Evaluation of 1‐D and 3‐D seismic models of the Pacific lower mantle with S, SKS, and SKKS traveltimes and amplitudes

Abstract

AbstractIn this study, we analyzed the seismic phases S, SKS, and SKKS from 31 deep‐focus earthquakes in the Tonga‐Fiji region recorded in North America between epicentral distances of 85° and 120°. The differential traveltimes and amplitude ratios for these phases reveal clear epicentral distance trends not predicted by standard one‐dimensional (1‐D) reference Earth models. The increase of the S/SKS amplitude ratio up to a factor of 10 is accompanied by an increase of the S‐SKS differential traveltime of up to 10 s. SKKS‐SKS differential traveltimes of 2–3 s and SKKS/SKS amplitude ratios of a factor of 2–4 across the epicentral range have maxima near 107°. We examined these observations using full (1‐D and 3‐D) waveforms for three 1‐D seismic velocity profiles for the central Pacific region and for the tomographic model S40RTS including modifications: different regularization parameters, great‐circle path azimuthal variation, strength of S wave velocity perturbations, S wave velocity gradients in the lower mantle, and ultra–low velocity zones. To explain these data, we constructed a hybrid model that combines both features of S40RTS and short‐wavelength features from the 1‐D profiles. The large‐scale seismic structure is represented by S40RTS. Embedded within S40RTS are a 20 km thick ultra–low velocity zone at the core‐mantle boundary near the source side and a 200 km thick negative velocity gradient zone near the receiver side of the paths. Our analysis demonstrates that the S wave velocity structure of the Pacific large low shear‐velocity province cannot be interpreted solely by global tomographic or regional modeling approaches in exclusion of each other.