In earthquake engineering, “engineering bedrock” is regarded as a stiff material (i.e., rock or rock‐like geological formation) that is characterized by a shear‐wave velocity greater than a target value (e.g., $800m/s$⁠; 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, $800m/s$⁠. In the latter case, seismic and engineering bedrocks coincided. Our mapping will be useful for future site response assessments, numerical modeling of seismic‐wave propagation, dynamic ground response analyses, and site‐specific seismic hazard evaluation at the basin scale.

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., $1s<T<3s$⁠) 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, 1D modeling 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).

We present the Istituto Nazionale di Geofisica e Vulcanologia Strong‐Motion Data‐quality (ISMDq)—a new automatic system designed to check both continuous data stream and event strong‐motion waveforms before online publication. The main purpose of ISMDq is to ensure accurate ground‐motion data and derived products to be rapidly shared with monitoring authorities and the scientific community. ISMDq provides data‐quality reports within minutes of the occurrence of Italian earthquakes with magnitude ≥3.0 and includes a detailed daily picture describing the performance of the target strong‐motion networks. In this article, we describe and discuss the automatic procedures used by ISMDq to perform its data‐quality check. Before an earthquake, ISMDq evaluates the selected waveforms through the estimation of quality indexes employed to reject bad data and/or to group approved data into classes of quality that are useful to quantify the level of reliability. The quality indexes are estimated based on comparisons with the background ambient noise level performed both in the time and frequency domains. As a consequence, new high‐ and low‐noise reference levels are derived for the overall Italian strong‐motion network, for each station, and for groups of stations in the same soil categories of the Eurocode 8 (Eurocode 8 [EC8], 2003). In absence of earthquakes, 24 hr streaming of ambient noise recordings are analyzed at each station to set an empirical threshold on selected data metrics and data availability, with the goal to build a station quality archive, which is daily updated in a time span of six months. The ISMDq is accessible online (see Data and Resources) from August 2020, providing rapid open access to ∼10,000 high‐quality checked automatically processed strong‐motion waveforms and metadata, relative to more than 160 Italian earthquakes with magnitude in the 3.0–5.2 range. Comparisons between selected strong‐motion data automatically processed and then manually revised corroborate the reliability of the proposed procedures.

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.