Effect of a plume on long period surface waves computed with normal modes coupling

The physical characterization and depth origin of mantle plumes are not well constrained. In order to address these issues, we look for observable effects of plumes on long period surface wave seismograms. The effect of a weak hut sharp heterogeneity on long period surface waves is computed by a first order normal mode theory using a generalization of the spherical harmonic summation theorem to reduce the number of operations. It turns out that it is necessary to couple very far along a given dispersion branch (I +/- 40 up to +/- 80) to remove spurious phases in seismograms. The coupling between different overtone branches (different overtone numbers) of the same kind (spheroidal or toroidal) and of different kinds are computed as well. By taking into account a large number of overtones, we are able to compute the effect of the heterogeneity on all seismogram phases from surface waves to body waves, including P-SH and SV-SH coupling. This technique is applied to different plausible models of mantle plume: a small vertical conduit down to 660 km depth, down to the core-mantle-boundary (CMB) and with or without head. We show that for a finite size plume the radiation pattern is essentially forward, which is not the case when the heterogeneity is considered as punctual. The scattered amplitude displays large variations for the different cases according to the temperature contrast, but an effect up to 10% of the incident amplitude can be expected, and should be observable on good quality seismic data. (C) 2000 Elsevier Science B.V. All rights reserved.