日本列岛下太平洋俯冲板块的精细结构

尽管许多学者对日本列岛下的太平洋俯冲板块做了大量的研究,但板块内部的结构(比如板块厚度,板块内地震波速度随深度的变化以及洋壳的俯冲情况等)仍然不太清楚。利用日本地区密集台网收集到的中深和深发地震到时数据来探讨上述问题。采用三维射线追踪正演模拟法,首先利用333个远震计算得到了日本地区太平洋板块的厚度为85km;然后利用3283个地震(震源深度大于40km)的130227条P波到时进一步研究板块内部的精细结构。结果显示,沿深度方向6个地层段(间隔100km)内的速度扰动值分别为5.5%,4.0%,3.5%,2.5%,2.0%和6.0%,在40～500km范围内速度扰动随深度的增加而减小,这与温度随深度的变化情况相一致。当深度大于500km时,速度扰动突然增大到6.0%,分析认为该异常可能由发生在东亚大陆边缘下方的深发地震无法精确定位导致的。最后利用40～500km深度范围内的近震测试得到日本东北和北海道地区下方洋壳俯冲的深度均为110km,平均厚度分别为7.5km和5km,相对于一维模型的速度扰动分别为1%和-3%。这说明洋壳在俯冲到110km以深时,由于受温度和压力的影响,逐渐脱水、变质,直至与板块融合。通过分析震源与洋壳的位置关系,本研究认为北海道地区比东北地区下方的俯冲洋壳可能含有更多的流体(比如水),导致两地区洋壳内的速度相差如此之大。此外,因为日本南部与洋壳对应的区域多为海洋,观测台站较少,所以本研究无法测试得到该区域内的洋壳俯冲情况。

Detailed structure of the subducting Pacific slab beneath the Japan islands and Japan sea

Although many studies have been made to image the subducting Pacific slab in and around the Japan islands,details of the slab structure(such as the slab thickness,its seismic velocity,the subducting oceanic crust,as well as the depth variations of these factors)are still unclear.In this study,we have addressed this issue by using arrival times of intermediate-depth and deep earthquakes recorded by the dense seismic network on the Japan islands.We adopted a forward-modeling approach with a 3-D ray-tracing technique.We first used 333 teleseismic events and found that the thickness of the Pacific slab beneath Japan is 85 km.We then used 130227 P-wave travel times from 3283 local and regional earthquakes with focal depths greater than 40 km to study the detailed structure of the subducting Pacific slab.Our results show that the average velocity perturbations of the slab in 6 depth ranges are 5.5%,4.0%,3.5%,2.5%,2.0% and 6.0%,respectively.The perturbations in the depth range of 40-500 km decrease with depth,which is consistent with the pattern of increasing temperature with depth.However,at depths greater than 500 km,the perturbation amounts to 6.0%,which may be caused by the large uncertainty in the hypocenter locations of the very deep earthquakes under the East Asia continent margin.Finally,we used the local and regional events with focal depths from 40 to 500 km to study the subducting oceanic crust beneath Northeast Japan and Hokkaido.Our results show that the oceanic crust extended down to 110 km depth under both regions,the average thickness of the oceanic crust is 7.5 and 5 km,and the velocity perturbation in the oceanic crust relative to the 1-D model is 1% and-3%,respectively.These results can be interpreted that the oceanic crust has dehydrated and metamorphosed gradually because of the increasing temperature and pressure with depth.After analyzing the relationship between the hypocenters and the oceanic crust,we consider that the large difference(1% and-3%)is probably caused by more fluids(such as water)in the oceanic crust beneath Hokkaido than that beneath Northeast Japan.The state of the oceanic crust beneath southern Japan was not determined because most of the region is covered by the ocean and so there are few stations there.