Complexity of Short Period P Seismograms: What Does Scattering Contribute?

In this report we review the work that has been done on the complexity of the P coda. The most difficult complex seismograms to explain turn out to be those from deep earthquakes with simple source functions and explosions in which the coda is made up of arrivals with high apparent surface speeds. Such seismograms seem to be best explained on the weak signal hypothesis; the seismograms look complex not because the arrivals in the coda are large but because, for the magnitude of the source, the first arrival is small. In order to explain complex explosion seismograms Douglas et al. suggest that direct P has been attenuated by passing through a region of low Q which has been missed by the arrivals in the coda. In order to explain complex seismograms from some deep earthquakes, Barley suggests that these are recorded when the direct P path at the source lies close to a node; P is then small for the magnitude of the source and the later (scattered) arrivals appear to be large by comparison.