Trans-Pacific whole mantle structure

Abstract

Recent reports on modeling USArray data reveal mostly vertical microplates with little resemblance to preliminary reference Earth model (PREM). Such complexity at plate boundaries makes it difficult to form reliable images of ocean basins using global paths. Here, we report on modeling stacked seismograms obtained from the first broadband array (TriNet) situated on the edge of the Pacific Plate, southern California, with no major subduction zone blocking its view. Extended records, including multi-S and ScS waves up to four bounces from 18 Tonga-Fiji deep events (140 to 620 km) are analyzed to check the validity of existing models and derive the whole mantle shear velocity structure along this corridor. Synthetics generated from 3-D tomographic models do not fit the upper mantle triplication data or the mantle reverberations associated with the ScS multiples as well as the 1-D model PAC06. We construct a hybrid model (HPAC), which remains one dimensional down to 800 km (PAC06). The lower portion of HPAC is essentially the tomography model S20RTS with velocity variation inflated by a factor of 2 for the lowermost 600 km. Thus, the mid-Pacific large low shear velocity province (LLSVP) has a lower shear velocity of about 2% relative to PREM and extends into the midmantle, similar to that beneath South Africa. Moreover, rapid changes in the differential (ScS-S) and (ScS2-S) times as a function of distance suggest ultra low velocity zones near the eastern edge and under the LLSVP, again similar to that found beneath Africa.