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Abstract

The observation of main seismic phases reaching the core shadow beyond 100º is known to traverse through the core of the Earth like PKP or PKiKP waves. Besides these, Pdiff wave that diffracts along the Core-Mantle Boundary (CMB) can also propagate into the core shadow. PKP is not the first wave that arrives in the shadow zone. Scattering of PKP at the CMB or in the whole mantle creates precursors to PKP arriving a few seconds before the actual PKP arrival. However, the scattering of P waves in the whole mantle allows waves to arrive more than 100 s prior to the PKP precursor if they do not enter the core but scatter around it through the fast mantle material. This energy arrives after the theoretical arrival of the Pdiff phase appearing to be the coda of Pdiff. We studied high-frequency (1-2 Hz) seismogram stacks of several large magnitudes (≥ 7.9 Mw) earthquakes. Individually Pdiff coda can be observed at very long distances beyond 150° in these events and deeper events allow for more clean observations. The Monte Carlo method is used to simulate the global earthquake scattering in a 1D spherically symmetric heterogeneous model. Simulation in an independently derived model of heterogeneity matches the observation of the Pdiff coda. An analysis of the origin of Pdiff coda is discussed using simulations with different scattering layers from the lithosphere to CMB. We demonstrate that the single scattering in the whole mantle explains the essential features of the observed Pdiff coda.