GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 2.1 on the infulence of ground motion predictive equationS on Probabilistic Seismic Hazard analysis, part 1: EFFECT OF horizontal GROUND SHAKING DEFINITION S. Barani 1 , D. Albarello 2 , D. Spallarossa 1 , M. Massa 3 1 Dipartimento di Scienze della Terra dell’Ambiente e della Vita, Università di Genova, Italy 2 Dipartimento di Scienze Fisiche della Terra e dell’Ambiente, Università di Siena, Italy 3 Istituto Nazionale di Geofisica e Vulcanologia, Milano, Italy Foreword and scope of work. Probabilistic Seismic Hazard (PSH) analysis aims at estimating the average annual exceedance rate relative to each possible (horizontal) ground motion threshold at a site. On this basis, the exceedance probability for the same threshold is computed for any fixed exposure time. Although this definition is widely known among seismologists and engineers, many neglect the meaning of the term “ground motion”, that is the impact of the definition of “horizontal component” on PSH estimates. As PSH results reflect a particular choice regarding the ground motion attenuation model, which in turn assumes a specific definition of horizontal component, hazard maps should clearly indicate what ground shaking definition is adopted. Although this may appear as a minor detail, at least at first glance, it may have a huge impact on risk mitigation strategies, urban planning, and building design. Private and public users must be aware of what kind of horizontal ground motion they are handling because differences in the definition of the considered horizontal component could imply differences in the hazard results as large as 20% or more. From another perspective, private and public users should demand that hazard results reflect a specific definition of horizontal component. This work wants to highlight the effect produced on the hazard by different definitions of the horizontal component considered in seismic hazard assessment. Given the tangible lack of awareness concerning this issue in both the engineering and seismological communities, the purpose of this work is to open a discussion aimed at defining what measure of the horizontal ground motion should be adopted for future seismic hazard assessments in Italy. Definition of horizontal component and impact on the hazard. While older ground motion predictive equations (GMPEs), such as those by Sabetta and Pugliese (1996) andAmbraseys et al. (1996), estimate the larger horizontal component (i.e., at each period the larger spectral ordinate of the x and y components is chosen; hereinafter ENV xy ), most recent GMPEs (e.g., Cauzzi and Faccioli, 2008; Akkar and Bommer, 2010; Bindi et al. , 2011, 2013; Akkar et al. , 2014) consider the geometric mean of the response spectra relative to the x and y components (hereinafter GM xy ). The former definition has larger variability than the latter since it takes into account the variability between the two orthogonal horizontal components (Ornthammarath et al. , 2014). However, the geometric mean exhibits two disadvantages. First, it depends on the orientation of the sensors as installed in the field. Second, near a nodal point or plane, one of the horizontal components can assume near-zero values. This would lead to a geometric mean which is near zero (e.g., Ripperger et al. , 2008; Dalguer et al. , 2012). To overcome these limitations, Boore et al. (2006) proposed the use of the median value of geometric mean spectral accelerations computed over all possible orientations for each response period (GMRotD50). An alternative definition, which is adopted by many GMPEs (e.g., Boore and Atkinson, 2008), is the one called GMRotI50 (Boore et al. , 2006). This definition is an approximation of GMRotD50, with a constant orientation axis for all periods. Specifically, GMRotI50 is defined as the geometric mean of the spectral accelerations computed along the direction that minimizes the sum of differences between GMRotI50 and GMRotD50 over the range of periods considered. More recently, the definition of RotD50 (i.e., median single-component horizontal ground-motion across all non-redundant azimuths) has been proposed (Boore, 2010). This definition is adopted by the NGA2 GMPE by Boore et al. (2013) and like GMRotD50, GMRotI50 and RotD50 is independent of the orientation of sensors. Moreover, compatibly with the definition of the