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

Abstract

SUMMARY 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 the observed (constant) coda level. The beamforming results show that the phases in the P coda all arrive along the great circle. As scattering calculations point out that body wave to body wave scattering is inefficient, it is concluded that the long-period P coda does not contain a significant amount of scattered energy. Synthetic seismograms obtained with the reflectivity method show that spherically symmetric upper mantle models can explain these coda waves. For events in the Hindu Kush region, an upper mantle with a thick lid overlying a pronounced low-velocity zone (LVZ) is necessary to explain the character of the P coda at GRF. Such an upper mantle agrees with previous studies of similar great circle paths. The strong coherence of the P coda at GRF is lost on the scale of NARS (station separation about 200 km). This suggests lateral variations in the upper mantle at a scale of about 200 km. It appears from previous studies of the upper mantle under Europe that these variations must be sought in the LVZ. It is shown that the long-period P coda is sensitive to variations in the P velocity structure of the LVZ. This suggests the P coda (i.e. PdP phases) as a tool for monitoring lateral variations in the LVZ and possibly to prove the existence or absence of a LVZ in the P velocity.