Seismic waves in 3-D: from mantle asymmetries to reliable seismic hazard assessment

A global cross-section of the Earth parallel tothe tectonic equator (TE) path, the great circle representingthe equator of net lithosphere rotation, shows a differencein shear wave velocities between the western and easternflanks of the three major oceanic rift basins. The lowvelocity layer in the upper asthenosphere, at a depth rangeof 120 to 200 km, is assumed to represent the decouplingbetween the lithosphere and the underlying mantle. Alongthe TE-perturbed (TE-pert) path, a ubiquitous LVZ, about1, 000-km-wide and 100-km-thick, occurs in the asthenosphere. The existence of the TE-pert is a necessary prerequisite for the existence of a continuous global flowwithin the Earth. Ground-shaking scenarios were constructed using a scenario-based method for seismic hazardanalysis (NDSHA), using realistic and duly validatedsynthetic time series, and generating a data bank of severalthousands of seismograms that account for source, propagation, and site effects. Accordingly, with basic selforganized criticality concepts, NDSHA permits the integration of available information provided by the mostupdated seismological, geological, geophysical, andgeotechnical databases for the site of interest, as well asadvanced physical modeling techniques, to provide a reliable and robust background for the development of adesign basis for cultural heritage and civil infrastructures.Estimates of seismic hazard obtained using the NDSHAand standard probabilistic approaches are compared for theItalian territory, and a case-study is discussed. In order toenable a reliable estimation of the ground motion responseto an earthquake, three-dimensional velocity models haveto be considered, resulting in a new, very efficient, analytical procedure for computing the broadband seismicwave-field in a 3-D anelastic Earth model.