ALBERT

Description: In this study, microseism recordings from a near coast seismic station and concurrent significant sea wave heights ( $H_{\frac{1}{3}}$ ) are analysed to calibrate an empirical relation for predicting sea wave height in the Ligurian Sea. The study stems from the investigation of the damaging sea storms occurred in the Ligurian Sea between 2008 October and November. Analysing data collected in this time frame allows identification of two types of microseism signal, one associated to the local sea wave motion and one attributable to a remote source area. The former is dominated by frequencies greater than 0.2 Hz and the latter by frequencies between 0.07 and 0.14 Hz. Moreover, comparison of microseism spectrogram and significant sea wave heights reveals a strong correlation in that the spectral energy content of microseism results proportional to the sea wave height observed in the same time window. Hence, an extended data set including also observations from January to December 2011 is used to calibrate an empirical predictive relation for sea wave height whose functional form is a modified version of the classical definition of $H_{\frac{1}{3}}$ . By means of a Markov chain Monte Carlo algorithm we set up a procedure to investigate the inverse problem and to find a set of parameter values for predicting sea wave heights from microseism.

Description: The impact on probabilistic ground-motion hazard of different definitions of the horizontal component of ground shaking is examined. The scope is to highlight how such a minor detail within the complex computation chain of a probabilistic seismic-hazard assessment can play a crucial role on final results. This is achieved by comparing hazard maps produced for Italy considering different definitions of the ground-motion component at different spectral periods. In our exercise, special attention is paid to the treatment of the aleatory variability of ground motion (sigma) when one switches from one metric to another. The results show that differences in the definition of the horizontal component could imply differences in the hazard results as large as 40%. Online Material: Color maps of geographical distributions of hazard value ratios.

Description: We extend some estimates of the right-hand side of Hermite-Hadamard-type inequalities for functions whose second derivatives absolute values are P-convex. Applications to some special means are considered.

Description: 〈span〉〈div〉Abstract〈/div〉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., 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 seismic‐wave propagation, dynamic ground response analyses, and site‐specific seismic hazard evaluation at the basin scale.〈/span〉

Description: This study compares 12 hazard models based on dated and recent ground-motion prediction equations (GMPEs) to evaluate the improvement provided by new equations on probabilistic seismic-hazard assessments in Italy. To this end, a statistical procedure is applied to score the outcomes of each hazard model at 56 different accelerometric sites that have been operating for at least 25 years. This procedure, which calculates the likelihood of the outcomes of the hazard models relative to available observations, evaluates the performance of each model and, indirectly, the influence of the selected GMPEs in providing effective hazard estimates. We have found that older GMPEs tend to yield high-frequency ground-motion hazard values that are overconservative at shorter mean return periods and underconservative at longer ones. To identify the sources of the different behavior between older and more recent equations, the biasing of each GMPE is evaluated by comparing median predictions with observations available at two accelerometric sites where a relatively large number of ground motions from different earthquakes have been recorded and local soil conditions are well established. Results indicate that two decades of research on GMPEs have resulted in a significant reduction of bias with an improvement in the accuracy of predictions. Major improvements have been observed from 2008 to 2010. These may be related to the increased completeness of regression data sets and to an increased effectiveness of functional forms, which allow a better modeling of the physical process governing the propagation of ground motions. Since then, the GMPE bias has remained almost stable and no significant improvement in the performances of the relative hazard models has been observed. Our results also indicate that worldwide GMPEs applied to Italy are less effective at providing hazard results corroborated by observations.

