A broad 660 km discontinuity beneath northeast China revealed by dense regional seismic networks in China

Abstract

We examined the P wave velocity structure around the 660 km discontinuity at the tip of the subducting Pacific slab beneath northeastern China by forward modeling waveform triplication data. A total of 742 broadband seismograms were recorded by dense regional seismic networks in China from a deep earthquake that occurred near the border of east Russia and northeast China, providing an unprecedented density of ray coverage near the front edge of the subducting Pacific slab. Multiple P waves were observed on single seismograms in the distance range of ∼14°-29°. The P wave triplication shows the following two features: (1) the direct arrival traveling above the 660 km discontinuity (AB branch) extends as far as ∼29°, approximately 6 further than the prediction from velocity model of International Association of Seismology and the Physics of the earth's interior (iasp91); (2) the refracted wave propagating through the lower mantle (CD branch) appears at a distance a few degrees greater than that of the iasp91 synthetics. Forward waveform modeling suggests that they are best explained by a high-velocity transition zone underlain by a ∼50-70 km thick 660 km discontinuity. The broadened discontinuity is likely caused by multiple phase transitions associated with the dissolutions of olivine and garnet components. Copyright © 2010 by the American Geophysical Union.