abstract
Record sections of long-period seismograms from shallow-focus events at distances between 80° and 120° commonly display significant amounts of spatially coherent energy between the arrivals of P and PP. While individual quality varies, the majority of observations share two characteristics: coherent arrivals have apparent ray parameters and amplitude attenuation rates more comparable to P than PP. The ray-kinematic restraints imposed by these statements suggest wave interactions in the upper mantle near the source or receiver. The dynamics of these phases are investigated by construction of synthetic seismograms for radially symmetric media by contour integration in the complex ray-parameter plane. The generated records containing contributions from waves reflected or converted at upper mantle discontinuities are found to be compatible with observations. The largest of these coda phases on vertical records are near-source-shear converted compressional waves of the types sdP and sdpP. Conjugate near-receiver phases are of less importance than near-source phases on vertical records but they can be expected to dominate on the horizontal component. The effect of transition zone thickness on these phases is determined by direct integration of the equations of motion. For a 10-km-thick transition, reflected amplitudes become negligibly small for waves with periods less than 3 sec. Transmitted converted waves, on the other hand, have significant amplitudes to periods as small as 1 sec. In the long-period passband, peaked near 15 sec, the transition behaves as a sharp interface. These signals are found to be highly sensitive to localized upper mantle structure. Further observations of these phases will provide a direct means of assessing the physical and regional extent of the discontinuities throughout the Earth.
Synthetic waveform modelling of SS precursors from anisotropic upper-mantle discontinuities
