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

Abstract

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.

Ocean expansion: The role of detachment faults

“…Plate growth rate is due to the difference between VA and VB. The plate separation increases at the half spreading rate hS = (VA−VB)/2, Vr being always faster than hS. Thickness of the oceanic lithosphere, estimated from geophysical data and confirmed with HP‐HT experimental results on mantle assemblages, is 100 km westwards far from the ridge and 80 km eastwards, and between 20 to 40 km depth at the ridge axis (Chalot‐Prat et al, 2010, 2013; Falloon, Danyushevsky, et al, 2007; Falloon, Green, & Danyushevsky, 2007; Green et al, 2014; Green & Falloon, 2015; Panza & Romanelli, 2014). Experimental results (Green & Falloon, 2015) demonstrate that MORB (Mid‐Oceanic Ridge Basalts) parental melts segregate from residual peridotite between 55 and 45 km depth (down to 65 km; up to 30 km) below the LAB (Lithosphere‐Asthenosphere Boundary) of the thinned lithosphere area.…”

Westward migration of oceanic ridges and related asymmetric upper mantle differentiation

“…Moreover, seismic images from Panza and Romanelli (2014) show an asymmetric shape of the LVZ, thicker and more elongated westward (2/3 in volume) than eastward (1/3 in volume) of ridge axis in all oceans. This spectacular and permanent asymmetry of the LVZ shape raises the question not only of the origin of partial melting, but also of its much greater development below the western part of the oceans.…”

“…From the top to the bottom of this layer, seismic waves gradually increase their speed with respect to the LVZ. Vs is much higher than in the upper asthenosphere and more or less similar to that of the lithosphere (Panza and Romanelli 2014). Its mineralogical and chemical composition is inferred from experimental petrology on MORB genesis (Green et al, 2014;Green, 2015), being the N-MORB mantle source located below 230 km.…”

“…As shown by Doglioni et al (2003), observations on the bathymetry of ridge flanks (shallower eastward) of Pacific and Atlantic oceans and on variable rates of oceanic spreading (Muller et al, 1997;Mallows and Searle, 2012) contradict the concept of full symmetry. Panza et al (2010) and Panza and Romanelli (2014) provided evidence for asymmetry in both shear wave velocity and thickness within mantle lithosphere and asthenosphere when comparing the two sides of most spreading ridges. In general, the old (>60 Ma) western lithospheric plates have a faster shear wave velocity and are thicker (≈100 km versus ≈80 km in the eastern flank).…”

“…In general, the old (>60 Ma) western lithospheric plates have a faster shear wave velocity and are thicker (≈100 km versus ≈80 km in the eastern flank). Seismic tomography (Thybo, 2006;Panza et al, 2010;Schmerr, 2012;Panza and Romanelli, 2014) provides robust evidence of an asymmetric LVZ at depths from about 80-100 km down to 180-225 km. The average Vs within the western LVZ is slower than the eastern one.…”