Stacking investigations of the dispersion of higher order mantle Rayleigh waves and normal modes

Using a number of stacking techniques (including non-linear stacking), we have systematically investigated, both in the travelling wave and normal mode formalisms, the dispersion of the third, fourth and fifth branches of Rayleigh overtones, which travel with identical group velocities at similar frequencies, and are a prominent part of the horizontal, away from the source, components of long-period seismograms from major earthquakes. Using data from six major events recorded on the Global Digital Seismological Network, we have extracted more than 100 normal modes from these three branches in the period range 82-160 s; our results are generally in excellent agreement with values computed from available models of the Earth, and also with previously published values for a well-separated part of the third branch.