The Seismic Hazard Harmonization in Europe (SHARE) project, which began in June 2009, aims at establishing new standards for probabilistic seismic hazard assessment in the Euro-Mediterranean region. In this context, a logic tree for ground-motion prediction in Europe has been constructed. Ground-motion prediction equations (GMPEs) and weights have been determined so that the logic tree captures epistemic uncertainty in ground-motion prediction for six different tectonic regimes in Europe. Here we present the strategy that we adopted to build such a logic tree. This strategy has the particularity of combining two complementary and independent approaches: expert judgment and data testing. A set of six experts was asked to weight pre-selected GMPEs while the ability of these GMPEs to predict available data was evaluated with the method of Scherbaum et al. (Bull Seismol Soc Am 99:3234-3247, 2009). Results of both approaches were taken into account to commonly select the smallest set of GMPEs to capture the uncertainty in ground-motion prediction in Europe. For stable continental regions, two models, both from eastern North America, have been selected for shields, and three GMPEs from active shallow crustal regions have been added for continental crust. For subduction zones, four models, all non-European, have been chosen. Finally, for active shallow crustal regions, we selected four models, each of them from a different host region but only two of them were kept for long periods. In most cases, a common agreement has been also reached for the weights. In case of divergence, a sensitivity analysis of the weights on the seismic hazard has been conducted, showing that once the GMPEs have been selected, the associated set of weights has a smaller influence on the hazard

No abstract

The time-averaged shear-wave velocity in the upper 30 m of a site (V S30 ) is commonly used for ground-motion prediction. When measured velocities are unavailable, V S30 is estimated from proxy-based relationships developed for application on global or local scales. We describe the development of a local relationship for Greece, which begins with compilation of a profile database (PDB) from published sources and engineering reports. The PDB contains 314 sites; 238 have profile depths ≥ 30 m and 59 are within 100 m of accelerographs. We find existing relations for extrapolating a time-averaged velocity for depths less than 30 m to V S30 to overpredict V S30 . We present equations for these extrapolations.We then compile proxies for PDB sites, including terrain type, surface geology, and surface gradients at 30 and 3 arcsec resolution (from radar-derived digital elevation models [DEMs]). When checked against ground survey data, we find ground elevations from 3 arcsec DEMs to be more accurate relative to survey data than alternative 30, 9, and 1 arcsec DEMs. Drawing upon expert opinion, we develop geologic categories based on age, gradation, and depositional environment and assign such categories to PDB sites. We find an existing 30 arcsec gradient-based global model to be biased relative to local V S30 data for gradients >∼0:05 m=m. Bias relative to a California model is also found for four of the eight well-populated geomorphic categories, and new (local) values are provided. We find statistically significant effects of the 3 arcsec gradient on V S30 for Quaternary and Tertiary materials but no gradient effect for those from the Mesozoic. Among Quaternary sediments, Holocene, mapped Quaternary (age unspecified), and mixed/fine-gradation materials exhibit consistent V S30 -gradient trends, whereas Pleistocene and coarse-gradation sediments have faster velocities. For the study region, we recommend use of the modified terrain-and geology-based methods in combination for proxy-based V S30 estimation.

The inversion of surface-wave dispersion curve to derive shear-wave velocity profile is a very delicate process dealing with a nonunique problem, which is strongly dependent on the model space parameterization. When independent and reliable information is not available, the selection of most representative models within the ensemble produced by the inversion is often difficult. We implemented a strategy in the inversion of dispersion curves able to investigate the influence of the parameterization of the model space and to select a "best" class of models. We analyzed surface-wave dispersion curves measured at 14 European strong-motion sites within the NERIES ECProject. We focused on the inversion task exploring the model space by means of four distinct parameterization classes composed of layers progressively added over a half-space. The classes differ in the definition of the shear-wave velocity profile; we considered models with uniform velocity as well as models with increasing velocity with depth. At each site and for each model parameterization, we performed an extensive surfacewave inversion (200,100 models for five seeds) using the conditional neighborhood algorithm. We addressed the model evaluation following the corrected Akaike's information criterion (AICc) that combines the concept of misfit to the number of degrees of freedom of the system. The misfit was computed as least-squares estimation between theoretical and observed dispersion curve. The model complexity was accounted in a penalty term by AICc. By applying such inversion strategy on 14 strong-motion sites, we found that the best parameterization of the model space is mostly three to four layers over a half-space; where the shear-wave velocity of the uppermost layers can follow uniform or power-law dependence with depth. The shear-wave velocity profiles derived by inversion agree with shear-wave velocity profiles provided by borehole surveys at approximately 80% of the sites.

Using a recently completed database of uniformly processed strong-motion data recorded in Greece, we derive a ground-motion prediction model (GMPM) for horizontal-component peak ground velocity, peak ground acceleration, and 5% damped pseudoacceleration response spectra, at 105 periods ranging from 0.01 to 10 s. The equations were developed by modifying a global GMPM, to account for more rapid attenuation and weaker magnitude scaling in the Greek ground motions than in the global GMPM. Our GMPM is calibrated using the Greek data for distances up to 300 km, magnitudes from 4.0 to 7.0, and time-averaged 30 m shear-wave velocities from 150 to 1200  m/s. The GMPM has important attributes for hazard applications including magnitude scaling that extends the range of applicability to M 8.0 and nonlinear site response. These features are possible because they are well constrained by data in the global GMPM from which our model is derived. An interesting feature of the Greek data, also observed previously in studies of mid-magnitude events (6.1–6.5) in Italy, is that they are substantially overpredicted by the global GMPM, which may be a repeatable regional feature, but may also be influenced by soil–structure interaction. This bias is an important source of epistemic uncertainty that should be considered in hazard analysis.