论文部分内容阅读
利用天然地震的走时数据和层析成像方法反演了四川西部及邻近地区的Pn波速度和各向异性,据此分析了龙门山的上地幔结构和汶川地震的深部构造特征.研究结果揭示了龙门山断裂两侧Pn波速度的差异:松潘-甘孜造山带速度偏低,四川盆地速度偏高;前者易于发生流动和韧性变形,后者显示出较强的刚性,反映出活动造山带和稳定地块岩石层地幔的不同性质.高速区和低速区的分界对应于龙门山断裂,大体上以汶川为界,北段相对平直且与余震的震中分布相一致,南段向四川盆地一侧凹进弯曲,汶川8级地震的震中位于北段和南段之间速度结构横向变化最大的区域,也是龙门山深层构造应力最易于集中和最易于产生应力差的地区,具有十分明显的深部构造特征.Pn波各向异性反映出的深部物质流动与青藏高原的向东运动相吻合,但是明显受到四川盆地刚性岩石层的阻挡,这一作用过程为龙门山地区的应力积累和汶川8级地震的发生提供了深部动力来源.龙门山断裂与上地幔顶部的各向异性无明显的对应关系,估计该断裂的活动仅限于地壳内部,而深部物质的流动与川滇地块沿着鲜水河断裂的旋转挤出有一定的相关性,据此推测龙门山断裂和鲜水河断裂具备不同的深层动力条件.
The Pn wave velocity and anisotropy of the western Sichuan and its adjacent areas were retrieved by the traveltime data and tomographic imaging of natural earthquakes, and the structure of the upper mantle and the deep structure of the Wenchuan earthquake were analyzed. The results reveal that Differences of Pn wave velocity on both sides of the Longmen Shan fault: the Songpan-Garzê orogenic belt is low in velocity and high in the Sichuan Basin; the former is prone to flow and ductile deformation, while the latter shows strong rigidity, reflecting active orogenic belt and stability The boundary layer between the high-velocity and low-velocity zones corresponds to the Longmenshan fault, which is generally bounded by the Wenchuan Mountains, with the northern section relatively flat and consistent with the epicenter distribution of the aftershocks, and the southern section to the Sichuan Basin side Recessing and bending. The epicenter of Wenchuan MS8.0 earthquake is located in the area with the most lateral variation of velocity structure between the north and the south sections. It is also the area where the deep tectonic stress in the Longmenshan area is most easily concentrated and the stress difference is most likely to occur. It has a very obvious deep structure The characteristics of Pn wave anisotropy reflect the deep material flow coincides with the eastward movement of the Qinghai-Tibet Plateau, but it is obviously blocked by the rigid rock layer in the Sichuan Basin, A process of action provides a deep source of power for the stress accumulation in the Longmenshan area and the occurrence of the Wenchuan Ms8.0 earthquake.The Longmenshan fault has no obvious corresponding relationship with the anisotropy of the upper mantle and it is estimated that the fault activity is limited to the interior of the crust, However, the flow of deep material is related to the rotation extrusion of the Xianshuihe fault along the Sichuan-Yunnan block. Based on this, it is speculated that the Longmenshan fault and the Xianshuihe fault have different deep dynamic conditions.