弧后逆冲断裂带深部结构与各向异性对比研究

As one of the significant characteristics and stress field models within the upper plate partially, back-arc thrust fault zones are the main information sources of seismogenic machnisms of and magmation in the back arc zone. The deep velocity structures and anisotropy of back-arc thrust fault zones are of great significance in science and practice for the study of the stress transfer and accumulation from plate subduction and back-arc spreading, seismogenic machnisms of back-arc earthquakes and subduction dynamic process. This study focuses on the Taranaki Thrust Fault in northern New Zealand and the Niigata-Kobe Tectonic zone in central Japan. We have collected the arrival times and waveform datum of local earthquakes and teleseismics, and will utilize a new tomographic methodology (joint nonlinear inversion for the velocity and Poisson’s ratio using a large number of arrival times from the same source-receiver pairs) to obtain simultaneously earthquake locations and 3-D images of P-wave velocity (Vp) and P- to S-wave velocity ratio (Vp/Vs) or Poisson’s ratio. We will interpret the relationships between the anomalies and the geologic structures. At the same time, we use the shear-wave splitting method to determine the polarization direction of fast shear-wave and the delay time of slow shear-wave in the crust and the upper mantle and the parameter variation characteristics of shear-wave splitting at the stations nearby the epicenter before and after the 2004 M6.8 Niigata Earthquake. This study centres on a comparative analysis of the deep structures, back-arc stress state and fluid behavior in the upper mantle on the different subduction system, and find the crustal deformation and the seismogenic machnisms in the back-arc thrust fault zone, and the dynamics relationship between that and deep magmation. By the above research, we will make and strengthen the understanding of the causing formation of the back-arc thrust fault zones and the dynamic process of subduction zones.

弧后逆冲断裂带作为部分俯冲带上覆板块弧后重要特征及应力场模型之一，是俯冲带弧后地震发震机制和岩浆活动的重要信息来源。研究弧后逆冲断裂带的深部速度结构与各向异性特征对认识板块俯冲和弧后扩张的应力传递和积累、弧后大震发震机理及俯冲带动力学过程具有重要的理论和实际意义。本课题选取新西兰塔拉纳基断裂带和日本新泻—神户构造带作为弧后逆冲断裂带的研究区域，利用近震远震到时及波形资料，采用速度和泊松比结构非线性联合反演，获得研究区多参数高分辨率深部结构，分析结构异常与地质构造之间的关系；同时利用剪切波分裂计算地壳、上地幔各向异性以及2004年M6.8新泻县地震前后剪切波分裂参数变化。对比两个研究区域不同俯冲系统的深部速度结构、应力状态空间分布差异和上地幔物质运动特征，探索俯冲带弧后逆冲断裂带的地壳形变、大震成因机制及其与流体和深部岩浆的动力学关系，进而加深俯冲带弧后逆冲断裂带成因机制和俯冲带动力学研究。

弧后逆冲断裂带作为部分俯冲带上覆板块弧后重要特征及应力场模型之一，是俯冲带弧后地震发震机制和岩浆活动的重要信息来源。研究弧后逆冲断裂带的深部结构与各向异性特征对认识板块俯冲和弧后扩张的应力传递和积累、弧后大震发震机理及俯冲带动力学过程具有重要的理论和实际意义。本课题选取新西兰塔拉纳基断裂带和日本新泻—神户构造带作为弧后逆冲断裂带的研究区域，选取环苏拉威西海多俯冲带地区研究多俯冲板片对上地幔形变相互影响，对比分析不同俯冲系统的应力状态空间分布差异和上地幔物质运动特征，探索俯冲带弧后地壳形变、大震成因机制及其与流体和深部岩浆的动力学关系。研究发现作为现今活跃的弧后逆冲断裂带，靠近断层的台站表现为构造引起的各向异性（快波方向平行于断层走向），远离断层的台站表现为应力引起的各向异性（快波方向与区域主应力方向一致）；2004年日本新泻逆冲型大地震引起的应力场变化对震源区微裂隙发育的影响会有所不同。而新西兰塔拉纳基断裂带古逆冲断裂带现今表现为北部拉张，南部挤压。对比近震S波分裂结果与远震SKS分裂结果的各向异性强度可以看出，研究区弧后地壳及地幔楔的各向异性强度较弱；塔拉纳基—鲁阿佩胡线两侧上地幔楔中各向异性快波方向明显不同，北部拉张和南部挤压对应的地幔楔对流表现出明显不同的方向。此外，通过远震SKS分裂计算环苏拉威西俯冲带上地幔各向异性结构，加深了对多俯冲过程中俯冲带的相互作用对上地幔形变动力学过程起到重要作用的认识，为复杂的构造背景下环苏拉威西海俯冲带的上地幔形变研究提供了新的地震学证据。