also observe that the overall offset of the stress-drop scaling is controlled by earthquake depth. We evaluate the performance of the proposed parametric models through the residual analysis of the Fourier spectra in the frequency range 0.5–25 Hz. The results show that the considered stress-drop scaling with magnitude and depth reduces, on average, the standard deviation by 18 per cent with respect to a constant stress-drop model. The overall quality of fit (standard deviation between 0.20 and 0.27, in the frequency range 1–20 Hz) indicates that the spectral model calibrated in this study can be used to predict ground motion in the L'Aquila region.

Description: In this study we derive a spectral model describing the source, propagation and site characteristics of S waves recorded in central Italy. To this end, we compile and analyse a high-quality data set composed of more than 9000 acceleration and velocity waveforms in the local magnitude ( M l ) range 3.0–5.8 recorded at epicentral distances smaller than 120 km. The data set spans the time period from 2008 January 1 to 2013 May 31, and includes also the 2009 L'Aquila (moment magnitude M w 6.1, M l = 5.8) sequence. This data set is suitable for the application of data-driven approaches to derive the empirical functions for source, attenuation and site terms. Therefore, we apply a non-parametric inversion scheme to the acceleration Fourier spectra of the S waves of 261 earthquakes recorded at 129 stations. In a second step, with the aim of defining spectral models suitable for the implementation in numerical simulation codes, we represent the obtained non-parametric source and propagation terms by fitting standard parametric models. The frequency-dependent attenuation with distance r shows a complex trend that we parametrize in terms of geometrical spreading, anelastic attenuation and high-frequency decay parameter k. The geometrical spreading term is described by a piecewise linear model with crossover distances at 10 and 70 km: in the first segment, the spectral ordinates decay as 〈 tex – mathid = " IM 0001" 〉 r – 1.01 while in the second as 〈 tex – mathid = " IM 0002" 〉 r – 1.68 . Beyond 70 km, the attenuation decreases and the spectral amplitude attenuate as 〈 tex – mathid = " IM 0003" 〉 r – 0.64 . The quality factor Q ( f ) and the high-frequency attenuation parameter k , are 〈 tex – mathid = " IM 0004" 〉 Q ( f ) = 290 f 0.16 and k = 0.012 s, respectively, the latter being applied only for frequencies higher than 10 Hz. The source spectra are well described by 2 models, from which seismic moment and stress drops of 231 earthquakes are estimated. We calibrate a new regional relationship between seismic moment and local magnitude that improves the existing ones and extends the validity range to 3.0–5.8. We find a significant stress drop increase with seismic moment for events with M w larger than 3.75, with so-called scaling parameter  close to 1.5. We also observe that the overall offset of the stress-drop scaling is controlled by earthquake depth. We evaluate the performance of the proposed parametric models through the residual analysis of the Fourier spectra in the frequency range 0.5–25 Hz. The results show that the considered stress-drop scaling with magnitude and depth reduces, on average, the standard deviation by 18 per cent with respect to a constant stress-drop model. The overall quality of fit (standard deviation between 0.20 and 0.27, in the frequency range 1–20 Hz) indicates that the spectral model calibrated in this study can be used to predict ground motion in the L'Aquila region.

Description: The goal of this article is to investigate the possibility of reducing the uncertainty of the ground motion predicted for a specific target area (Po Plain and northeastern Italy), by calibrating a set of ad hoc ground-motion prediction equations (GMPEs). The derived GMPEs account for peculiarities that are not generally considered by standard predictive models, such as (1) an attenuation rate dependent on distance ranges and geological domains; (2) enhancement of short-period spectral ordinates, due to the reflection of S waves at the Moho discontinuity; and (3) generation of surface waves inside an alluvial basin. The analyzed strong-motion dataset was compiled by selecting events in the 4.0–6.4 magnitude range, records with distances shorter than 200 km, and focal depths shallower than 30 km; the major contribution comes from the recent 2012 Emilia sequence (first mainshock, 20 May 2012 M w  6.1; second mainshock, 29 May 2015 M w  6.0). The GMPEs are derived for the geometrical mean of horizontal components of peak ground acceleration, peak ground velocity, and 5% damped spectral acceleration in the 0.04–4 s period range. The derived region-specific models led to a reduction of the hazard levels for several intensity measures, with respect to the values obtained by considering the reference Italian attenuation model ( Bindi et al. , 2011 ), as exemplified by the comparison of the hazard curves computed for two specific sites. Online Material: Database of Northern Italy (DBNI) flat-file and tables of northern Italy ground-motion prediction equations (GMPEs) (NI15) regression coefficients and variability components for use with Joyner–Boore and hypocentral distances.

