GNGTS 2013 - Atti del 32° Convegno Nazionale

Effects of the rupture directivity on the Probabilistic Seismic Hazard Maps in the Southern Apennines, Italy E. Spagnuolo, A. Akinci, and A. Herrero Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italy Introduction. In planning the design of structures in a region of potential seismic activity, a specification of the earthquake ground motion “intensity” is strongly needed. The earthquake occurrence and its effects on ground shaking is described as a stochastic process and thus its analytical representation is based on a number of state variables defined over a known space. Those variables which are called explanatory variables are linked together by a continuous function, the ground motion predictive equation (GMPE). The GMPEs have a significant role in the standard hazard practice as the hazard maps are based on a probabilistic assessment of such an empirically derived prediction. However, despite the relevance of the GMPEs in capturing first-order effects of the ground motion variability (e.g. attenuation with distance, dependence on magnitude, modulation due to the faulting style) and the fact that their characterization has greatly improved within the NGA project (Power et al. , 2008) GMPEs are still incomplete; large azimuthal variations, ascribed to the complexity of the rupture propagation on a finite fault, are predicted and observed in the ground motion distribution (e.g., Benioff, 1955; Kasahara, 1960). Those spatial-temporal features of the ground motion variability generally lump together into the term “directivity effect” (Ben-Menahem, 1961). Several authors have introduced directivity in PSHA computations (Somerville et al. , 1997; Abrahamson 2000; Rowshandel et al. , 2006; Chiocciarelli and Iervolino, 2012 and references therein). In this work we follow the strategy proposed by Spagnuolo et al. (2012) who use the corrective factor for directivity proposed by Spudich and Chiou (2008). The advantage of the latter method is that the Spudich and Chou’s corrective factor is based on the physical meaning of rupture directivity that is: the energy radiated between the hypocenter and the closest point to the site is modulated as a function of the elapsing time; if this time is short the energy is time compressed, a directivity pulse is formed and the spectral amplitude is amplified. In order to retain this essential physics, Spudich and Chiou (2008) have developed a predictor for directivity by simplifying the computation of synthetic seismograms to an analytical expression. This analytical expression allows to make predictions even in absence of a direct empirical specification of the occurrence of a directivity effect. The correction for rupture propagation. The analytical model of Spudich andChiou (2008, hereafter SC2008) acts as a corrective factor - an additive term to the logarithmic expression of the ground motion intensity measure, i.e. spectral acceleration (SA) - in the GMPEs and its functional form is searched over both an empirical data-set and a database of synthetic simulations: the use of synthetic simulations guarantees a complete azimuthal coverage and, more generically, an efficient sampling that accounts for a statistically consistent combination of source parameters. SC2008 is based on a formalism derived from the isochrones theory (Bernard and Madariaga, 1984; Spudich and Frazer, 1984) - a high frequency representation of the seismic rupture where the source contribution at any given time and station is represented by a line on the fault plane called an isochrone. The isochrone velocity is directly proportional to the isochrone spacing and it is defined along the entire fault. In the two-dimensional case, the reciprocal of the iscrohone velocity, the isochrones slowness, is equal to the usual seismic directivity function. The finite fault radiation pattern is approximated by a single point source radiation pattern corresponding to the hypocenter. In summary, the SC2008 corrective factor accounts for the fraction of rupture surface lying between the hypocenter and the point on fault closest to the site (s), for the isochrone velocity ratio (c’) and for the radiation pattern (p) which spatially modulates the ground motion amplitudes. The SC2008 depends on the spectral period (T), ranging between [0.5-10] seconds. It can be synthesized as follows: f SC008 (T) = f M (M) f R (R) [a (T) + b (T) IDP] (1) 142 GNGTS 2013 S essione 2.1