Hunting for azimuthal anisotropy beneath the Pacific Ocean region

Abstract

Lateral variations in azimuthal anisotropy cause significant waveform anomalies in long‐period surface waves (ƒ < 15 mHz, T ≳ 70 s), as a result of coupling between fundamental branch Rayleigh and Love waves. These anomalies, termed “quasi‐Love” (QL) waves, have elliptical polarization, arrive slightly behind the Love wave but prior to the Rayleigh wave, and are observed on many propagation paths in the Pacific Ocean region. Our observations of quasi‐Love waves indicate the existence of strong lateral anisotropic gradients in the western Pacific Ocean, in particular, near Hawaii and seaward of the Tonga‐Kermadec, Kurile and Marianas‐Izu‐Bonin subduction zones. A lack of quasi‐Love generation beneath the Phillipine Sea suggests weak azimuthal anisotropy in the region. In the southwest Pacific Ocean, the long‐period quasi‐Love waveforms recorded at station SNZO (South Karori, New Zealand) can be fit well with a simple anisotropic Earth model with a 90° rotation in the orientation of the fast P velocity axis near the boundary between the Indo‐Australian and the Pacific plates. In this model, 6% P wave velocity anisotropy from the Moho to 210 km depth, with a NNE‐SSW fast direction (roughly parallel to the plate boundary), extends 500–1000 km east of the Tonga‐Kermadec trench, where it shifts to a WNW‐ESE orientation (parallel to fracture zones in the southernmost Pacific plate). This solution is nonunique, however, as a similar lateral variation of 2% S wave anisotropy at asthenospheric depths (100–300 km) generates similar waveform anomalies. We attribute the laterally varying anisotropy to either (1) flow variations in the asthenosphere, consistent with lateral shear detected in deep Tonga‐Kermadec seismicity, (2) variations in fossil spreading direction in the Cretaceous long normal polarity interval, (3) the disturbance of fossil anisotropy caused by the passage of the Louisville Ridge hotspot, and (4) lithospheric compression associated with continental collision along the Alpine Fault. The apparent location of the Love‐to‐Rayleigh “scatterer” favors options 1 and 3. Gradients in azimuthal anisotropy are inferred in the northwestern Pacific, associated with one or more of these mechanisms. A quasi‐Love wave observed on a path from Alaska to PPT (Papeete, Tahiti) is bandlimited to ƒ > 8 mHz, suggesting lateral variations with wavelength twice the spacing of fracture zones in the north central Pacific.