基于波形拟合的中国东海地区410km间断面附近速度结构研究

Velocity structure around the 410km discontinuity beneath the East China Sea area based on the waveform fitting method

410km间断面是地幔转换带的顶界面,对其速度结构和起伏形态开展地震学探测有助于认识地球内部物质组成和相关的地球动力学过程.本文选取了由中国数字地震台网记录到的位于琉球俯冲区的一个中源地震P波宽频带波形资料,利用三重震相波形拟合研究了中国东海地区410km间断面附近的精细速度结构.结果表明:中国东海地区下方410km间断面整体表现为一尖锐的速度界面且有8~15km的小幅抬升;该间断面之上存在52~62km厚的低速层,其P波速度降低0.5%~1.6%;440km深度以下存在1.0%~3.0%的P波高速异常.结合前人在该地区的层析成像结果,我们推测该高速异常体与西太平洋俯冲板片在中国东海地区地幔转换带内的滞留有关;板片内水相E分解使得转换带内水含量增加,这引发了410km间断面的抬升;410km间断面之上的低速层应与含水矿物脱水导致的地幔橄榄岩部分熔融有关.

The 410 km discontinuity is the upper boundary of the mantle transition zone. Seismic detections on the structure and morphology of the 410 km discontinuity are helpful to understand the compositions of the Earth＇s interior and the relevant geodynamics. In this paper, we select the broadband P waveforms of an intermediate earthquake that occurred in the Ryukyu subduction zone and retrieved from the China Digital Seismograph Network （CDSN）, and study the fine velocity structure around the 410 km discontinuity by matching the observed triplicated waveforms with the theoretical ones. Our results reveal that （1） the 410 km discontinuity beneath the East China Sea area is a sharp boundary with a small-scale uplift of 8~15 km, and （2） there exist a low velocity layer atop the 410 km discontinuity with the thickness of 52~62 km and P-wave velocity decrease of 0.5%~1.5%, and （3） a high velocity anomaly with P-wave increase of 1.0%~3.0% below 440 km. According to the previous tomographic results in this area, we speculate that the high velocity anomaly is relevant to the stagnancy of the western Pacific slab in the mantle transition zone, the decomposition of phase E in the slab results in the increase of water content, which would cause the uplift of the 410 km discontinuity, and the low velocity layer atop the discontinuity should be related to the partial melting of the mantle peridotite induced by the dehydration of the hydrous minerals.