Understanding anomalies in Rayleigh phase velocities: Insight from slip dynamic modelling of crust-mantle interface

Accurate modelling of surface wave dispersion relations (i.e., phase velocity dependence of elastic wave on the frequency), is a useful tool for detecting shallow tele-seismic activities and precisely locating the event site. However, past accounts of seismic data from earthquake events often report anomalies in dispersion relations when compared with standard reference Earth models, at regional as well as at global scale. Although causes for these anomalies remain obscure, but factors like anisotropy, Moho depth, plate tectonics interactions could be responsible. Present study suggests an explanation for apparent anomalies in Rayleigh wave dispersion relation, by modelling the slip waves appearing due to in-plane steady state frictional sliding at the crust-mantle interface (representatively modelled as a finite elastic layer sliding over an infinite elastic half-space). For frictionless interface, such wave is known as generalized Rayleigh waves in literature. In presence of frictional interface, dispersion relations of slip waves agree very well with the reported field observations in low seismic frequency range (1-100 mHz). Overall, we conclude that signals often reported on seismograms as Rayleigh waves, could in-fact be the slip waves with phase velocities similar to Rayleigh waves; and the regional or the global anomalies reported in their dispersion relations could be attributed to the behavior of the frictional law applicable at the crust-mantle interface at regional scale.