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Modeling the ratios of SKKS and SKS amplitudes with ultra‐low velocity zones at the core‐mantle boundary
Author(s) -
Zhang Yang,
Ritsema Jeroen,
Thorne Michael S.
Publication year - 2009
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2009gl040030
Subject(s) - geology , core–mantle boundary , seismology , mantle (geology) , amplitude , geophysics , physics , quantum mechanics
Between 105–115° degrees, the SKS waveform is complicated by the formation of SP d KS, a wave that has segments of P diffraction along the core mantle boundary. While previous studies have primarily focused on the move‐out of SP d KS 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 SP d KS.