Regional difference of total scattering coefficients revealed from the envelope analysis of ScS waves

Coda envelope reflects the medium heterogeneity in the Earth. The change of coda decay gradient associated with the ScS arrival becomes smaller as a period becomes shorter and a focal depth becomes shallower around the world. We discovered an offset behavior with coda decay rate change around the ScS arrival for deep events. To quantitatively explain the regional difference of coda envelope characteristics, we examined seismogram envelopes of regional earthquakes registered by IRIS stations in Kyrgyzstan, Sakhalinsk, and Bolivia with focal depths greater than 150km ranging from 1s to 20s periods for a wide lapse time range up to 2000s. We find that the offset before and after the ScS arrival is prominent in Kyrgyzstan and Bolivia at 10s period band. We estimated total scattering coefficients using the Direct Simulation Monte Carlo method based on the radiative transfer theory for the two-layer attenuation structure modified from the PREM. The total scattering coefficients of the upper layer for depths shallower than 670km are 7.520\times10^-4\sim1.129\times10^-3km^-1 and 4.510\times10^-4\sim6.770\times10^-4km^-1 for 4s and 10s, respectively, and the values of the lower layer for depths up to the CMB are 4.160\times10^-4\sim6.230\times10^-4km^-1 for 4s at all three places. In Kyrgyzstan and Bolivia, where show the offset, for the lower layer at 10s, we obtained total scattering coefficient as 2.710\times10^-4km^-1, which is larger than that of Sakhalinsk on the order of ten. It tells us that much stronger scattering in the lower mantle beneath Kyrgyzstan and Bolivia could be a cause of the offset behavior and coda decay gradient change after the ScS arrival.