Albarello, Dario

We present a probabilistic seismic hazard analysis (PSHA) for the entire Po Plain sedimentary basin (Italy)—one of the widest Quaternary alluvial basins of Europe, to evaluate the impact of site-response characterization on hazard estimates. A large-scale application of approach 3 of the U.S. Nuclear Regulatory Commission (NRC) to include seismic amplification in the hazard is presented. Both 1D amplification related to stratigraphic conditions and 3D amplification due to basin effects are considered with the associated uncertainties, and their impact on the hazard is analyzed through a sensitivity analysis. Whereas 3D basin effects are considered through the application of an empirical, spatial invariant correction term, 1D amplification was estimated throughout the study area by means of dynamic (equivalent linear) ground-response analysis. To separate aleatory variabilities and epistemic uncertainties related to site response, a partially nonergodic approach is used. The results provide a finer picture of the actual seismic hazard, highlighting those areas where the ground motion is affected by amplification effects due to local or regional geological features. We found that, for a return period of 475 yr, neglecting basin effects produces a 30% underestimation of the seismic hazard in the long-period ( > 1 s) range. Moreover, with reference to the hazard model adopted, such effects are responsible for most of the epistemic uncertainty (up to 80%) in the results. Therefore, such effects deserve special attention in future research related to PSHA in the Po Plain sedimentary basin.

A methodology to detect local incompleteness of macroseismic intensity data at the local scale is presented. In particular, the probability that undocumented effects actually occurred at a site is determined by considering intensity prediction equations (in their probabilistic form) integrated by observations relative to known events documented at surrounding sites. The outcomes of this analysis can be used to investigate how representative and known the seismic histories of localities are (i.e., the list of documented effects through time). The proposed approach is applied to the Italian area. The analysis shows that, at most of the considered sites, the effects of intensity ≥ 6 should most probably have occurred at least once, but they are not contained in the current version of the Italian macroseismic databases. In a few cases, instead, the lack of data may concern higher intensity levels (i.e., ≥ 8). The geographical distribution of potentially lost information reflects the heterogeneity of the seismic activity over the Italian territory.

The geographic distribution of earthquake effects quantified in terms of macroseismic intensities, the so-called macroseismic field, provides basic information for several applications including source characterization of pre-instrumental earthquakes and risk analysis. Macroseismic fields of past earthquakes as inferred from historical documentation may present spatial gaps, due to the incompleteness of the available information. We present a probabilistic approach aimed at integrating incomplete intensity distributions by considering the Bayesian combination of estimates provided by intensity prediction equations (IPEs) and data documented at nearby localities, accounting for the relevant uncertainties and the discrete and ordinal nature of intensity values. The performance of the proposed methodology is tested at 28 Italian localities with long and rich seismic histories and for two well-known strong earthquakes (i.e., 1980 southern Italy and 2009 central Italy events). A possible application of the approach is also illustrated relative to a 16th-century earthquake in the northern Apennines.

This study investigates and quantifies the influence of the shallower deposits (down to few hundreds of meters) of the Po Plain sedimentary basin (northern Italy) on the long-period component (i.e., 1 s < T < 3 s) of seismic ground motion, in which amplification effects due to the soft sediments above seismic bedrock were observed. A new seismostratigraphic model of the shallow deposits of the entire basin is provided with an unprecedented detail by taking advantage of recently acquired geophysical data. The seismostratigraphic model is used to simulate the ground motion amplification in the Po Plain by means of extensive 1D ground response analysis. Results are compared with seismic observations available at a number of sites equipped with borehole seismic stations, where earthquakes have been recorded both at the surface and at the seismic bedrock depth. Despite the general agreement with observations concerning the seismic resonance frequencies, our model may fail in capturing the amplitude of the actual seismic amplification of the basin in the long-period range. We observe that 3D basin effects related to surface waves generated at the edge of the basin may play a significant role in those zones where seismic hazard is controlled by distant sources. In these cases, 1Dmodeling leads to average underestimations of 30%, up to a maximum of 60%. The amplification functions need to be corrected for a basin-effects correction term, which in this case is provided by the ground-motion prediction equation of the study area. The corrected amplification functions agree with the empirical observations, overcoming the uneven distribution of the recording stations in strong-motion datasets. These results should be taken into account in future seismic microzonation studies in the Po Plain area, where the 1D approach is commonly adopted in ground response analyses, and in site-specific seismic hazard assessments aimed at the design of structures that are sensitive to the long-period component of seismic ground motion (e.g., long-span bridges and tall buildings).

