Trans-Pacific Upper Mantle Shear Velocity Structure Along A Corridor From Tonga-Fiji To Alaska

Two geodynamic models,half-space cooling and plate cooling models,have been proposed to describe the age dependence of oceanic lithospheric thickness.However,it is difficult to distinguish two models with current seismic studies,which provide conflicting views.Tan and Helmberger(2007)developed a pure path upper mantle shear velocity model,PAC06,along the corridor from Tonga-Fiji to southern California.Although the sampled oceanic lithospheric age ranges from?10 Ma to 125 Ma,the 1D PAC06 model explains the S wave multiples up to S5 well.The similarity between PAC06 and PA5,sampling old oceanic lithosphere(100-125 Ma),indicates a more uniform oceanic lithosphere structure along the whole path from Fiji-Tonga to southern California.However,the PAC06 model is still debatable because of the possible path average effect.To further test the applicability of the PAC06 model,we study an older corridor from Tonga-Fiji to Alaska,sampling oceanic lithosphere from?55 Ma to 125 Ma,to investigate possible age dependence of the oceanic lithosphere structure.We selected 48 events of different depths occurring since 2016 in Tonga-Fiji including rich multibounce SH phases.Verified by modeling waveforms and travel times of G-phase and S multiples turning at different depths simultaneously,the PAC06 model can describe the lithospheric structure of the corridor we studied.The similar lithospheric structures of two corridors with different ranges of age suggests little variation in the mantle velocity structure beneath the Pacific with very different lithospheric ages.For deeper structure,using earthquakes with different depths and locations at Tonga-Fiji region,we find that the travel times of SS and SSS gradually delay to?10 s from the south to the north,which can be interpreted as a low-velocity anomaly located along the profile form Tonga-Fiji to Alaska.This slow anomaly varies at different depths,which extends from 17°S to 5°S with a-3%S-wave velocity perturbation(?Vs)in the transition zone depth.Beneath the transition zone,this slow anomaly extends from 12°S to 2°S at the depth of 660?800 km with ?Vs of-3%,and from 12°S to 2°S at the depth of 800?900 km with ?Vs of-5%.We conclude that this slow anomaly may be related to the Samoa plume.