Surface waves

Abstract

Surface waves propagate along the surface of the earth with exponentially decreasing energy with depth, with waves of longer period reaching greater depths. They contain most of the elastic energy generated by earthquakes at periods greater than 20 s and are dispersive. There are two types of surface waves, Rayleigh and Love waves, with different polarization properties. The fundamental mode surface waves are well separated from other energy arrivals in the time domain and provide the best constraints to date on continental scale and global scale structure. They play a particularly important role in constraining the upper mantle structure in the ocean basins where few seismic stations have been installed. While the dispersion of fundamental mode surface waves is a classical tool for the investigation of upper mantle structure, higher modes, or overtones provide constraints at transition zone depths (400–700 km) and deeper. They require more sophisticated analysis tools because they cannot be distinguished from each other readily on the seismogram. Waveform inversion approaches are increasingly favored to handle both fundamental mode and overtone interpretation, and hold increasing promise as numerical methods for the computation of the seismic wavefield in arbitrary 3D structures are being implemented, and can account for complex scattering and focusing effects accurately. In addition to isotropic shear velocity, surface waves provide constraints on the distribution of polarization and azimuthal anisotropy in the upper mantle, as well as on anelastic attenuation. When path effects have been corrected for, surface waves provide robust constraints on the source depth and radiation pattern, as expressed by its moment tensor. The earth’s continuous background noise consists primarily of surface waves, at least at periods longer than 5 s. Notable is the microseismic noise peak around 6–7 s. Most of this energy is generated in the oceans by nonlinear interactions involving wind-driven ocean waves, and the seafloor. Recently, a method based on the cross-correlation of noise records has gained popularity and has been applied successfully to resolve crust and uppermost mantle structure. It is particularly useful in seismically quiet regions.