华北克拉通东部滞留板块下方低速异常的地震三重震相探测

Seismic detection of a low-velocity anomaly under the stagnant slab beneath the eastern North China Craton with P-wave triplication

本文基于中国数字地震台网记录的中国东北与俄罗斯交界地区发生于2011年5月10日的一个深源地震的P波宽频带波形资料,研究了华北克拉通东部660km间断面附近的P波速度结构.通过一维射线追踪拟合P波三重震相的相对到时,并进行观测波形与理论地震图的对比,发现华北克拉通东部660km间断面下沉约15～20km,其上方存在厚度约115～120km的高速异常,P波速度升高1.5%～2.0%,应为滞留的太平洋俯冲板块;660km间断面下方存在局部的低速异常,P波速度降低0.6%～0.9%,该异常可能与滞留板块从其底部向下地幔顶部脱水或坍塌进入下地幔深处滞留体的脱水有关,也可能与板块深俯冲及板块碎片崩塌所引起的地幔热物质上涌有关.

The broadband P waveforms from a deep earthquake of 10 May 2011 at the border of Northeast China and Russia,recorded by the China Digital Seismograph Network,are used to study the P-wave velocity structure around the 660 km discontinuity beneath the eastern North China Craton.Through relative travel-time fitting of P-wave triplication using the 1Draytracing method and comparison between the observed and synthetic seismograms,we find a～15～20km depression of the 660 km discontinuity.A high-velocity layer is revealed above the 660 km discontinuity with the thickness of～115～120km and velocity increase of 1.5%～2.0%,and the layer should be the stagnant slab of the Pacific Plate.A local low-velocity anomaly is also revealed below the 660 km discontinuity with the velocity decrease of 0.6% ～0.9%.The low-velocity anomaly is possibly related with the dehydration of the stagnant slab from its bottom to the top of lower mantle or the dehydration of the 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 collapse of slab fragments.