Spatial variations of the 660-km discontinuity in the western Pacific subduction zones observed from CEArray triplication data

We examined the spatial variation of velocity structures around the 660-km discontinuity at the western Pacific subduction zones by waveform modeling of triplication data. Data from two deep earthquakes beneath Izu-Bonin and Northeast China are used. Both events were well recorded by a dense broadband seismic network in China (CEArray). The two events are located at approximately the same distance to the CEArray, yet significant differences are observed in their records: (1) the direct arrivals traveling above the 660-km discontinuity (AB branch) are seen in a different distance extent: ~29° for the NE China event, ~23° for Izu-Bonin event; (2) the direct (AB) and the refracted waves at the 660-km (CD branch) cross over at 19.5° and 17° for the NE China and the Izu-Bonin event, respectively. The best fitting model for the NE China event has a broad 660-km discontinuity and a constant high velocity layer upon it; while the Izu-Bonin model differs from the standard IASP91 model only with a high velocity layer above the 660-km discontinuity. Variations in velocity models can be roughly explained by subduction geometry.