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.

Probabilistic seismic hazard analysis is a process that integrates over\ud aleatory uncertainties (e.g., future earthquake locations and magnitudes) to calculate\ud the mean annual rate of exceedance (MRE) of given ground-motion parameter values\ud at a site. These rates reflect the contributions of all the sources whose seismic activity is\ud deemed to affect the hazard at that site. Seismic hazard disaggregation provides insights\ud into the earthquake scenarios driving the hazard at a given ground-motion level. This\ud work presents the disaggregation at each grid point of the Italian rock ground-motion\ud hazard maps developed by Gruppo di Lavoro MPS (2004), Meletti and Montaldo\ud (2007), and Montaldo and Meletti (2007). Disaggregation is used here to compute\ud the contributions to the MRE of peak ground horizontal acceleration (PGA) and\ud 5%-damped 0.2, 1.0, and 2.0 sec spectral acceleration values corresponding to different\ud mean return periods (MRPs of 475 and 2475 yr) from different scenarios. These sce-\ud narios are characterized by bins of magnitude, M, source-to-site distance, R, and\ud number, ε, of standard deviations that the ground-motion parameter is away from its\ud median value for that M R pair as estimated by a prediction equation. Maps showing\ud the geographical distribution of the mean and modal values of M, R, and ε are presented\ud for the first time for all of Italy. Complete joint M–R–ε distributions are also presented\ud for selected cities. Except for sites where the earthquake activity is characterized by\ud sporadic low-magnitude events, the hazard is generally dominated by local seismicity.\ud Moreover, as expected, the MRE of long-period spectral accelerations is generally con-\ud trolled by large magnitude earthquakes at long distances while smaller events at shorter\ud distances dominate the PGA and short-period spectral acceleration hazard. Finally, for a\ud given site, as the MRP increases the dominant earthquakes tend to become larger and to\ud occur closer to the site investigated

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.

The scope of this work is to examine the influence of the uncertainty in soil modeling on numerical ground response estimates through a comprehensive sensitivity analysis. This allows identification of those parameters with the largest effect on both soil amplification (quantified here by a frequency‐independent factor, F _a ) and fundamental frequency, f ₀ . Although extensively examined in previous articles, the effect of the input motion is also analyzed. The uncertainty affecting the shear wave velocity ( V _S ) profile was found to be the factor contributing most to the uncertainty of both F _a and f ₀ . The soil thickness was also found to play a key role, particularly in those cases where bedrock depth is unknown or largely uncertain. Although of secondary importance compared to the effect of V _S on F _a and f ₀ , the influence of the input motion cannot be neglected; rather it becomes predominant with regard to the uncertainty affecting frequency‐dependent shaking parameters.

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.

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.