This paper presents an overview of the geophysical activities for the seismic microzonation of 138 municipalities belonging to four Italian regions (Abruzzo, Lazio, Marche and Umbria) that were severely damaged by the seismic sequence of Central Italy (August 2016–January 2017). This study is the result of a collaborative effort between research Institutions and professional geologists with the support of local Administrations and the Italian Civil Protection Department and sets an unprecedented large-scale example of geophysical investigations supporting detailed seismic microzonation studies. This manuscript presents the methodological approach adopted for the geophysical activities, including the technical protocols and procedures, the best practices, the final products and the results supporting a detailed microzonation study of III level. The first step of the study was the collection and critical review of all available geophysical and geological information for planning the new geophysical surveys (specifically their type and location), in order to assess the subsoil geometry and the seismic characterization of the areas under investigation. Integration with the newly acquired geophysical data allowed the identification of zones with homogeneous local seismic hazard as well as the reference seismo-stratigraphy for each area, defining for each geological unit the ranges of the relevant properties in seismic amplification studies: layering and thicknesses, density, P-wave and S-wave seismic velocity. We also present a few representative case studies illustrating the geophysical investigation for different geomorphological situations. These examples, together with the findings of the entire project, are discussed to point out the strength points and the criticalities, as well as the necessary requirements in the application of geophysical methods to detailed microzonation studies.

We present a methodology for the selection of accelerometric time histories as input for dynamic response analyses over vast areas. The method is primarily intended for seismic microzonation studies and regional probabilistic seismic hazard assessments that account for site effects. It is also suitable for structural response analyses if one would like to use a fixed set of ground motion records for analyzing multiple structures with different (or unknown) periods. The proposed procedure takes advantage of unsupervised machine learning techniques to identify zones (i.e., groups of sites) with homogeneous seismic hazard, for which the same set of earthquake recordings can be reasonably used in the numerical simulations. The procedure consists of three steps: (1) data-driven cluster analysis to identify groups of sites with comparable seismic hazard levels for a specified mean return period (MRP); (2) for each zone, definition of a single, reference uniform hazard spectrum (UHS) corresponding to the MRP of interest; (3) selection of a set of accelerometric recordings that are consistent with the magnitude-distance scenarios contributing to the hazard of each zone, and meet the spectrum-compatibility requirement with respect to the reference UHS. An application of the procedure in the Po Plain (Northern Italy) is described in detail.

