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 lowvelocity 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 asthenosphere.The existence of the TE-pert is a necessary prerequisite for the existence of a continuous global flow within the Earth.Ground-shaking scenarios were constructed 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,propagation,and site effects.Accordingly,with basic selforganized criticality concepts,NDSHA permits the integration of available information provided by the most updated seismological,geological,geophysical,and geotechnical databases for the site of interest,as well as advanced physical modeling techniques,to provide a reliable 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,analytical procedure for computing the broadband seismic wave-field in a 3-D anelastic Earth model.