Are long period body wave coda caused by lateral heterogeneity? Geophys

Data from two broad-band arrays in western Europe (NARS and GRF) are used to study the character of long-period body wave coda. The events studied are at epicentral distances of 40 " to 60", in the Hindu Kush region and on the Mid-Atlantic Ridge, sampling the upper mantle to a depth of about lo00 km. The periods studied are 5-50 s. The long-period coda of P at GRF (interstation distance about 10 km) are strongly coherent, whereas the long-period coda of PP-, PPP- and S-waves are incoherent. This indicates that the latter coda consists of scattered waves. To investigate the nature of the scattering process, the data of GRF are analysed for slowness and azimuth variations in the coda intervals. A new beamforming algorithm is presented to deal with the low frequences and relatively short time intervals. The method is based on Backus-Gilbert inverse theory. The results show that the incoherent long-period coda intervals almost entirely consist of surface waves; these waves are scattered from the preceding body waves. Some calculations with linearized theory for body wave to Rayleigh wave conversion at topography at the surface or at the Moho show that realistic scatterers can account for th