Azimuthal distortion of the seismic focal sphere: application to earthquakes in subduction

Abstract

Abstract New broadband seismographs enable us to analyse in more detail mantle waves perturbed by the complexity of the medium around the source. We propose to apply ray tracing for complex media around the source and more efficient ray tracing for a spherically symmetric Earth through the mantle up to the stations. We show on synthetic examples that the complexity around the source may indeed modify considerably the seismograms. We analyse theoretical teleseismic waveforms and relate modulation of the amplitude to the medium's structure around the source. For the example of the Romanian earthquake of 30 May 1990, we analyse three seismograms which were not modelled correctly with a spherically symmetric Earth model. By adding some near-surface interfaces, we were able to improve the fit between observed and computed seismograms. The deduced hypothetical model, which may not be the only possible model, is in agreement with proposed published models in the area.