Numerical simulation of high- and low-frequencyLg-wave propagation

Abstract

SUMMARY Lg waves are sometimes extinguished when traversing geological structures such as mountain ranges or grabens. Previous investigations have applied numerical, full-waveform simulations at relatively low frequencies (on the order of 1 Hz) to investigate the relationship of different structural models and the amplitude anomalies of the Lg waves. These studies often fail to link specific known structures to the amplitude anomalies. However, the observed waveforms also often contain data of significantly higher frequency than the full waveform simulations. We compare such low-frequency simulations computed with the boundary-integral-equation (BIE) technique with synthetic seismograms generated by dynamic ray tracing (DRT). Beginning with a plane-layered model of the crust in central France, we show that the general characteristics of the Lg wave train over distances ranging from 100 km to around 500 km can be modelled by computing a relatively small number of rays reflected between the free surface and the Moho. Matching the details of the waveforms is more difficult, but can be improved by the addition of reflections between other interfaces in the model. Considering a one-layer model of the crust with a step-like change in Moho depth that leads to a rapid decrease in crustal thickness, we show again that the BIE and DRT solutions are very similar in character. Significantly, both solutions predict that the Lg wave will not be extinguished on traversing the Moho step. Finally, we consider a more complete and detailed model of the Pyrenees. Though it is more difficult to match exactly the ray-and integral-equation solutions, the general trends continue to be the same, and the Lg waves are still predicted to cross the tectonic structures associated with the mountains. Therefore, it appears that observed amplitude changes must be caused by something other than the general structure of the Pyrenees, such as scattering by smaller scale features within the lower crust.