It is widely recognized that a significant proportion of the variability of earthquake ground motion is related to local geological conditions, which can modify the ground-motion amplitude, duration, and frequency. In particular, several investigations of deep sedimentary basins have highlighted that thick and soft sediments can strongly amplify long-period ground motion (> 1 s) (e.g., Anderson et al., 1986; Joyner, 2000; Milana et al., 2013; Massa and Augliera, 2013). The characteristics of long-period ground motion have gained growing interest in the civil engineering community because of the increase in the number of large-scale structures (i.e. tall buildings, long-span bridges etc.). In the framework of the site response estimation, both the S-wave velocity profile and the thickness of the soft sedimentary cover are considered fundamental parameters. At this regard, in a deep sedimentary basin at least three different kind of bedrock can be identified: geologic, engineering and seismic bedrock. The definition of bedrock is critical since it may provide very different reference conditions. The ‘geologic bedrock’ can be identified in correspondence with rock formations, whereas the ‘engineering bedrock’ can be identified based on the shear-wave velocity (Vs) value indicated in the current seismic code. Both the European and Italian seismic codes have defined the Vs transition from soft to stiff soil or rock (i.e., soil category A) at 800 m/s (European Committee for Standardization, 2004; Ministero delle Infrastrutture e dei Trasporti, 2018). In this sense, the Vs threshold of 800 m/s marks the top of engineering bedrock. However, this is a conventional value and it might not correspond to significant variation in the mechanical properties of the subsoil materials. On the other hand, the seismic bedrock is defined by a marked seismic impedance contrast between soft and hard soils (or rock), which results in a considerable ground motion amplification at the surface. In deep and wide sedimentary basins, geologic, engineering and seismic bedrocks do not always coincide, and the threshold of 800 m/s indicated in the current seismic codes (i.e., engineering bedrock) might not be significant for site amplification if does not also correspond to a seismic impedance contrast. In this sense, Mascandola et al. (2019) identifies the seismic bedrock of the Po Plain - one of the deepest and widest alluvial basin worldwide where several cities and critical facilities are present - in correspondence with a marked increase in the mechanical properties of the subsoil materials, which produces a measurable resonance effect at the surface in the medium-to-long-period range (i.e., 3.33 -1 s, that is 0.3-1 Hz). This corresponds to a marked seismic impedance contrast where the shear-wave velocity approaches, or exceeds, 800 m/s. In detail, to map the seismic bedrock depth we rely on an extensive collection of both existing and newly acquired ambient vibration measurements, with the aim of defining the soil resonance frequencies and the shear-wave velocity gradients within the soft sediments above seismic bedrock. Based on the collected data, an empirical regression model that relates the thickness of the soil deposits above the seismic bedrock to their resonant frequency is defined and applied to map the seismic bedrock depth in the Po Plain area. The resultant seismic bedrock map is correlated with the main unconformities recognized inside the Quaternary succession (Regione Emilia-Romagna,ENI–AGIP, 1998; Regione Lombardia, Eni Divisione Agip,2002). With the aim of providing a long period soil amplification model for the Po Plain, a regional shear-wave velocity model of soft sediments above seismic bedrock is performed through the interpolation of 54 S-wave velocity profiles selected after a quality check on the available data. The velocity gradients highlight two different zones inside the study area: one at Northwest and another at East-Southeast with higher and lower velocity gradients respectively. To compute the soil amplification functions, the velocity model is discretized into a 0.1° x 0.1° grid (~ 8 km x 11 km). For each grid node, a 1D soil model is defined, and a numerical ground response analysis is carried out to compute site amplification functions. The numerical model is verified at those sites with borehole seismic stations, where recordings of the same earthquake are available both at the surface and bedrock depth. Finally, with the aim of determining the influence of the deep geophysical discontinuities on the long-period hazard up to 3 s, the probabilistic seismic hazard assessment is computed at regional scale with the partially non-ergodic approach (e.g., Rodriguez-Marek et al., 2014; Kotha et al., 2017), and considering the most updated scientific improvements in seismogenetic zonation and ground motion prediction equations. The results will be compared with conventional PSHA estimations that accounts for site effects through the application of frequency-independent soil factors derived from seismic codes (European Committee for Standardization, 2004; Ministero delle Infrastrutture e dei Trasporti, 2018), or by adopting ground motion prediction equations (GMPEs) for specific ground types (Barani and Spallarossa, 2017).

The aim of this study is to provide a seismo-stratigraphic model of the Po Plain sedimentary basin (Northern Italy), to be implemented in soil hazard studies at regional scale. The proposed model characterizes the subsoil up to the seismic bedrock depth. Mascandola et al. (2018) identifies the seismic bedrock of the Po Plain in correspondence with a marked increase in the mechanical properties of the subsoil materials, which produces a measurable resonance effect at the surface in the medium-to-long-period range. To map the seismic bedrock depth we relies on an extensive collection of both existing and newly acquired ambient vibration measurements, with the aim of defining the soil resonance frequencies and the shear-wave velocity gradients within the soft sediments above seismic bedrock. Based on the collected data, an empirical regression model that relates the thickness of the soil deposits above the seismic bedrock to their resonant frequency is defined and applied to map the seismic bedrock depth in the Po Plain area. The resultant seismic bedrock map is correlated with depth of the main unconformities recognized inside the Quaternary succession (Regione Emilia-Romagna,ENI–AGIP, 1998; Regione Lombardia, Eni Divisione Agip,2002). The shear-wave velocity model above seismic bedrock is derived through the interpolation of 51 S-wave velocity profiles selected after a quality check on the available data. The velocity gradients highlights two different zones inside the study area: one at Northwest and another at East-Southeast with higher and lower velocity gradients respectively. To compute the soil amplification functions, the velocity model is discretized into a grid. For each grid node, a 1D soil model is defined and a numerical ground response analysis is carried out. The gridded soil amplification model is checked at those sites with both borehole and surface seismic sensors by comparing the theoretical and empirical soil amplification functions. These results will be included in regional seismic hazard studies, to account for soil amplification in seismic hazard estimates.

