Low-velocity layer atop the upper mantle transition zone in Northwest Pacific subduction zone
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摘要:
自20世纪90年代首次探测到410-km间断面上覆低速层以来，全球多个俯冲带和大陆克拉通地区都陆续发现了该低速层结构.对其特性及形成机理的探讨是深部地幔结构、物性和动力学研究的热点问题.本文聚焦于西北太平洋俯冲地区410-km间断面上覆低速层的探测及特性研究上.通过对发生于日本北海道地区两个中等深度地震区域波形资料的分析，利用三重震相波形拟合方法获得了我国东北及日本海西北部下方410-km间断面附近的P波速度结构.速度模型明确显示，410-km间断面上方存在厚~47±14 km，异常值~2%的低速层，横向展布近700 km.结合区域地震层析成像、矿物高温高压物理实验及动力学模拟结果，我们否定了"从下至上"的上涌热物质导致410-km间断面上覆低速层的模型；认为较老且快速俯冲的太平洋板块在地幔过渡带顶部脱水导致硅酸盐矿物的部分熔融，由于熔体密度较大能够稳定存在于410-km间断面之上，从而产生了观测到的横向展布较广的410-km间断面上覆低速层结构.
Abstract:Since the first detection of the low-velocity layer atop the 410-km discontinuity in 1990s, the seismic structure has been revealed in many subduction zones and continental craton regions. Its characteristics and mechanism of formation are among one of the hottest topics in studies of deep mantle structure, physical properties of mantle material, and geodynamic processes. We focus on the detection and feature investigation of the low-velocity layer atop the 410-km discontinuity in the northwestern Pacific subduction zone in this study. By modeling P triplicate waveforms from two intermediate-depth earthquakes, which are recorded by regional network stations, we obtain the P wave velocity structure near the 410-km discontinuity beneath northeast China and northwest area of Japan Sea. A low-velocity layer with thickness~47 ±14 km and~2% velocity anomaly is detected above the Mantle Transition Zone, covering a lateral range around 700 km. Combined with results from regional seismic tomography, high pressure and high temperature experiments and geodynamic simulations, we regard the "bottom-to-up" upwelling model an unlikely mechanism for the observed low-velocity layer; instead, we propose that the low-velocity layer may represent the partially melted silicates caused by dehydration of the cold and fast subducting Pacific slab near the 410-km discontinuity. The higher density makes it stable at the 410 km depth and produces the observed widely distributed low-velocity layer.
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图 1
三重震相射线路径(a)及理论走时(b)示意图
Figure 1.
Schematic ray paths of the triplicated phases (a) and theoretical travel times of these arrivals (b)
图 2
不同速度模型及其对应的P波理论地震图(垂向位移记录)
Figure 2.
Synthetic seismograms (vertical displacement components) calculated from different models
图 3
地震事件及台站分布
Figure 3.
Map showing location of deep earthquakes and regional seismic stations used in this study
图 4
事件20080604(a)和20090824(b)波形拟合图及P波速度模型图(c)
Figure 4.
Comparison of the observed (black solid line) and synthetic (blue dashed line) vertical waveforms computed from our preferred P velocity model for events 20080604 (a) and 20090824 (b)
图 5
事件20080604(a)和20090824(b)观测波形与拟合波形之间的相关系数随震中距变化情况
Figure 5.
Cross-correlation between the observation and synthetics for events 20080604 (a) and 20090824 (b) calculated
表 1
地震事件信息
Table 1.
Information of earthquakes used in triplication waveform modeling
事件 时间 纬度/(°N) 经度/(°E) 震级/MW 深度/km PDE CMT ISC 选定深度 20080604 17:03:07.40 41.53 139.05 5.7 212 204 210 212 20090824 05:25:16.53 41.05 140.18 5.3 176 170 171 171 注：PDE，Preliminary Determination of Epicenters；CMT，Centroid Moment Tensor；ISC，International Seismological Centre. 
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