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

A global cross-section of the Earth parallel to the tectonic equator (TE) path, the great circle representing the equator of net lithosphere rotation, shows a difference in shear wave velocities between the western and eastern flanks of the three major oceanic rift basins. The low-velocity layer in the upper asthenosphere, at a depth range of 120 to 200 km, is assumed to represent the decoupling between the lithosphere and the underlying mantle. Along the TE-perturbed (TE-pert) path, a ubiquitous LVZ, about 1,000-km-wide and 100-km-thick, occurs in the astheno-sphere. The existence of the TE-pert is a necessary pre-requisite for the existence of a continuous global flow within the Earth. Ground-shaking scenarios were con-structed using a scenario-based method for seismic hazard analysis (NDSHA), using realistic and duly validated synthetic time series, and generating a data bank of several thousands of seismograms that account for source, propa-gation, and site effects. Accordingly, with basic self-organized criticality concepts, NDSHA permits the inte-gration of available information provided by the most updated seismological, geological, geophysical, and geo-technical databases for the site of interest, as well as advanced physical modeling techniques, to provide a reli-able and robust background for the development of a design basis for cultural heritage and civil infrastructures. Estimates of seismic hazard obtained using the NDSHA and standard probabilistic approaches are compared for the Italian territory, and a case-study is discussed. In order to enable a reliable estimation of the ground motion response to an earthquake, three-dimensional velocity models have to be considered, resulting in a new, very efficient, ana-lytical procedure for computing the broadband seismic wave-field in a 3-D anelastic Earth model.