Pdiff Coda Waves as the Result of Distributed Whole-Mantle Scattering

Seismic energy that follows the theoretical arrival time of the P_diff phase is usually regarded as P_diff coda. This implies its generation by scattering of diffracted P-waves. Such waves can theoretically be observed in the core shadow from 100° up the antipode in the time window extending from the theoretical arrival of the P_diff phase until the arrival of the next direct phase which is PP or a core phase.

However, scattered energy is also observed at frequencies above 1Hz where diffraction is inefficient. We present observations of scattered energy arriving more than 100s prior to PKP at distances exceeding 150° with an emergent shape and in the complete absence of a direct P_diff arrival. These observations exclude a connection to a diffracted P-wave. Modelling of the seismic energy propagation with radiative transfer theory in an independently established model of mantle heterogeneity confirms that the scattered seismic energy in the P_diff coda time-distance window has its origin in scattering of P-waves in the whole mantle. We demonstrate that different depth layers contribute to different arrival times in the scattered wave train which explains the emergent shape of the wave train and provides means to improve the depth resolution of current heterogeneity models. 

These findings confirm earlier interpretations that connected P_diff coda with mantle scattering. They are also compatible with array observations that show an extinction of the direct P_diff phase towards 107° above 1Hz, because even the seemingly direct arrival of P_diff at distances shortern than 130° can be mimicked by mantle scattering, as our modelling shows. The observed energy is thus more directly related to P-coda or PP-precursors than to the P_diff phase.