Modeling the ratios of SKKS and SKS amplitudes with ultra-low velocity zones at the core-mantle boundary

Between 105-115 degrees degrees, the SKS waveform is complicated by the formation of SPdKS, a wave that has segments of P diffraction along the core mantle boundary. While previous studies have primarily focused on the move-out of SPdKS from SKS, we analyze the concomitant reduction of the SKS amplitude. Long-period SKKS/SKS amplitude ratios present a coherent global pattern. SKKS/SKS is relatively large in North and South American recordings of deep Tonga-Fiji earthquakes but PREM-like in European recordings of earthquakes in South America and North American recordings of earthquakes in Indonesia. Modeling of SKKS/SKS indicate that Ultra-Low Velocity Zones (ULVZs), layers at the base of the mantle with a thickness of about 10-20 km and a shear velocity reduction between 20-30%, are required to explain high SKKS/SKS ratios and the early move-out of SPdKS. Citation: Zhang, Y., J. Ritsema, and M. S. Thorne (2009), Modeling the ratios of SKKS and SKS amplitudes with ultra-low velocity zones at the core-mantle boundary, Geophys. Res. Lett., 36, L19303, doi: 10.1029/2009GL040030.