Description: In this note, we derive an attenuation function for computing magnitude values equivalent to M w using strong-motion data. We analyze 106 earthquakes of the 1 April 2014 M w  8.1 Pisagua sequence, which occurred along the 1877 seismic gap in northern Chile. We considered both foreshocks and aftershocks with moment magnitude available from moment tensor inversion in the GEOFON bulletin and recorded by the Integrated Plate boundary Observatory Chile strong-motion network. The maximum peak displacement measured over the double integrated traces is used to construct the magnitude scale, following a nonparametric approach. A bootstrap analysis is performed to assess the uncertainty of the model parameters, and cross-validation tests are performed to proof the suitability of the derived model in predicting the M w in the analyzed area, with an uncertainty of 0.2 magnitude units. The derived scale is applied to an early aftershock, which occurred about 155 s after the mainshock, initially missed in bulletins published by rapid global earthquake monitoring agencies (e.g., National Earthquake Information Center and GEOFON), because its phase arrivals at regional/teleseismic distances mix with those of the mainshock and its later arrivals. The estimated magnitude equivalent to M w is 6.6±0.3, which rank this event as the second largest aftershock of the sequence, after the M w  7.6 earthquake that occurred on 3 April 2014.

Description: Since August 2016, central Italy has been struck by one of the most important seismic sequences ever recorded in the country. In this study, a strong-motion data set, consisting of nearly 10,000 waveforms, has been analyzed to gather insights about the main features of ground motion, in terms of regional variability, shaking intensity, and near-source effects. In particular, the shake maps from the three main events in the sequence have been calculated to evaluate the distribution of shaking at a regional scale, and a residual analysis has been performed, aimed at interpreting the strong-motion parameters as functions of source distance, azimuth, and local site conditions. Particular attention has been dedicated to near-source effects (i.e., hanging wall/footwall, forward-directivity, or fling-step effects). Finally, ground-motion intensities in the near-source area have been discussed with respect to the values used for structural design. In general, the areas of maximum shaking appear to reflect, primarily, rupture complexity on the finite faults. Large ground-motion variability is observed along the Apennine direction (northwest–southeast) that can be attributed to source-directivity effects, especially evident in the case of small-magnitude aftershocks. Amplifications are observed in correspondence to intramountain basins, fluvial valleys, and the loose deposits along the Adriatic coast. Near-source ground motions exhibit hanging-wall effects, forward-directivity pulses, and permanent displacement.

Description: A fundamental problem for site-specific ground-motion prediction, commonly required in seismic-hazard assessment, lies in the fact that ground-motion observations over long enough time periods are unavailable at the vast majority of sites. For this reason, most of the ground-motion prediction equations have been derived using observed data from multiple stations and seismic sources, and the standard deviation (sigma) is related to the statistics of the spatial variability of ground motion instead of temporal variability at a single site (ergodic assumption). In this paper, we explore the variability at single sites, decomposing sigma into different parts so that the various contributions to the variability can be identified and the standard deviation for empirical ground-motion prediction models quantified by removing the ergodic assumption. The analysis was conducted using three different data sets. Sigma obtained for Italy using the ergodic assumption is about 0.35log10 units ( Bindi, Pacor, et al. , 2011 ) and decreases to about 0.3 when single stations are considered (15% reduction). The values of single-station sigma obtained in this study for multiple-source data sets are rather stable, in the range 0.18–0.2log10 units, comparable to the findings of previous studies. The reduction of the epistemic uncertainty achieved through the restriction of the analysis to a particular seismic source leads to a sigma of about 0.25log10 units when the ergodic assumption is removed, suggesting that sigma at a particular site, due to a particular earthquake source, may reduce the sigma obtained for the Italian territory by Bindi, Pacor, et al. (2011) by about 30%. Online Material: Tables of ground-motion prediction equation coefficients, site terms, and event-corrected single-station standard deviations.