P Signal Complexity Re-examined

The early recordings made by short-period seismometer arrays showed clearly that P signals differ very widely in their complexity. Many attempts have been made to explain this complexity and here evidence is presented to show that greater absorption of the direct signal relative to the later arrivals may be the principal explanation of complexity at least for explosion signals. Deconvolution and synthesis of explosion signals is used to demonstrate that multipathing also contributes to complexity. From a comparison of explosion signals from sites in shield areas with those from sites in or near fold belts it is suggested that fold belts are underlain by regions of much lower Q than shield areas. Thus methods of discriminating between earthquake and explosion signals that are based on the observed differences between explosion signals from sites in shield areas and earthquake signals from foci in fold belts, may not then be measuring differences in the two sources but rather differences in Q in the upper mantle in the source regions.