As it is well known, the severity and frequency content of the ground shaking at a site are significantly dependent on the soil characteristics and local geomorphological features (e.g., Stone et al. 1987; Massa et al. 2014). It follows that neglecting site response may result in a severe underestimation of the local ground motion hazard. Therefore, probabilistic seismic hazard analysis (PSHA) based on the assumptions of level ground and exposed bedrock defines only a basic level for the definition of the expected ground motion. Besides studies for critical facilities, where the integration of site-effects into a PSHA has become a standard practice (e.g., Abrahamson et al. 2004; Rodriguez-Marek et al. 2014), detailed hazard mapping inclusive of site-effects is nowadays possible in many regions of the world where extensive seismic microzonation and/or scientific studies have been carrying out, leading to large-scale evaluations of seismic amplification effects. With the aim of performing a soil hazard analysis in the Po Plain sedimentary basin (Italy), one of the deepest and widest alluvial basin worldwide where several cities and critical facilities are present, we present the results from pseudo-3-D modelling of soil amplification, computed using a new shear-wave velocity model for the study area. The proposed model characterizes the subsoil up to the seismic bedrock depth. Mascandola et al. (2018) identifies the seismic bedrock of the Po Plain in correspondence with a marked increase in the mechanical properties of the subsoil materials, which produces a measurable resonance effect at the surface in the medium-to-long-period range. This corresponds to a marked seismic impedance contrast where the shear-wave velocity approaches, or exceeds, 800 m/s. The pseudo-3-D shear-wave velocity model is derived through the interpolation of several S-wave velocity profiles obtained from joint 1D inversion of H/V and array data. Locally, the model shows a good agreement with the existing shear-wave velocity profiles. To account for soil amplification in regional PSHA, the model was discretized into a grid. For each grid node, a 1D soil model is defined and a numerical ground response analysis is carried out to compute site amplification functions. The regional soil hazard is computed by coupling the hazard curves on rock with the gridded amplification functions (e.g., Bazzurro and Cornell, 2004; Barani and Spallarossa, 2017). To avoid double counting of uncertainties related to site response, a partially nonergodic approach will be used (e.g., Rodriguez-Marek et al., 2014) and to propagate the epistemic uncertainty of Vs through the hazard, a multi-SAF approach (Barani and Spallarossa, 2017) will be adopted by random sampling of the soil column. The results will be compared with conventional PSHA estimations that accounts for site effects through the application of frequency-independent soil factors derived from seismic codes (European Committee for Standardization, 2004; Ministero delle Infrastrutture e dei Trasporti, 2018), or by adopting ground motion prediction equations (GMPEs) for specific ground types.

In earthquake engineering, “engineering bedrock” is regarded as a stiff material (i.e., rock or rock-like geological formation) that is characterized by a shearwave velocity greater than a target value (e.g., 800 m=s; current Italian and European seismic codes). In the case of deep basins, the identification of engineering bedrock is problematic, because it can lie well below the penetration depth of most common prospecting methods (i.e., a few tens of meters). Moreover, the depth of engineering bedrock might not represent an effective proxy of the sedimentary thickness responsible for site amplification. The Po Plain sedimentary basin (northern Italy) is one of the deepest and widest worldwide, and it presents such problems. The aim of this work is to estimate the sedimentary thickness responsible for ground-motion amplification at medium and long periods in the Po Plain. Passive seismic prospecting methods based on ambient-vibration measurements using single-station and array configurations were considered to map “seismic bedrock” depth. This corresponds to a marked seismic impedance contrast where the shear-wave velocity approached, or exceeded, 800 m=s. In the latter case, seismic and engineering bedrocks coincided. Our mapping will be useful for future site response assessments, numerical modeling of seismicwave propagation, dynamic ground response analyses, and site-specific seismic hazard evaluation at the basin scale.