Near-vertical multiple ScS (S waves reflected at the core-mantle boundary) phases are among the cleanest seismic phases traveling over several thousand kilometers in the Earth's mantle and are useful for constraining the average attenuation and shear wave speed in the whole mantle. However, the available multiple ScS pairs are limited. We took advantage of the recent dramatic increase in the number of global broadband stations and made a thorough computer-assisted search for high-quality data of multiple ScS pairs. We could find 220 station-event pairs which provided us with robust local estimates of average Q (quality factor) and two-way shear wave travel times. With the assumption that geometric focusing caused by lateral velocity heterogeneity does not seriously affect the amplitude measurements, the Q values exhibit strong short-range lateral variations, with very high and very low Q regions adjacent to each other. The mantle beneath seismic station KIP (Hawaii) has normal Q and shear wave speed, which supports the result of earlier studies. The mantle beneath station AFI (Samoa Islands) has a very high Q , possibly larger than 1400, and the slowest shear wave speed. The stations on the upper plate of the Tonga and Japan subduction zones yield average to low Q values. In contrast, the stations on the trenchward side of the upper plate of some subduction zones, e.g., station LVC (Chile) and station PET (Kamchatka, Russia), indicate high Q values, larger than 1000. We found no obvious correlation between Q and shear wave speed, which suggests that different factors like temperature, composition, anisotropy, etc., are controlling these properties in the mantle of different tectonic environments.

Seismic anisotropy is an efficient way to investigate the deformation field within the upper mantle. In the framework of rigid tectonic plates, we make use of recent tomographic models of azimuthal anisotropy to derive the best rotation pole of the Pacific plate in the uppermost 200 km of the mantle. It is found to be in good agreement with current plate motion (NUVEL1, HS3, and NNR). However, when dividing the Pacific plate into two subplates separated by what we refer to as the megagash, an east-west low-velocity and low-anisotropy band extending across the Pacific plate from Samoa-Tonga to the Easter–Juan Fernández Islands, the rotation pole of northern Pacific is still in agreement with current plate motion but not the rotation pole of the southern part of the Pacific, far away from the “classical” rotation pole of the Pacific plate. This result suggests a differential motion between the North and South Pacific and an ongoing reorganization of plates in the Pacific Ocean. The megagash might be a future plate boundary between the North and South Pacific plates, associated with the intense volcanism along this band.