基于三重震相波形非线性反演的俯冲带410-km间断面起伏研究

利用NECESSArray宽频带地震台阵记录的P波三重震相波形资料，采用遗传算法，对千岛俯冲板块内部及附近410-km间断面的结构进行了非线性反演.其中，选取了发生在俯冲带的发震时刻为2009年10月10日21时24分（GMT时间）震级为Mw5.9的地震；其三重震相的射线回折点处射线路径的方向与俯冲板块的走向大致一致，克服了间断面在俯冲板块内部沿俯冲方向起伏剧烈、不易识别的困难，设计以震中为顶点、方位角范围分别为275°~280°、269°~274°、264°~266°的北、中、南三个扇形区域，用于研究410-km间断面逐渐靠近俯冲板块直至处在其中的起伏情况；"先对齐、后反演"的具体计算方案极大地减小了浅部结构不确定性对反演结果的影响；同时，整体归一化策略充分利用了台阵的振幅信息，有效地加强了对深部结构的约束.反演结果显示，"410-km间断面"在北区抬升了10~20 km，在中区抬升了20~30 km，在南区抬升了60~70 km，与橄榄石-瓦兹利石平衡态相变界面的矿物物理学预测结果一致；其波速跃变在北区为10%，在中区为10%，在南区为7%.扣除了前人在层析成像显示的地震源区及目标区速度异常的影响后，约4%的波速跃变可能由橄榄石-瓦兹利石的相变所产生，与IASP91模型的速度跃变值相当.目前的研究结果表明俯冲带内部似乎不存在大量的亚稳态橄榄石.基于更多资料并对波形细节进行拟合，可望刻画俯冲板块内部410-km间断面的精细结构，给出关于这一问题的确定性回答.

Topography of the 410-km discontinuity in and around subduction zone from nonlinear inversion of triplicated waveforms

Using triplicated P-waves recorded by the NECESSArray broadband seismic array,we applied the genetic algorithm to perform nonlinear inversion for the structure of the 410-km discontinuity in and around the Kuril subduction zone.We focus on the waveforms from an M w5.9 earthquake that occurred in the Kuril subduction zone at 21∶24(GMT),October 10,2009.The ray paths are roughly parallel to the subducting slab′s strike direction,overcoming the difficulty that,in the dip direction,the discontinuities fluctuate too sharply to be identified.We divided the region into northern(275°~280°),central(269°~274°),and southern(264°~266°)regions,according to their azimuthal ranges.Therefore,we could study the undulations of the 410-km discontinuity as it gradually approaches and eventually penetrates the subducting slab.In order to minimize the influence of the shallow structure on the results,we aligned waveforms on the P-wave arrival prior to inversion.Additionally,we apply an array normalization strategy which allows us to make full use of triplication amplitudes to resolve discontinuity structure.Our inversion results show that the“410-km discontinuity”is uplifted by 10~20 km in the north,20~30 km in the central region,and 60~70 km in the southern area.This uplift is consistent with mineral physics predictions of the equilibrium phase transition from olivine to wadsleyite.The jump in compressional wavespeed across the discontinuity is 10%in the north,10%in the middle,and 7%in the south.After accounting for the influence of the wavespeed anomalies both in the earthquake source side and our research region,shown by previous tomography results,the remaining~4%of the wave speed jump is comparable to the value in the IASP91 model.Our current results show no conclusive evidence for the presence of a metastable olivine wedge in the subducting slab,but future studies aimed at detailed waveform fitting of transition zone triplications may give a deterministic answer to this issue.