郯庐断裂带鲁苏皖段及邻区上地幔顶部Pn波速度与各向异性

为着重探讨郯庐断裂带鲁苏皖段的分段特征及其深部结构特征,本研究搜集增加了研究区内近几年的Pn震相数据.经挑选后采用了由2163个地震和301个台站构成的9156条射线,重新反演构建了郯庐断裂带鲁苏皖段及邻区上地幔顶部Pn波速度及方位各向异性分布.尔后,将速度反演结果与地形地貌、大地热流、强震活动及地壳厚度等进行了综合对比和分析;此外,还将Pn波各向异性与地震各向异性其他观测手段的结果展开了对比和讨论.反演结果的可靠性和分辨能力较前文有所提高,研究区上地幔顶部的非均匀性特征揭露得更为清楚,尤其是由Pn波速度揭露的郯庐断裂带分段特征与地质构造特征吻合得更佳.结果表明:(1)Pn波高、低速的分布与地形地貌呈负相关的关系,即山地隆起区呈低速分布,而平原地区表现为高速异常.(2)Pn波速度沿郯庐断裂带的分布具有明显的分段特征,本文的研究结果支持郯庐断裂带在研究范围内的上地幔顶部可细分为4个亚段.由上地幔顶部Pn波速度揭示的郯庐带分段特征与前人根据郯庐断裂带各段的地表地质构造特征给出的分段结果吻合,深、浅耦合的现象表明郯庐断裂带是一条贯穿地壳、深抵Moho面的幔源深大断裂带.(3)Pn波速度与大地热流呈负相关的关系.(4)Pn波速度的分布与强震的发生具有一定的关联性,强震大部分发生在Pn波高、低速过渡地带或者低速区域的地壳内.(5)在南黄海海域新发现一条NNE-SSW向展布的弱高波速异常带,该异常带被强震震中清晰地勾勒了出来,据此推测其下方存在一条切割Moho界面的幔源深大断裂.(6)Pn波各向异性的强弱与地质构造的活动性相关,活动性越强则各向异性强度越大.(7)由地震台站的时间延迟分布可以看出,研究区地壳厚度总体上自SE往NW逐渐增厚;最大厚度位于渤海湾盆地的北西部和西部.

Uppermost mantle Pn-wave velocity and anisotropy structure beneath the Shandong-Jiangsu-Anhui segment of the Tan-Lu fault zone and its adjacent regions

In order to focus on the sectional characteristics and deep structural characteristics of the Shandong-Jiangsu-Anhui section of the Tan-Lu fault zone, this study collects and adds Pn seismic phase data in the study area in recent years. After selection, 9156 rays composed of 2163 earthquakes and 301 seismic stations were used to reconstruct the Pn wave velocity and azimuthal anisotropy distribution at the top of the upper mantle in the Shandong-Jiangsu-Anhui segment of the Tan-Lu fault zone and its adjacent areas. Later, the velocity inversion results were comprehensively compared and analyzed with topography and geomorphology, geothermal flow, strong earthquake activity and crustal thickness. In addition, the results of Pn wave anisotropy and the results of other observational methods of seismic anisotropy were compared and discussed. The reliability and resolution of the inversion results are better than that of the previous paper, and the heterogeneity at the top of the upper mantle in the study area is more clearly revealed, especially the segmentation characteristics of the Tan-Lu fault zone revealed by Pn wave velocity are better consistent with geological structure characteristics. The result show that: (1) The distribution of Pn wave with high and low velocity is negatively correlated with topography and geomorphology. (2) The Pn wave velocity distribution has obvious segmentation characteristics along the Tan-Lu fault zone. The results of this study support that the Tan-Lu fault zone can be subdivided into 4 sub-segments within the study range and the top of the upper mantle. The segment characteristics of the Tan-Lu fault zone revealed by Pn wave velocity at the top of the upper mantle are consistent with the segmentation results given by the predecessors according to the geological structure of each section of the Tan-Lu fault zone. The phenomenon of deep-shallow coupling indicates that Tan-Lu fault is a mantle-derived fault zone penetrating the crust and reaching the Moho surface. (3) The Pn wave velocity is negatively correlated with the geothermal flow. (4) The distribution of Pn wave velocity has a certain correlation with the occurrence of strong earthquakes. Most of the strong earthquakes occur in the crust of the transition zone of Pn wave high and low velocity or in the crust of the region with low Pn velocity. (5) An anomaly zone with weak high wave velocity in the NNE-SSW direction was newly discovered in the South Yellow Sea. The anomaly zone was clearly outlined by the epicenter of the strong earthquakes, and it was speculated that there was a deep fault cutting the Moho interface below it. (6) The strength of Pn wave anisotropy is related to the activity of geological structure. The stronger the activity is, the stronger the strength of anisotropy is. (7) From the time delay distribution of the seismic stations, it can be seen that the crustal thickness from SE to NW is gradually thickened in the study area; the largest lies in the northwest and west of the Bohai Bay Basin.