Lg group velocities are slower for very shallow events than for deeper crustal events in both southern California and Israel. An Lg group velocity depth discriminant applied to data below 1 Hz was proposed and then abandoned two decades ago, but a more precise measure of the energy distribution within Lg leads to the conclusion that higher frequency Lg group velocity may identify event depth. Because of its practical importance to nuclear monitoring and its implications for the generation and propagation of Lg, we quantify and model the delay of shallow-event Lg.
We consider two mechanisms. First, shallower, slower modes are more strongly excited by shallow events, while deeper events excite faster modes. Second, Rg-to-Lg scattering for shallow events could introduce a time delay into Lg relative to Lg generated directly by the source. Further, since Rg propagates close to the surface, Rg scattering would also preferentially scatter into the shallower, slower modes. The depth dependence of modal excitation is therefore important for either mechanism, but Rg scattering could increase the time delays.
Mode and finite-difference simulations demonstrate the feasibility of both hypotheses. We consider the implications of related observations for the source of the Lg delay, including predictions and observations of the frequency dependence of Lg delays.
We conclude that Lg group velocity variation with event depth can be explained by preferential modal excitation and may have the potential to identify very shallow events and events too deep to be man-made. Practical application of the procedure will require regionalization to account for variations in Earth structure.