Description: The paper presents an extensive review of topographic effects in seismology taking into account the knowledge of 40 yr of scientific literature. An overview of topographic effects based on experimental observations and numerical modelling is presented with the aim of highlighting meaning and causes of these phenomena as well as possible correlations between site response (fundamental frequency, amplification level) and geometrical (width and shape ratio of a relief) parameters. After a thorough summary of topographic effects, the paper focuses on five Italian sites whose seismic response is potentially affected by local morphology, as already evidenced by previous studies. In this study, seismic data recorded at these sites are analysed computing directional spectral ratios both in terms of horizontal to vertical spectral ratios (HVSRs) and, wherever possible, in terms of standard spectral ratios (SSRs). The analysis lead to the conclusion that wavefield tends to be polarized along a direction perpendicular to the main axis of a topographic irregularity, direction along which ground motion amplification is maximum. The final section of the article compares and contrasts different spectral ratio techniques in order to examine their effectiveness and reliability in detecting topographic effects. The examples discussed in the paper show that site responses based on HVSRs rather than SSR measurements could lead to misinterpretation of ground response results, both as concerns the definition of the site fundamental frequency and amplification level. Results and findings of this work will be used as starting point to discuss the influence of topographic effects on ground motion prediction equations and regulations for design. These topics will be discussed in the companion article.

Description: This study examines the role of topographic effects on the prediction of earthquake ground motion. Ground motion prediction equations (GMPEs) are mathematical models that estimate the shaking level induced by an earthquake as a function of several parameters, such as magnitude, source-to-site distance, style of faulting and ground type. However, little importance is given to the effects of topography, which, as known, may play a significant role on the level, duration and frequency content of ground motion. Ridges and crests are often lost inside the large number of sites considered in the definition of a GMPE. Hence, it is presumable that current GMPEs are unable to accurately predict the shaking level at the top of a relief. The present work, which follows the article of Massa et al. about topographic effects, aims at overcoming this limitation by amending an existing GMPE with an additional term to account for the effects of surface topography at a specific site. First, experimental ground motion values and ground motions predicted by the attenuation model of Bindi et al. for five case studies are compared and contrasted in order to quantify their discrepancy and to identify anomalous behaviours of the sites investigated. Secondly, for the site of Narni (Central Italy), amplification factors derived from experimental measurements and numerical analyses are compared and contrasted, pointing out their impact on probabilistic seismic hazard analysis and design norms. In particular, with reference to the Italian building code, our results have highlighted the inadequacy of the national provisions concerning the definition of the seismic load at top of ridges and crests, evidencing a significant underestimation of ground motion around the site resonance frequency.

Description: 〈span〉〈div〉Abstract〈/div〉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., 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 seismic‐wave propagation, dynamic ground response analyses, and site‐specific seismic hazard evaluation at the basin scale.〈/span〉

Description: This study deals with the impact of deep-soil discontinuities on ground-motion amplification in the Po alluvial basin (northern Italy), and consequently, on long-period probabilistic seismic-hazard analysis estimates. The focus is on Castelleone, where a seismic station of the Italian National Seismic Network has been deployed since 2009. To define a detailed shear-wave velocity ( V S ) profile, extensive active and passive geophysical surveys were carried out. The active measurements included seismic profiles examined through standard refraction techniques and multichannel analysis of surface waves. The passive methods included ambient vibration measurements in both single and array configurations. In particular, three microtremor arrays with increasing apertures were performed to capture the experimental dispersion curve down to 0.2 Hz. The horizontal-to-vertical spectral ratio (HVSR) showed two main peaks, at 0.17 and 0.70 Hz. A joint inversion of the experimental phase-velocity dispersion and HVSR curves was performed to obtain the V S profile. The data show two main discontinuities at ~160 and 1350 m in depth. According to the Italian and European seismic codes, the shallow V S discontinuity can be ascribed to the seismic bedrock (i.e., V S 〉800 m/s), whereas the deeper one is observed at the transition between Pliocene–Quaternary deposits and Miocene marls. Preliminary site-specific seismic-hazard analysis in terms of displacement response spectra for periods up to 10 s shows that neglecting the effects of the deeper discontinuity implies underestimation in hazard estimates of up to about 49% for mean return period (MRP) of 475 years and about 57% for MRP of 2475 years, with possible consequences regarding the design of very tall buildings and large bridges.