Velocity structure of upper mantle discontinuities from waveform modelling of triplicated phases using Korean station

Abstract

The mantle transition zone and its discontinuities are keys to understand the complex nature of mantle convection. Northwest Pacific region has many earthquakes and seismic stations, so it is one of the best locations to study the mantle transition zone. Triplicated phases travel and carry the velocity information near the discontinuities. Waveform modelling of these phases can reveal detailed structure near the mantle transition zone. Korean seismic stations and two earthquakes from the Izu-Bonin-Mariana arc are used for the triplicated waveform modelling. Synthetic waveforms are calculated by Kennett reflectivity code. Grid search of models and calculating variance reductions to compare synthetics and observations is applied for each event. The best result for each event showed slow upper mantle and deep discontinuities. To make better model that explains both events, models that matches arrival times best are found by trial and error. First, the arrival times of phases travel above the discontinuity are matched with the models first. Then those travel deeper regions are matched by trial and error. The best model has 2% slow upper mantle, 440km deep 410 discontinuity, and 3% fast layer below 530km depth. This is consistent to the mantle wedge and stagnant slab found beneath the northern Philippine Sea.