SEISMIC DETECTION OF A LOW-VELOCITY ANOMALY UNDER THE STAGNANT SLAB BENEATH THE EASTERN NORTH CHINA CRATON WITH THE P-WAVE TRIPLICATION

We study the P-wave structure around the 660 km discontinuity (660) beneath the eastern North China Craton (NCC), using a deep earthquake occurred at the border of northeast China and Russia and recorded by the China Digital Seismograph Network (CDSN). Best-fitting models are obtained with relative travel-time fittings and waveform comparisons. We find that the 660 km discontinuity is depressed by 15∼20 km, atop which there is a high-velocity layer about 115∼120 km thick with a velocity increase of 1.5%∼2.0%, which should be the stagnant slab of the Pacific plate; and below the 660, there is a local low-velocity anomaly with a velocity drop of 0.6%∼0.9%, which is possibly related to the slab dehydration from its bottom to the top of lower mantle or the dehydration of slab fragments that have collapsed into the deep lower mantle, or probably associated with the hot material upwelling induced by the slab deep subduction and the collapsing of slab fragments.