P-WAVE AND S-WAVE SCATTERING FROM MANTLE PLUMES

We discuss a new method for seismically imaging mantle plumes by model ing the plume as a,vertical elastic cylinder and analyzing the effects of the cylinder on an incoming teleseismic plane wave. The incident p lane wave decomposes into cylindrical waves for which the scattering c oefficients can be computed by imposing continuity of stress and displ acement at the cylinder boundary. We extended this development to cyli nders with gradational boundaries by numerically integrating the elast o-dynamic equations. Synthetic seismograms calculated using this techn ique for a cylinder with. a range of parameters appropriate for a mid- Pacific mantle plume (epicentral distance Delta 50 degrees-90 degrees, Delta T = 200-400 K, boundary width 0-60 km) have the following chara cteristics: (1) a delay of the P wave arrival for an observational poi nt directly behind the plume, (2) the focusing of energy into the regi on directly behind the plume where the focal length is similar to 2 ti mes the plume diameter, and (3) the presence of secondary phases, and especially the excitation of a transverse component whose amplitude is up to 25% of the radial amplitude. Most of the phases arise from P-to -S conversions and reflections at the boundary of the cylinder. Increa sing the epicentral distance reduces the focal length and strengthens the focusing effect for receivers near the plume. Increasing the therm al anomaly reduces both the focal length and the amplitude of the seco ndary phases. Using gradational boundaries does not affect the scatter ing of low-frequency waves, but it does cause the reduction or disappe arance of the secondary arrivals at high frequency because of the redu ced efficiency of the conversions. Synthetic seismograms for an incide nt S wave arrival show a time delay more than twice as long as for the P wave and stronger focusing but only weak secondary phases. Consider able coupling takes place between SV and SH. On the basis of these fin dings we outline an observational program to image the Hawaiian plume: stations should preferably be installed close to but not directly on top of the most Likely position of the plume axis.