Waveform inversion of SS precursors: An investigation of the northwestern Pacific subduction zones and intraplate volcanoes in China

Abstract

The arrival time and amplitude of underside reflections from mantle seismic discontinuities (SS precursors) have made major contributions to the understanding of mantle composition and dynamics. In this study, we introduce a nonlinear waveform inversion technique to simultaneously constrain shear velocities and discontinuity depths beneath the northwestern Pacific subduction system. Based exclusively on a large SS precursor waveform dataset, we are able to clearly delineate the morphology of the descending Pacific plate, which flattens at the base of the upper mantle and extends westward by ~1500km toward northern-central China. Our grid search yields the maximum correlation between shear velocity and transition zone thickness at an angle of ~30°, consistent with the reported average slab dip beneath the study region. The strongly positive correlation suggests predominantly thermal, rather than compositional, variations along the descending Pacific plate. The joint depth-velocity solution also shows a 5–10km depression of the 410km discontinuity and an average decrease of 1.2% in upper mantle shear velocity beneath the intraplate volcanic fields in northeastern China. This anomaly, which reaches the middle of the upper mantle transition zone beneath the Changbai hotspot, initiates at a significantly shallower (~320km) depth beneath the Wudalianchi region. High amplitude reflections at depths greater than 410km suggest a water-poor melt layer in possible association with 1) decompression melting from passive upwelling and/or 2) active upwelling through a slab window.